Welcome to our resource for MG Car Information.
MG MGB GT V8 Factory Originals Technical - V8 Engine Breathing
|I have a 74.5 MGB with a 69 Rover 3.5.|
I have the crankcase breather air inlet at the back of the engine blanked off. Before I blanked it off it was blowing oil out.
I have a PCV valve in one rocker cover and it is going to the carb via the SS Braided vacuum line.
The other rocker cover has a air filter and should be acting as an air intake. I understand the air should be sucked in via this filter and then the PCV valve should be sucking the fumes out of the engine and through the carb to be re-burnt.
However I am getting a lot of blow by past the air intake filter and was wondering why?
I have tested the PVC valve and it appears to be working and the line is clear.
I have changed the PVC valve and air intake filter from one side (rocker cover) to the other with no difference.
If I pull the PVC valve off the rocker cover there is no noticeable change in engine sound or speed, the fumes will stop going out the air filter (intake) and now rise out of the rocker cover hole where the PCV valve resides?
I should have mentioned I recently did a compression check and the lowest was 165, the highest 180.
I also did a leak down test and the average was 16%, which is within the normal range.
The PCV valve is hooked up to the PCV valve port
The PVC vale is just a stock valve off the shelf! Should I be using a special type????
My oil pressure also seems a little high, about 60#'s when warm and idles at 25#'s and i am suing the lowest spring that came with the uprated oil pump kit
It used to be that the fresh air inlet for the pcv would be placed in the air filter housing just outside the air filter and this accomplished at least one good thing: whenever there was a lot of blowby, such as under WOT the fumes would be sucked into the intake and recycled. With the open element filters we see so often these days the only equivalent setup is to use the fitting that comes with some of these filter bases which allows you to attach a hose to an opening inside the filter element and run it to the valve cover, and that should remedy your problem. Even the best engine is probably going to generate more blowby under heavy accelleration than the suction from the PCV can process so some fumes will of necessity come out of the inlet side. It may be possible to retain the charcoal canister and plumb it into the inlet side, allowing the canister to collect the blowby and then release it back into the engine under normal throttle. I'd expect the charcoal could become contaminated with oil pretty quickly that way but it gives you another alternative. However if you vent your fuel tank to the charcoal canister that means you'd be running gas fumes through the crankcase to some limited degree. Probably OK, but I don't know for sure.
|Thanks for replying Jim|
I might have to run a line from the breathing rocker cover to the carb filter base.
Before I plugged off the rear engine air intake the blow by would expel there. I guess all I have done is move the blow by over to the rocker cover filter now which is more obvious.
The PVC valve I have is just a cheap generic, off the shelf one.
I will check with the local parts store and see if it can be improved upon.
Is there a PVC valve for a V8? Is there one for a 4 cylinder? Maybe I have the wrong one.
There certainly is enough suction at the carb, it held a business card in position quite nicely while I checked other parts of the running engine.
|That solution should work Bruce. Although PCV valves come in many different shapes and sizes I believe they do all work essentially the same way, however just exactly how they work under all conditions seems to remain a bit of a mystery. Manufacturers seem to build a different one for each engine application though so there should be several to choose from for your engine.|
I just did some further checking.
According to the local parts store, if the PCV valve rattles, it's good. Mine rattles, mine goes up and down as the engine RPM changes, (up at idle), at about 3000RPM the filter breather on the other rocker cover starts to smoke.
But when I take the PCV valve out of the system and just shove the hose (from the PCV valve port) into the hole in the rocker cover and run the engine up to 3000RRM the filter breather on the other rocker cover doesn't smoke.
Makes me feel like it is the PCV valve. I guess a cheap way to find out is just to buy one.
I wonder what would happen if I bought an adapter and just ran the hose from the PCV valve port into the rocker cover, no PCV valve at all? From what I have been reading, having a little negative pressure in the crankcase is a good thing?
|I'm embarassed to be asking such a basic question but it's on my list of things to figure out.|
What is a PCV valve?
