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MG MGA - Tappet cover / breather pipe
| My '56 roadster has the early style front tappet cover where the breather pipe comes straight out and down. I've found, as BMC did years ago, that this configuration tends to allow oil to drip out. I'd like to change to the later design where the pipe comes out at an upward anlge before bending back down. Unforunately it's no longer available from the usual suppliers. Just wondering if anybody has a spare that they could do without or knows where I might find one. |
| Andy Bounsall |
| Andy An option would be to rotate the cover 180 degrees and then connect to the rocker box vent pipe as I have done. See here: http://www.mgaroadster.co.uk/hot_press.htm Steve MG |
| Steve Gyles |
| I have the rocker cover with the breather but somebody put it on the back with the breather pointing downward - I am sure that is a source of a bunch of my oil all over the bottom of the car. The other Steve's solution looks good. You could turn your cover over and solder together a copper pipe that makes a 180 degree turn using two els. I also have heard of people bending soft copper by putting sand in it to keep the tube from crimping. Just some ideas. |
| Steve |
| Andy, I have a spare you're welcome to. I don't have the pipe itself however. ddsjostrom (at) comcast (dot) net. Doug |
| D Sjostrom |
| Do not connect the tappet cover vent to the air cleaner without a fresh air inlet for the crankcase. Connecting both the tappet cover vent and valve cover vent to the air cleaner defeats (disables) the intended crankcase ventilation function. This results in stagnant vapor in the crankcase and all of the cylinder blow-by fumes being forced into the carburetor intake. This will oil up the front carburetor, possibly carbon up the front two cylinders, and allow accumulation of water vapor and other nasty acids and sludge in the crankcase. Frequent oil changes may keep cam and tappet and and bearing wear to a minimum, but the engine still gets dirty inside. The draft tube works as a venturi when the car is in motion to draw a slight vacuum on the crankcase and produce air flow. The combination of valve cover vent and draft tube moves fresh air into and through through the crankcase to ventilate it to purge the blow-by gasses and reduce the accumulation of water vapor, acids and sludge in the crankcase. Later model cars have the tappet cover vent connected to the inlet manifold vacuum (with or without a PCV valve) to produce similar crankcase ventilation, along with restricted fresh air intake to the valve cover. All engines should have some form of crankcase ventilation with fresh air throughput. To do this the crankcase vent (front tappet cover tube) must either have the draft tube or be connected to a vacuum source. |
| Barney Gaylord |
| Barney This modification was recommended to me by Bob West. I have never had any of the symptons you talk about in the 8 years I have been running the modification. Surely, the suction through the air filter provides the necessary vacuum you talk about. Steve MG |
| Steve Gyles |
| i bought a good used one, including the proper pipe and bracket from Scarborough Faire in Rhode Island, USA. They charged me something like 20 bucks. |
| brucep |
| Steve G, Suction inside the air cleaner (pressure drop across the filter element) is very small compared to the vacuum level created by the draft tube. With the draft tube in place and the car moving air goes in through the air cleaner, then through the vent hose to the valve cover, through the crankcase, and out the draft tube. This is amazingly effective at road speed moving a lot of ventilation air through the crankcase. When you connect both pipes to the air cleaner you have no pressure differential to drive the air flow, so you get no crankcase ventilation at all, only exhaust of the blow-by products and crankcase fumes into the carburetor intake. How well your engine fares with that arrangement depends on how much you drive, how many short cold-run trips you make, and how often you change the oil. With no crankcase ventilation you may expect to find some sludge accumulating inside, some in the oil and some adhering to all internal engine parts. With the crankcase exhaust directed into the carburetor intake you should also expect to have carbon deposits accumulating in the combustion chambers. If this doesn't bother you that's fine, but I wouldn't recommend it to anyone else. Internal combustion engines need crankcase ventilation and don't like oil in the air intake. |
| Barney Gaylord |
| For what it's worth to the discussion, Doug Jackson from British Automotive (mgbmga.com) is clearly in the camp that advocates blocking off the tappet cover vent altogether and just using the rocker (valve) cover vent for crankcase ventilation. (See links below...) As far as I know, all modern cars use closed crankcase ventilation (i.e., no atmospheric vent) with a PCV connected to the intake. Usually the PCV hose is connected to the valve cover that either has a non-vented cap, or, as in the case of SAABs, no cap at all. http://www.mgbmga.com/tech/mgb3.htm http://www.mgbmga.com/tech/mgb3a.htm Steve |
