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MG MGB Technical - oil going somewhere

Hello everybody, I

have a problem which is driving me up the wall.
I rebuilt the engine on my 78 roadster and I am losing oil. The plugs are dry, the compression is 55 lbs per cylinder. There is no appreciable smoke from the exhaust, and there are no oil leaks under the car.
I can't figue it out. Any suggestions anyone?

Jerry
J W Hopkins

Jerry. If you compression is actually 55 psi on each cylinder, you are blowing oil out through the crankcase. My latest rebuild, with less than 500 miles on it is showing 168-170 psi on each cylinder. This is with a stock cam and LC pistons. With 55 psi compression, you are getting a significant amount of blowby and overpressure in the sump. How is your vent line to the front tappet cover hooked up? Les
Les Bengtson

I hope Jerry meant he has 155 lbs of compression per cylinder. Jerry, how many miles on your motor? Is it possible the rings haven't seated?
Matt Kulka

Sorry Guys, Can't type either. I meant 155 lbs on the compression
I put 30 wt. non detergent oil in and ran it for 800 miles without any problems. The rebuild has about 1300 miles on it at this time.I am using 20x50 castrol now.
J W Hopkins

You didn't mention how much oil you're using. Others may chime in to disagree, but here's my amateur opinion: As long as you don't see it leaking anywhere I'd give it another few hundred miles and see if consumption decreases. Granted your rings should be seated by now, but perhaps you have some nice hard rings and they're taking a little longer. If you can, measure how many miles it takes you to drop a pint of oil. Then keep track and see if it decreases over the next several hundred miles.

Now that "as long as you don't see it leaking anywhere" is important. I don't just mean on the garage floor, but have you crawled underneath to see that there's no wet stain? A small leak can blow backwards on the chassis when the car's moving but not build up enough to drip on the floor when the car's parked.
Matt Kulka

Thanks guys for the comments and suggestions. The car is using a pint every 60 miles opr therabouts.
I haven't seen a lot of spray underneath the car. I will keep an eye on it and hope for the best. Will report later.
Jerry
J W Hopkins

Jerry. How are you measuring your oil? I, too, am breaking in a rebuilt engine for my daughter's car. Last time I drove it, I let the engine stand for a half hour after returning home, then checked the oil. Level was mid-way between min and max. Let the engine sit over night and checked the next morning. Oil level was right at the max line. Hence, it would seem that all of the oil up top takes several hours to fully drain into the sump before it will give a truly accurate reading.

As to where the oil is going, there are only three choices. First it is being burned because the oil scraper rings are not removing it from the cylinder walls or because of valves sucking it in. This, normally, shows up on the spark plugs and you have already checked this. Secondly, it is being leaked out due to loosening of the tappet cover bolts, timing chain cover bolts or the sump. This, generally, produces drips. Thirdly, it is being blown out of the engine due to some pressurized source.

Oil being blown out of the engine will, normally, either be the result of some form of polution control problem or a leak somewhere in the high pressure oil system. A bad PCV valve will cause this as will a loose oil galley plug or the pressure relief valve screw cap. The last reason is a piston that is collapsing because the block was bored with insufficient clearance. This latter will, normally, show up when a compression check is done, but not always with a static compression check. (The one we normally do with the spark plugs out and the engine turned over on the starter to get the compression readings.) You may wish to run a dynamic compression check to see what readings you get.

To do a dynamic compression check, pull one spark plug out, insert the spark plug gauge (one of the screw in types works best), then start the engine and read the compression. If you have one of the screw in type gauges, release the pressure and take a couple of readings. You should have about 80 psi running compression. My daughter's rebuild shows 90 psi running. This may give some additional data which we can look at. Les
Les Bengtson

Les, Have you found any variation in your running cylinder pressure from a warm or cold engine? I would suspect higher readings from a warm engine, but don't really know. I also understand that with correct cam timing, cylinder pressure will be at its highest therefor producing the most power?

Jeff Schlemmer

Sorry I got off topic a bit. I also noticed increased oil consumption after a fresh rebuild. I used a K&N filter on the breather cover, but it didn't seem to help much. I also noticed a little oil being vented out of the oil fill cap vent hole. I'm thinking about going back to a PCV setup and sucking the oil back throught the intake rather than vent it in my engine compartment.