Where does it go?
Is it essential?
I have the pipe at the rear of the bloke breathing thru a generic plastic fuel filter. One rocker cover has a PCV valve going to the PCV inlet on the base of Holley carb, the other rocker has hose going to the base of 14" air cleaner base. Car does not have any blowby or fume issues. I may be wrong but don't PCV valves suck? one way only? maybe yours is around wrong way.
|Can't spell, bloke = block.|
|PCV: positive crankcase ventilation, is designed to remove combustion byproducts from the crankcase and a hose going to the carb base would do that, however at idle this would add a lot of O2 to the fuel mix and make mixture control tricky. If you suck on a NEW pcv valve you will discover that it never completely blocks off the flow in either direction but it does restrict the flow in one direction if you suck on it hard enough and that's the end that goes to the carb base. So it's not a check valve, but is a variable flow restrictor. At low vacuum (WOT) it flows freely but at high vacuum it provides a restriction because: 1) there is not as much blowby and 2) it makes mixture and idle control much easier, especially on carburated cars. At WOT most of what it lets through is burned gasses which do not affect mixture anyway, and the flow needs to be greater to prevent fumes from escaping from the breather. The rattle you hear when you shake it is a metal plug of a calculated weight which at a specific flow rate is lifted to partially block a seat to reduce flow at idle. Undoubtedly some EFI systems have replaced this simple device with a solenoid valve having an orifice bypass. So much for American practice.|
On the original MGB 4 cylinder it was a bit different. Do you recall the 1/2" diameter spigot at the top of the valve cover which had a 1/16" hole in the end? Yep, pcv restrictor but it worked differently. This was only used on cars with the evaporative canister and was the inlet to the pcv system rather than the exhaust, drawing (relatively) fresh air through the charcoal canister. The exhaust was via the tappet cover to constant vacuum ports on the carbs thereby eliminating the need for a pcv valve, as the constant metered leak could be calibrated for. If excessive blowby overcame the crankcase vacuum no more fresh air would flow through the orifice and the mixture would become richer but this would generally only occur at high throttle settings and was not a concern. Fuel vapors in the canister would likewise enrichen the mixture though so the system was less than ideal as it simply moved unburned hydrocarbons from being evaporated into the air to the tailpipe. Also since excessive blowby had to pass the orifice to vent to atmosphere and a buildup of crankcase pressure would increase gas flow into the carbs, most if not all blowby was prevented from escaping and was recycled through the combustion chambers, where aside from perhaps burning a small amount of oil spray carried in with it, it did little more than water down the air/fuel mixture.
So that's the gist of it, although later on racers learned that they could evacuate the crankcase by using the headers to generate a vacuum and piping it to the crankcase, but this only works well with open headers or a very free flowing exhaust. The benefit was a small horsepower improvement but it was essentially free.
Now at some point blowby does become excessive but since the only cure is a new set of rings, the smart action is to dispose of it as long as compression is acceptable. If this means recycling it into the air filter then that is what you should do. If it reaches the point that it is carrying enough oil into the carb that you can smell it on the exhaust then it may be time to think of a ring job, or it may still be possible to use an oil separator on the PCV line as was done on several systems. This basically consists of a chamber filled with a metal mesh and a drain line back to the crankcase which, if it's done right can be the pcv line itself.
Older American V8's had an oil separator in the lifter valley connected to a port at the rear of the block and a large diameter 'road tube' running from there to the bottom of the car where an angle cut at the tip helped disperse the fumes when the car was moving. On inline engines this tube was at the side. This worked well enough until blowby became great enough to overcome the oil separator, at which point oil began finding it's way to the road and showed up as a dark streak in the center of the traffic lane which could be quite slick in a light rain. So in the final analysis although the value of the pcv system may be questioned up to the point where catalytic converters are added, it has proven to have a quite beneficial safety value. Not at all what you would have expected is it?
|As a footnote, a high enough volume of blowby also has the potential to lift the plunger in the pcv valve so finding one with a heavier plunger can help. Also the 'gulp valves' used on earlier British systems used a vacuum command to open the valve to full flow and may be less susceptible to closing under high blowby conditions provided the outlet to the intake manifold has less restriction than the inlet to the valve cover, thereby maintaining the vacuum signal necessary to open the valve.|
|Nothing to be embarrassed about Liam. I too am just learning about PCV valves and their functions. |
The explanation of Jims is an excellent, easy to understand piece of writing. I know I am going to store it in my personnel archives.