| Stewve Brandt |
| After posting, I realized that the crankcase ventilation systems are not truly closed. (Think first, type second...) The ones I'm familiar with have a charcoal canister that is connected to some type of valving arrangement for pressure balancing the crankcase. I did, however, find a Goggle reference that might be helpful to the discussion, and as a bonus, it is written by someone with multiple LBCs. http://www.teglerizer.com/triumphstuff/emissions/index.htm Steve |
| Steve Brandt |
| Barney - Wow - I assumed the tube from the valve cover vented from the valve cover to the carb where the engine would be sucking in air, not the other way with air going to the valve cover. So what you are saying is the air comes in through the air cleaner and through the valve cover and blows-by down to the crankcase and out the tappet cover vent. If a car is not driven at good speeds very much it would seem there is not a lot of vacuum to pull the fumes, moisture, etc. from the crankcase. In my case, with no tube from the tappet cover, my engine has no vacuum. Hmmmm - gotta get one of those tappet cover vent tubes! |
| Steve |
| Wow. Thanks much for all the replies and info about how the crankcase ventilation system works. Fascinating stuff. Doug, thanks again for your generous offer. |
| Andy Bounsall |
| Barney Sorry, I totally disagree with the thrust of your argument. You mention, quite rightly, that there is a pressure drop in the filter box caused by air being sucked through the carbs. Bearing in mind that the rocker box and crankcase are running at a greater than ambient pressure there is no way that air travels up the pipe to the rocker box from low to high pressure. That defies the rules of science. I also suggest that any pressure drop in the tappet cover pipe due to the car's speed will always be less than the pressure drop in in the air filter box. Therefore, airflow will always be from the engine to the filterbox. Having looked at Steve Brandt's links, the one concession I may make to my system is to also fit a front air filter canister to the rear carb, plus a linking pipe, thereby equalising the venting to all 4 cylinders instead of just to numbers 1 and 2. Steve |
| Steve Gyles |
| Steve, I suppose that eventually someone will have to use a manometer and measure the pressures at all the different points involved. You say that the rocker box and crankcase are at greater than ambient pressure. This is not so, and must never be the case. You would have oil coming out of every seal the and crankcase fumes going up past the piston rings etc. It is essential that the crankcase and rocker box be at reduced pressure to remove the contaminants that are created in them. You can tell which way the air flow is by observing the flow on older engines of the combustion products (fumes, moisture etc.) from the bottom of the tappet cover tube. If the flow was towards the air cleaner you would have all these contaminants clearly visible inside the air cleaner and carburettor throat (in front of the jet but not behind it where the fuel mixture would clean it). I have never observed this mess on the inside of my air cleaner elements. A final compelling fact is that if the air flow was from the tappet tube to the air cleaner, dust and grit would be drawn from the road, through the crankcase and valve gear. You must have clean filtered air drawn through the rocker box and crankcase. Mick |
| Mick Anderson |
| Barney Re-reading your words a few more times, I am not actually miles apart with your analysis of my set-up. What I cannot get my mind round is your view about the the down pipe acting as a very efficient venturi. For me, it does not have correct profile to make it a venturi. I am certainly playing it through my mind about the direction of airflows with the rocker box vent, but here again I struggle to come to terms with your logic. The crankase without ventilation becomes a high pressure area. This is made up of blow-by gasses from the combustion chambers and evaporation of the very hot engine oil. The oil drain holes from the cylinder head valve gear would allow the air pressure to equalize in the rocker area. To me this logically says that air would then normally migrate from both areas through their respective vent pipes. Therefore, I really do struggle to see how air can travel from the low pressure area in the air filter box to the higher pressure area in the rocker box. I can see the logic of your venting cycle and would agree that it is desirable. It is just that my brain tells me that this does not happen in the MGA engine compartment with its venting set-up. Maybe I am wrong, but I can't get my logic to see it another way. Incidently, I have the 1800 engine with the vent tube coming upwards infront of the manifold, so I don't have the downpipe unless I retrofit the MGA plate. Steve |
| Steve Gyles |
| Whilst on the subject of these tappet covers and vent tubes, anybody have a vent tube for sale? Thanks, Steve |
| Steve |