Jeff Schlemmer

Jeff. My daughter's engine was warm and showed 90 psi. This was part of a diagnosis for a non-firing number three cylinder. There was a clear miss and, when I put the car on the engine analyzer, the scope showed all cylinders sparking. But, when I shorted the cylinders, in turn, there was a engine speed drop on 1,4 and 2 and no drop on 3. Went to a mechanic friend who demonstrated the running compression check, then checked the engine on his analyzer. Same readings as mine. Problem turned out to be a bad gulp valve which, on the emissions equipped Weber adaptations, is plumbed into the leg of the intake serving cylinders 3 and 4. Mechanic crimped off the line, all four cylinders started firing and all was well. Hence, the running compression check is something new to me and I will have to play with it for a while before I am fully comfortable with it. It does, however, seem to offer some interesting information which I would like to research such as why is the running pressure significantly less than the non-running pressure?

Not sure what you mean by "with correct cam timing, cylinder pressure will be at its highest therefore producing the most power". I always have used a dial indicator and a degree wheel when installing a new cam over the last several years. All I have learned is that aligning the dots would have been sufficient with the cams I was using. All were within one degree of the specified full open reading. More information on what you are asking? Les
Les Bengtson

Les, just a naive guess here, but would running compression be less than non-running because a running compression test is not done at WOT?
Rob Edwards

Jerry,

It may just be a cultural idiosyncrasy, but your answers are a bit indistinct: "No *appreciable* smoke in the exhaust" and "I *haven't seen a lot* of spray underneath the car." I don't say this to criticize, but to help narrow down your problem: Do you have ANY smoke in the exhaust? Do you have ANY moisture under the car? Either of these is a clue to where the oil is going.

60 miles/pint is a lot for any motor. And if it's leaking that amount, you'll see it somewhere. This leads me to believe you're burning it - as Les said - either through the valve guides or piston rings.

If you have your oil ring in upside down, it will draw oil into the combustion chamber like a pump. Were you careful to note the markings on the rings to get them in right side up?

Loose valve guides can let oil in, but I doubt to this degree. Did you have headwork done? Valve guide seals installed?


Matt Kulka

I think you mean compression rings rather than oil rings. But that much oil burning in the combustion chamber would foul the plugs after a short while.

Were valve seals installed on the exhaust valves?

Wayne
Wayne Pearson

I had high oil consumption after an engine rebuild - was losing about 1 quart every 200 miles without much smoke except at gearchange. Problem was from the crummy stock-type valve stem o-rings. Had them replaced with cup-type seals and problem went away. In my experience, rate of oil use caused by new rings not seating is much less than 1 pint/60 miles or 1 qt/200 miles
WL Leong 1

Okay Guys, I'll try to answer all the questions. When I say no appreciable smoke, I mean that there is a little grey smoke at idle and none at speed. The car was desmoged and the vent runs to atmosphere. The head was reworked and new cup seals installed on the new valves.
I looked at the vent hose awhile ago and found no evidence of oil. The underside of the transmission is damp above the drain plug. Thats all.
After reading the comments I am getting the idea that I didn't keep the non- detergent oil in long enough. Should I revert back to it and start over.
J W Hopkins

Les, theoretically, when a cam is positioned for the greatest power output, it will also give the greatest cylinder pressure. If it is advanced or retarded from the optimal position, then valve opening/closing timing will not be properly set in accordance to TDC. If the intake valve closes too early and the exhaust valve opens too early, the engine won't make the most of its compression stroke, thereby having a lower cylinder pressure. Cylinder pressure is directly related to power output and throttle response. This is where an infinately adjustable timing set like APT offers can really help tune an engine for the greatest output. Going by the timing card sent with a cam is great for a ballpark setting, but if you degree your cam for the highest cylinder pressure you're likely to get more hp and torque, at least with performance cams. I haven't actually had time to dig into this theory and tune my own cam any better than I could with a degree wheel, but I'd be interested to hear any (dyno)research anyone has done on this. What I was originally asking is if you had noticed an increase or decrease in running cylinder pressure when the engine was warm -vs.- cold. I would suspect a greater pressure when warm, and a drop in pressure when cold could lead to some blow-by? (the morning puff of smoke seen in worn engines)

I'd have to guess that the lower cylinder pressure on a running engine could be a measure of engine efficiency. It could be the result of exhaust backpressure, blowby, valve overlap, backfeeding gas in to the intake manifold, etc. I would guess it would diminish at rpms higher than idle, but that may be offset by lack of power production in the tested cylinder. At WOT it may match a static compression test.(?)
Jeff Schlemmer

Jeff. You seem to have a better handle on the cam timing issue than I. Please let me know what your research turns up.