I had another talk with the local parts store and bought a PCV valve which is commonly used in a 350. It is possible the one I have is from a 4 cylinder. With the new valve I immediately noticed how much easier it was to draw/blow air through it. The old one(2 months old)seem sort of plugged/restricted. I will give it a try tomorrow and report back.
|Thanks - I understand what it does better now, but where in the system do I insert it?|
On my engine I have two small 5-6 mm breather tubes, one on each rocker cover, and a larger screw in flame trap on one of the rocker covers.
Do I join the two breather pipes together, and run them to the carb via a PCV valve, or do I replace the flame trap with a PCV valve and just run the breather pipes direct to the carb (its an edelbrock).
I can't recall my donor having anything like a PCV valve when I removed the engine.
if you use a Holley or Edelbrock, it is the easy to use the covers from a P5 or P6B or early Range Rovers.
Try to pick these items with the hoses and the flame taps and route the breather pipes to the underside of the pancake filter using a T connector just underneath the filter housing.
This is the most foreward way to solve the problem given by the SD1 style breathing layout with expensive filters etc.
BTW. the works V8 used the same layout as the early Rover V8 cars but with unique covers as reproducted by Clive Whelthy meanwhile (but these are extremely expensive for beeing labled 'MG' instead of 'ROVER')
|Liam, be particularly watchful for a restrictor or orifice in your system. You are likely to find one on the inlet side, perhaps built into the flame trap. The two small tubes very likely went to connections on the SU carbs, one on each, thereby eliminating the need for a PCV valve due to the orifice and constant vacuum. The same setup as was used on the later MGB's. If you use a 4bbl carb a PCV valve is needed and should be in the hose from one rocker to the fitting at the base of the carb, whilst a breather or flame trap should be on the other rocker, preferably plumbed into the underside of the air filter housing. Usually the PCV valve is simply inserted into a grommet in the valve cover and a hose then run to the carb.|
Do you mean using something like this : Ebay Item number: 120019568392. instead of the flame trap, then run the thin pipe on the other rocker cover straight to the carb with no flame trap or pcv valve?
Well with the replacement of the PCV valve my problems now appear to be solved.
Thanks for all the help
What part number did you use? Sometimes I think the one that I have is not adequate for 6 grand. Maybe it's the blowby at the dipstick that makes me think so. ;)
Yes, blow by at the dipstick would get me thinking too.
I didn't really use a part number to purchase the part. I talked to the parts guy about what I currently had, what the problems seemed to be and what I thought I thought I needed from a PCV valve. He said the one for a 350ci engine should be adequate.
Again, the first time I went in and brought the PCV valve, he said "if it rattles, it's usually ok" It wasn't. I could immediately tell from the amount of air I could blow through the new one in comparison to the old one. Today, a typical drive to work and back home and no smell of fumes.
I love a simple fix.
It is a Fram PCV Valve FV184
|Close Liam. That is the right part, use as they recommend it and run a hose from it to the base of the carb. On the other valve cover you should use a larger hose (5/8" ID is typical) and connect it to a fitting on the bottom of the air filter housing. Most chrome open element filters include a plastic fitting that can be attached with a pair of screws and a knock-out that is removed to open the passage. Or, you can use a breather or flame trap instead.|
This thread was discussed between 15/08/2006 and 18/08/2006
This thread is from the archive. The Live MG MGB GT V8 Factory Originals Technical BBS is active now.