| I have to ask what may seem to be a "dumb" question. Years ago I installed a Moss aluminium valve cover, it has no vent pipe to the air cleaner. I have since installed the Moss supercharger and am not using the vent port on the supercharger that the stock valve cover would be vented to (rather, I installed a small K&N air cleaner on to that port as closing it kills the supercharger). The engine is otherwise a stock 1600 (late 1959 roadster). I have never noticed any oil leaking from anywhere, the oil filler cap must be vented and I have never had any oil blowing up on the inside of the bonnett. When I installed the valve cover I never gave it any further consideration and that was in 1980 or so........ What am I missing? Ted |
| Ted Persons |
| Eliminating the valve cover vent also defeats crankcase ventilation (while retaining exhaust from the tappet cover vent). A valve cover with no vent pipe should be used with a vented oil filler cap. The vented cap should contain an intake air filter and needs to be cleaned or replaced periodically. MGA and MGB through 1963 used the draft tube and free intake air vent from the air cleaner to valve cover. If you have no valve cover vent with these models you should use a large unrestricted free flow filter type oil filler cap. From 1964 on the MGB used manifold vacuum to induce crankcase ventilation. 1964-1969 models drew air in through a vented oil filler cap with flow restrictor and internal filter. These filter type caps were supposed to be replaced periodically. From 1970 on crankcase vent air was drawn through the carbon canister with restrictor at the valve cover inlet pipe. The carbon cannister did a decent job of filtering crankcase inlet air. Operation of the draft tube as a vacuum venturi is easy to demonsrate in your kitchen sink. Fill a tall glass with water and drop in a soda straw (not more than two inches from top of straw to water level). Cut the top end of the straw at 45 degree angle and turn it so the opening faces away from you. Blow across the tip of the straw at a right angle and the vacuum generated can draw water up the straw to produce an atomizing spray. The draft tube on the MG engine does not produce a very high vacuum level, but with the 1/2 inch bore pipes it does draw a moderately large volume of air. The manifold vacuum setup on the MGB produces crankcase ventilation any time the engine is running. The draft tube draws air through only when the car is moving. For the purpose of purging fumes and acids and water vapor from the crankcase it is sufficient to have the fresh air ventilation only while moving. With the draft tube setup, when standing still idling some blow-by gasses will be pushed gently from the valve cover vent into the air cleaner. If you spend a lot of time stopped and idling you may notice a little oiling of the front air cleaner cannister, which should be very minor and of no consequence to the health of the engine. If the piston rings are badly worn producing copious amounts of blow-by, the air cleaner cannister may accumulate a larger amount of oil. In this case you needn't worry much about it being detrimental to the engine, because it already needs a ring job. Anything you do with crankcase venting from that point on is a BandAid patch job while you procrastinate about doing the engine work. |
| Barney Gaylord |
| Barney, Is "valve cover" the common US term for what we and WM call the "side cover" exposing the push rods? Mike |
| Mike Ellsmore |
| Mike, The "valve cover" is what it says, the cover over the valves, or more accurately the cover over the valve springs and rocker arms. The "side cover" is again what it says, the cover on the side of the engine, covering the pushrods. Sometimes called the tappet (cam follower) cover because of their close proximity. Mick |
| Mick Anderson |
| Steve, Leaving aside for the moment the views on the direction of travel of the gases in the rocker cover and crankcase. In your photo the pipes from your side cover and the rocker box cover join together and then the single pipe goes to the air cleaner. In this situation the slight lower pressure inside the air cleaner is applied to the crankcase and rocker box equally. This will create a slightly lower than atmospheric pressure inside the crankcase and rocker box. This will be OK for one task, preventing pressure forcing oil out through seals and other openings. However, there will be almost no crankcase ventilation, which is the more important task, to remove water vapour, acid vapour etc. A small amount of ventilation will be provided by air leaking in through the front and rear crankshaft seals, oil filler cap, and dipstick hole, but not nearly enough. I would be very concerned about corrosion in your engine. Mick |
| Mick Anderson |
| Mick Thanks for your comments. It is something I am obviously mulling over in my mind. As a matter of interest what set-up are other MGA 1800 engine convertees using? Are you using the original 1500/1600 tappet (side) cover or are you using the full MGB enclosed ventilation system? If I was to alter my system, what is the best way to proceed? Steve |
| Steve Gyles |