As to "the morning puff of smoke seen in worn engines", I have seen this in rebuilt engines with silicone bronze valve guides. On my 79, with a rebuilt engine and Mike Brown cylinder head, I will quite frequently get a puff of smoke on start up. This, I believe, is due to a little oil dripping down the valve guide when the engine has been shut down and allowed to sit for a while. No other sign of oil related smoke is present, including clean spark plugs. As I remember it, Mike had made a point of mentioning that he did not use oil seals on the exhaust valve stems because a small bit of oil, for lubrication, was desirable. Thus, when I am looking at oil loss through burning, I look for the oily plugs or a consistent blue-gray smoke.

All. My belief is that the current problem we are dealing with is due to the fact the front tappet cover is not connected to a slight, positive vacuum source. On either my daughter's or my car (cannot remember now) the fellow who sold the Weber conversion kit recommended running the hose down from the front tappet cover and letting it dangle to act as a road draft tube. This form of crankcase ventilation was common back in the mid 60's. It was not, however, a very effective method and many of the high performance cars of that era tended to "blow oil". I had a similar problem with the car I was working on and went to BAP which had a K&N style filter with an elbow to hook the hose from the tappet cover to. Not a great deal of suction produced, but it was sufficient to take care of the problem with oil loss that I had been experiencing with the road draft tube. Anyone else had any similar experiences? Les
Les Bengtson

Oil loss on this scale on a new engine is serious. It is due to bad fitting -like way too much piston clearance, broken rings, excessive guide clearance, completely wrong cylinder wall finish; or it is due to pressurization of the crankcase. Since there is no sign of excessive oil burning or leakage, the oil is most likely exiting after the combustion chamber into the exhaust. Even very badly worn exhaust guides will not show this level of oil use WITHOUT PRESSURE, even with no or "Crummy" O ring oil seals. At these levels, burning in the chambers would be very obvious, smokey, and plug fouling. Engines that are badly worn and have been overheated to the point of having NO tension or friction at the rings do not usually use oil at this level.

On an engine that runs well, I would expect to find some malfunction or incorrect plumbing in the breather circuits, as Les suggests. Any setup that vents the crankcase into the intake will show the same oil burning signs as a worn out unit, so you are left with pressurization. This will cause leaks, but with new seals and gaskets, they may hold. Final seating of rings will reduce the problem as there will be less blowby. So you are left with oil out the exhaust.

Worn valve guides or bad seals may contribute, and better ones may hide problems related to crankcase pressure, but they are neither the cause nor the cure.
Engines with no seals don't use oil like this; the
"Crummy" originals mentioned by WL Leong above have worked OK on millions of engines. As a long time student/mechanic on these cars I do take some umbrage at this characterization. Modern seals are certainly better at sealing, they contribute to cleaner running engines; they are not the "cure" for this problem and they may cause wear and seizure with some materials and fits used on some engines, as suggested by Les re bronze guides. Loose intake guides cause oil ingestion under high vacuum conditions, as do worn rings. Under these conditions there is NO blowby pressure so the actual oil loss is fairly small as regards the intake guide contribution. A long downhill with closed throttle causes oil to be sucked into the cylinders, followed by a cloud of smoke when you open the throttle. Loose exhaust guides allow pressure from blowby with open throttle to force oil into the exhaust where it burns incompletely, but is diluted by the large volume of combustion gases so that smoke may not be obvious. Any loose guides/seals may cause a puff on startup as mentioned by Les, this is not a cause of significant oil consumption.