| Steve G: Did you go through the entire site for the GT6 evap. system that I posted earlier in the thread? The 5th version of his system looks very well-thought out. Be sure and drag the cursor over the pictures at the end of this page. Very well-done site... http://www.teglerizer.com/triumphstuff/emissions/emissions_system5.htm Steve |
| Steve Brandt |
| Steve Yes, had a good look. I like the effect with the pictures. Based on the balance of opinion I reverted my venting to standard this afternoon. Took me a couple of hours to remove the carbs and manifold, swap the late 1800 side cover with my 1500 cover, make a new cork gasket and reassemble. I will mull over other options in slower time. Most interesting on these forums what can come out of an initial question for a spare part etc. I had never given crankcase venting any major thought before this thread. I had simply followed a tip from an expert for a simple modification to 'clean up' the engine and reduce the likliehood of oil drips etc. There is certainly more to the subject than meets the eye. Steve |
| Steve Gyles |
| Just to add to it all-- I replaced the front 'tappet chest cover' and road draught tube on my '58 MGA 1500 with the front cover having upward tube from an early MGB. Connected the tube through a PCV valve (1980s Mazda unit) to a new connection at the centre of the intake manifold (similar to MGB). I measured pressures at both sides of the PCV valve and found minimal vacuum at the crankcase side, as I'd expect. My engine has an aluminium rocker cover with a 'vent' tube going to the front air cleaner. The adapter I made for the rocker cover has a restrictor orifice (#40 drill hole). It seems logical that since the rocker cover tube is connected to the clean side of the air cleaner, that the designers expected air to be drawn into the engine this way. The road draught has no filtering on it, so MUST be an exhaust. As Barney says, the road draught tube works only when the car is moving. I have a clear section of tubing on the crankcase side of the PCV valve and have found very little oil deposits in it. BUT this engine has only about 5000 km on new rings, pistons and bores so shouldn't have much blow-by. I've never seen oil deposits inside the air cleaner. Fraser |
| Fraser Cooper |
| I have followed this discussion on venting and I know that if I take the front air cleaner off there will be a dark area of oil fume staining on the filter near the breather tube area as it enters the air cleaner. There will also be a small amount of oil dripping from the pipe. I assume that this occurs when the car is either stationary or going at a sufficiently slow speed and the crankcase breather pipe does not develop a negative pressure (via the venturi effect) higher than that existing in the air cleaner. I can see how this could be the case but I keep thinking that at high revs there must be a very high negative pressure in the air cleaner due to the large engine suction - can the venturi effect of the crancase breather tube really exceed this? Perhaps only when going at speed but with the foot off the accelerator? |
| J H Cole |
| There is never a very high negative pressure in the air cleaner (enless the filter element is very dirty and clogged). Any significant vacuum inside the air cleaner would have the same effect as a mechanical choke plate, restricting inlet air flow and enriching fuel mixture. The filter surface area is large compared to the amount of air passing through it, so air flow speed through the filter is relatively slow with very little pressure differential. The draft tube can easily generate a larger depression. |
| Barney Gaylord |
| Just out of idle curiosity, could the tapped plug at the rear of the manifold be adapted for venting purposes? Obviously this would also entail an alternative anchoring arrangement for the accelerator bracket. Or, as I suspect, if one was to go down this route, better to acquire an MGB manifold? Steve |
| Steve Gyles |
| Ok everyone, Barney especially, I hold my hand up and say I have been incorrect in my analysis. I now agree that when moving the air goes from the air filter to the rocker box, down through the engine and out of the draft tube. You will have read from my post yesterday afternoon that I have reverted my system to standard. This morning I have been for a long drive along the coast in the damp and then rain with increasing strong winds. The fumes smelt a bit questionable during the first 10 minutes as the car heated up, mainly due, I think, to disturbance of all the bits yesterday. However, I cannot rule out some of the internal oily residues beeing vented off because, when I looked inside the filler cap after the run, I have never seen the area so clean. If you look back at a thread of mine a year ago, I commented on the milky coloured sludge around the filler cap. Maybe this was partly caused by my then vent system. Another item of note was that at the end of the run the engine was idling 250RPM higher than normal, However, this could be partly attributable to taking the carbs off and then back on, although I did not disconnect any cables or alter any settings. Steve |
| Steve Gyles |
| Barnet , I love all your patient and clear explanations - especially your little experiments to prove a point - you should have been a physics teacher !! - cheers Cam (retired physics teacher.) |
| Cam Cunningham |
| Steve G, - The milky coloured sludge inside the rocker cover is emulsified water in oil condensed inside the relatively cooler cover area. Good to hear you have bannished the sludge. Also good to hear how fast the stuff gues away with proper venting. The bottom end of the draft tube is close to the exhaust down pipe, so it is not uncommon to notice a little warm oil smell under the right conditions. A bit of crankcase mist near the hot pipe can do that. It would be most likely to occur when you stop after a hard run and the fumes can waft around the car rather than blowing away in the breeze. Consider it to be one of the charms of an MGA. |
| Barney Gaylord |
| G/day all, Where does all this leave me? I have an 1800 motor fitted to the MGA and have an alloy valve cover, no vent! I have mounted an MGB one-way valve in the tube from the tappet cover,(upward pointing pipe) then on to a nipple fitting attached to the forward end of the MGA inlet manifold. It is a closed system and cannot drip oil or emit fumes. I assume the pressure built up in the valve cover passes to the sump and is vented through the tappet side cover aided by a slight vacuum provided by the suction of the inlet manifold. Any suggestions ? (especially Barney) Happy MotorinG - A, Barry |
| BM Gannon |
| The alloy covers use a vented cap |
| dominic clancy |
| Dominic, My apologies, You are correct, I have found that my valve cover cap is in fact vented!! Gentlemen, our opinions please. Barry |
| BM Gannon |
| The vented cap acts in the same way as a hose to the air cleaner in a normal setup. With a MGA side down pipe acting as a suction venturi air will be drawn in through the cap. The cap should have air filtration. Usually wire gauze which is oiled. This must be properly cleaned often. However, you have a mixture of MGB and MGA parts and anything could be happening. I would settle on one type or the other. A full MGB system or a MGA system. The MGB system has many parts which you don't mention, but all play an important part in the overall operation. Mick |
| Mick Anderson |
| A check valve in the hose between tappet cover and intake manifold serves only one function. It will prevent momentary pressurization of the crankcase in the event of an intake backfire which pressurizes the intake manifold (spitting back through the carburetors). Any engine which is not terribly sick should never have an intake backfire (but it might happen on cold start with lean mixture). Otherwise a check valve in that location allows full forward air flow toward the intake manifold, same as if it was a plain open hose (unless the check valve also has a restrictor). If you have an open vent on the valve cover this will allow copious amounts of air (with no fuel) to flow into the intake (through the crankcase), the fuel mixture will go very lean, the engine will run badly and may not run at idle. If you put a restrictor on the valve cover intake it will cause very high vacuum level in the crankcase, close to intake manifold vacuum level. Bad news, as that much crankcase vacuum will suck air in past the crankshaft seals allowing entry of dirty air and possibly allowing too much raw air into the intake. You could put a restrictor in the hose between the tappet cover and intake manifold which would limit maximum air flow at highest manifold vacuum. That works at idle and low throttle settings but produces only a tiny vacuum level in the crankcase (unless you also have a restrictor at the valve cover inlet). Under heavy load with high throttle settings manifold vacuum drops dramatically, and crankcase "exhaust" draw would be greatly reduced just when you need it most. Result could be some crankcase exhaust going out the valve cover vent, oiling up the filter cap or air cleaner, and defeating fresh air ventilation in the crankcase. Loss of crankcase vacuum also allows the oil leaks we seek to banish. The primary goal of crankcase ventilation is to pass a little fresh air through the crankcase. The original draft tube setup will do that, but it drips. Reasons for connecting crankcase vent to the intake system is to recycle the crankcase fumes to be burned, and to apply vacuum to the crankcase to avoid the drips. If you are going to do this, what you really want is to maintain a low level vacuum in the crankcase, on the order of 1 to 4 psi below atmospheric pressure (or lower). A flow restrictor alone will not do this because of varying manifold vacuum and varying blow-by. The key to making this work properly is to control/limit the vacuum level at the suction end of the system. I will put this in a separate post to follow. |
| Barney Gaylord |