As the car in question has been "desmogged" consider this. Early atmosphere vented cars with road draft tubes had large clear passages that allowed plenty of pressure/volume out, and also allowed quantities of crap to be sucked in to your nice engine, one reason why frequent oil changes were more common in times of yore. Many modern engines, including later MGB, have carefully restricted passages designed to work with various negative pressure schemes. Later Bs have a 1/16 hole restrictor in the 1/2 in pipe at the rocker cover, and some have similar restrictions in the oil fill cap. When these systems are altered or made up of mixed bits from the several designs used, all sorts of things don't work. Some are this pressure problem, others give strange idle characteristics which lead to "fixes" that make the engine run badly at anything but idle. SU and ZS carbs are set at idle, any deviation here destroys all calibrations above idle. Ignition timing is based on static settings in conjunction with vacuum characteristics mostly at idle as well. Crankcase vent system changes primarily affect things at or near idle, so if it is wrong, everything is wrong. Find out what is needed for the car/engine setup you have. Simple PCV valves are usually suitable for even modified engines. They are not hard to figure out, they make it run cleaner, and they keep dirt out. I have frequently gone so far as to fit them to early engines. I will make this point: While I have nothing against changing things to make it work better, faster, more reliable, changing it so it is worse, slower and more polluting is stupid and IT WILL BRING THE ANTI=POLLUTION PEOPLE DOWN HARD ON US ALL.

There is a lot of research on the crankcase pressure thing. Cummins manuals tell how to determine blowby and ring seating on new or used engines by putting pressure gauges on the crankcase. David Vizard (Tuning BL A-Series) and Bill Jenkins (How to Hotrod Small Block Chev)have superb books which cover all aspects of engines from both theoretical and practical informed points of view. They both have a lot to say re crankcase ventilation systems, quite astounding and known since the 70s at least. It is very instructive to plumb a sensitive pressure gauge directly into the rocker cover and read actual blowby/crankcase pressure under operating conditions as Cummins do. Poke a hole in the oil filler cap and glue a pipe in; go driving. Ideally, you should always have negative to zero pressure. I strongly suggest that anyone who decides to muck with his engine's design by changing stuff study one or both books.

Most ringsets have symetrical top compression and oil control rings, so upside down is not a problem. Sometimes top rings have features that mean they must be upright. Intermediate one or two rings are usually taper faced scraper/compression and must be fitted right way round. If upside down they will pump oil up and cause compression loss.

Non detergent oil use as mentioned is a myth. The detergents serve to keep dirt in suspension in the oil, rather than plating out or settling. There are other additives which are anti-wear, but only the extreme ones like teflon, moly etc will interfere with ring seating. Mostly non detergent oil today is not very good oil, not having the additive package known to work. Use modern oil from first start-up!

A "collapsed" piston results from seizing due to insufficient clearance or overheat. It will not normally cause oil control difficulties unless the seizure has caused metal to be dragged across the ring grooves making the rings stick-quite rare. Primarily affects the skirt, not the ring area.It will make a rattling noise, esp. on acceleration and at low speed when cold. I have seen a number of engines run with this for many trouble-free years.

Low running compression is due to closed throttle as Jeff guesses. The cylinder pressure mentioned by Jeff re efficiency is "Mean effective cylinder pressure" not running compression. It is a function of overall engine characteristics and is not affected by tuning as it is usually meant. It is what knowledgeable designers and modifiers are changing, but is really a quasi-theoretical measurement of efficiency, similar to a steam engine "card". Cam timing for any given cam/engine is within a narrow range. The cam design if matched to the precise engine spec, can be changed a few degrees sometimes, for very fine adjustments, if you really know what you are about. I set a lot of cams by factory marks before I started degreeing them in. I recently did a Spitfire cam that turned out to have been about 9 deg retarded from the factory (marks were wrong). On rebuild, I timed it 1.5 deg advanced to allow for chain wear in and initial stretch. It gained 5mpg -27 to 32- and very noticeable power. The engine was not worn out, but had suffered an odd failure, so this is not due to other factors. It had full emission controls and cat convertor, and I did not even touch the carb.
Sorry it got long, but there are a lot of myths and partial truths in this thread.
Have fun!
FRM
http://www.usachoice.net/gofanu
FR Millmore

One issue nobody has raised is that of cylinder wall porosity.

If the cylinders have been rebored in the rebuild then is is possible that the new surface contains small but significant pockets of porous metal - these will show up as brown stains on the cylinder walls.

The factory was aware of this casting problem, which is why all Gold and Silver Seal factory rebuilt engines were sleeved.

The only way to check for this is to pull the cylinder head and examine the bores with a flashlight.
Chris at Octarine Services

This thread was discussed between 05/05/2004 and 07/05/2004

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