| MGB used two different methods to produce a low level vacuum in the crankcase (along with ventilation). I will cover the later method first because it is simpler to explain (and less likely to be used on the MGA). From 1970 models onward the tappet cover vent was connected to the venturi area of the carburetors. This provides a low level vacuum at idle, and slightly more suction at higher speed, but will never produce the very high vacuum levels present in the intake manifold. Because this is a low level vacuum source, no further pressure regulation is required. The plumbing is split to flow equal amounts of crankcase air into each of two carburetors, because the venturi porting effects fuel mixture. To use this method you need the carburetors that have the venturi vacuum port. 1970-1971 used SU HS carbs. 1972-1974 used SU HIF carbs. From 1975 onward there was the single Z-S carb which also used venturi vacuum for this function. To install any of these carburetors you need the matching MGB intake manifold. The 1964-1969 MGB connects the crankcase vent to the intake manifold, but the tappet cover hose is not connected directly to the high vacuum source. In between there is a special PCV valve. This is the wide flat round thing with a spring and a rubber diaphragm inside. See item 2 here: http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=28990 This device is actually a pressure regulated flow control valve. It uses the inlet pressure signal (crankcase absolute pressure) to regulate flow. This device will control the flow volume to accept passage of any amount of crankcase exhaust available. The inlet restrictor in the valve cover limits the fresh air inlet flow to a relatively small amount sufficient for crankcase ventilation (not to overly affect fuel mixture). The PCV valve will accept this flow plus any amount of cylinder blow-by gasses in variable volume, while all the time maintaining a narrow range of low level vacuum in the crankcase. This is a slick little device which I highly recommend using if you are going to port the crankcase vent into the intake manifold. If someone wants a layman's technical explanation how the pressure regulated PCV valve works, I could make another post. |
| Barney Gaylord |
| Thanks Mick and Barney, I will run with my setup for a few miles yet (the motor is quite new) and do a complete appraisal when it gets to oil change, grease time etc. Barry |
| BM Gannon |
| Barney I would appreciate the layman's description of the PCV valve workings Mike |
| Mike Ellsmore |
| Okay, Mike. Grab your favorite beverage and take the desk in the front row. This will come in three moderate size postings. Functional description for the Positive Crankcase Ventilation valve, late 60's to early 70's vintage from BMC: I have not found a good cross section drawing of this part yet. If anyone knows of one, do tell. Meanwhile, the setup as installed on the MGB is here: http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=28990 For a larger view of the part see item 70 here: http://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29286 From top down the parts are assembly clip, metal cover plate, large rubber diaphragm, small metal poppet valve, compression spring, and valve body. The top cover has a small hole for atmospheric air vent. The diaphragm seats and seals around the top rim of the body. The side port is below the diaphragm and is the inlet connection from the crankcase. The bottom port connects to the intake manifold with high vacuum. The spring force is small, so you could push the diaphragm down with one finger. In its rest state with engine off, no vacuum and no flow, the spring holds the diaphragm up and the poppet valve open. The other key ingredient is the inlet restrictor on the valve cover (and some form of inlet air filter). When the engine starts vacuum in the intake manifold begins to draw air through the valve body. Since there is some restriction at the valve cover inlet, a small vacuum develops in the crankcase. As soon as there is vacuum present in the valve body it uses atmospheric air pressure to push the diaphragm down to close the poppet valve. As the poppet approaches the valve seat it restricts air flow. The internal cavity of the valve body is then isolated from manifold vacuum and is exposed to the lighter vacuum level in the crankcase. If the poppet was to closed all the way it would stop air flow completely, resulting in no vacuum in the valve body, and the spring would push the valve open. So it settles into a state of equilibrium with a very small air flow and a light vacuum in the valve body. The pressure differential between atmospheric pressure and the lower crankcase absolute pressure pushes the diaphragm with just enough force to compress the spring. The light vacuum in the valve body and the crankcase is a result of air flow and the inlet restriction at the valve cover. The magnitude of that low vacuum level is determined by the strength of the spring working against the diaphragm. A stronger spring would hold the valve open a bit more making more air flow and higher crankcase vacuum. Result of all this is, with engine idling there is a small air flow going through the crankcase. The carburetor will be adjusted to enrich fuel mixture slightly to accommodate this extra air input and result in correct overall air fuel ratio. (Continue with next message). |
| Barney Gaylord |
| (Continued from prior message). When you hit the throttle to accelerate manifold vacuum drops. This would result in slightly less air flow through the PCV valve. Less flow decreases vacuum in the crankcase. Since the crankcase vacuum is controlling the diaphragm, the valve opens a bit more to admit more air flow. Increased flow increases crankcase vacuum due to the inlet restrictor, and the valve closes a little. End result of this balancing act is, with lower manifold vacuum the valve is open slightly more, but the crankcase vacuum and air flow remain about the same as when it was idling. Since the diaphragm is controlled by crankcase vacuum, the crankcase vacuum level and ventilation air flow remain fairly constant over a wide range of operating conditions and manifold vacuum. The flow volume of ventilation air going through the crankcase is determined by the vacuum level and the size of the inlet restrictor orifice. Blow by gasses from the pistons into the crankcase would want to increase crankcase absolute pressure (still below atmospheric pressure). Increasing crankcase pressure allows the diaphragm to lift opening the valve poppet a bit. This allows more air to flow from crankcase to inlet manifold, and the PCV valve will be passing both the ventilation air and the blow by gasses. When valve flow has increased enough to match the blow by volume, the crankcase vacuum will then draw the valve diaphragm down to regulate the original vacuum level in the crankcase, resulting also in the same original ventilation flow through the inlet restrictor. Hitting the throttle harder you get lower manifold vacuum and more blow by making higher crankcase pressure. Both of these things conspire to open the PCV valve more to allow more flow until the original crankcase vacuum level is restored. End result of all this is that the PCV valve will open as much as is required to allow flow of blow by gasses in any variable amount while retaining the same vacuum level in the crankcase. When you close the throttle at high engine speed you get very high vacuum in the intake manifold and virtually zero blow by. This causes higher vacuum at the PCV valve, so the diaphragm pulls down to close the valve some to reduce flow. The reduced flow brings down the vacuum at the crankcase side of the valve until it gets back to equilibrium again with the same original crankcase vacuum and ventilation rate. So the PCV valve is doing two things. It is maintaining a constant light vacuum level in the crankcase, and it is modulating air flow to exactly match the crankcase ventilation volume and blow by volume combined at all times. The proper description for this device is then "vacuum modulated flow control valve". Or if you think in terms of crankcase absolute pressure you can call it "pressure modulated flow control valve". For the common folk we just call it the PCV valve. (Continue with next message). |
| Barney Gaylord |
| (Continued from prior message). There are a few unnatural operating conditions for this valve. First, if you remove the oil filler cap when running you lose the crankcase vacuum. With increased absolute pressure in the crankcase the PCV valve will open wider in attempt to process this perceived excess "blow by". When the valve cannot increase flow enough to restore crankcase vacuum, the best it can do is to go wide open. When allowing maximum flow of air through the PCV valve the engine idle will speed up a little, and the fuel mixture will go lean, and it should have a slightly rough fast idle. Second, putting your hand over the oil filler port and closing off the valve cover inlet vent will stop all ventilation air flow through the crankcase. This will cause higher vacuum in the crankcase that will make the PCV valve go completely closed, except it may stay open just a crack to process blow by gasses. Stopping the fresh air ventilation flow will make the engine idle slightly slower, and the fuel mixture will be a little rich, so it should then have an abnormally slow rough idle. Third, if the valve cover inlet vent should happen to get clogged up during normal operation, the results will be similar to the second case with rich running. This can foul spark plugs and carbon up combustion chambers as well as accumulating water and other nasty things in the crankcase. Resulting damage could lead to a large repair bill. This could happen if the inlet filter in a vented oil filler cap becomes clogged. That is why service instructions call for changing the vented type oil cap periodically. Fourth, if the piston rings or cylinder walls are badly worn, beyond the normal expected range of normal engine life (or if the piston rings are stuck in the grooves), the blow by volume may be more than the PCV valve can process. In this case the PCV valve will be wide open and there may also be positive pressure in the crankcase. This would result in reverse flow through the valve cover inlet vent and possible expulsion of oil past the normal engine seals. Then you can hang up your jock and go home while you contemplate the cost of the required engine overhaul. Thank you for attending, and you can pick up your diploma on the way out. |
| Barney Gaylord |
| Thanks Barney I owe you a beer! Mike |
| Mike Ellsmore |
This thread was discussed between 28/12/2006 and 06/01/2007
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