Burning oil, then stopped

[shoup]

I have an e30 endurance race car with stock engine. A few hours into our last race the car started burning oil excessively. We went through about 5 quarts in a two hour period then the oil level stabilized for the last 4 hours. While it was loosing oil we had lots of smoke out of the tailpipe especially on startup.

Here are the compression numbers:

1: 167, 2: 156, 3: 161, 4: 160, 5: 170, 6: 165

Not very consistent and I plan to do a leak down tomorrow.

Right now I suspect valve stem seal. Not sure what else could leak for a while then stop.

Any more ideas?

[AndrewBird]

Old engine or rebuilt?

[shoup]

Rebuilt a couple race seasons ago but probably has a few hundred hours on it.

[shoup]

So I finally got a leakdown tester and was able to get some time to work on the car. Here are the numbers:

1: 7.5%
2: 5%
3: 5%
4: 7.5%
5: 3.75%
6: 7.5%

Some cylinders were initially higher until I tapped the intake valves or moved the piston slightly up/down. Based on this I would say the engine is in pretty good shape and I am hesitant to tear it down. All the air was leaking out the dipstick/valve cover which is expected since rings can never seal 100%. Engine was also not completely up to operating temp.

I still have no idea what could have caused the excessive oil consumption last race. There is oil residue and oil has collected in cavities in the intake manifold, but it does not seem excessive.

[PKile]

When I first read the title of this thread, all I could think of in terms of explanation was...you've run out of oil!!! :)

[TobyB]

Did you add oil before it started smoking?
You don't have an accusump, do you?
What is the airspeed of an unladen swallow?

I agree, that doesn't make a ton of sense, unless it got
overfilled at first, and then you kept filling it.
5 quarts in 2 hours is nuts.

The plugs probably won't tell you a thing, since it ran clean after that.

I guess I'd rule out the PCV, just because it's the only thing that makes sense...
... I can't even see a valve stem seal using THAT much!

no help here
t

[shoup]

Did you add oil before it started smoking?
You don't have an accusump, do you?
What is the airspeed of an unladen swallow?

I guess I'd rule out the PCV, just because it's the only thing that makes sense...
... I can't even see a valve stem seal using THAT much!

no help here
t

I do not have an accusump and we started the race ~1qt overfull. We had been racing for several hours before we noticed the low oil when oil temp started to rise and oil pressure began to drop. This all seemed to begin after a red flag and re-start.

Crankcase ventilation is just a hose from the valve cover to intake manifold on these cars. I did find some oil in my intake but it seemed reasonable.

These are the possible causes I can think of:

Excessive crankcase pressure due to blow-by from stuck ring pressurizes valve cover pushing oil into intake (could have gotten stuck and then dislodged)
Head gasket (leaking then closed back with temperature change)
Valve stem seal (dislodged, leaked, then re-seated)

None of these seem to be capable of leaking that much oil other than head gasket, but I don't think they are self healing.

[TobyB]

high temp/low pressure are classic signs of aeration- when the oil gets whipped
up around the crank. And yes,
it then gets sucked into the intake as an oily vapor.

On the M20, you plug the intake and then vent the crankcase into a container.
Bonus points for a flowmeter on the catchcan- 2-3 cfm being the upper 'acceptable' limit for a generic race engine, per the Prodcar guys... if this fixes it, it was a pcv problem...

I agree- none of the obvious choices would seem to heal themselves.

I run about 1/4 qt over full, and haven't had trouble with a baffle and a scraper (both IE).

DID have trouble with theM10 several times when the accusump got undercharged- I then overfilled the oil, and it flooded the engine when I stalled in a spin at Spokane. Then the oil aerated, and temps and pressures went to hell in a hurry. Also had it UNDERFILL when I shut down without properly recharging it with oil... and resultant low pressures from underfill... also at Spokane.

But that's not yourproblem. From the volume you're losing, if it was me I'd vent the crankcase externally....

hth

t

[shoup]

high temp/low pressure are classic signs of aeration- when the oil gets whipped
up around the crank. And yes,
it then gets sucked into the intake as an oily vapor.

We do have an IE crank scraper (no baffle).

The car typically does loose/burn some oil during the race. So is your theory that low oil -> higher temp/lower pressure -> more aeration -> higher rate of oil loss through PCV domino effect?

The oil loss did occur during the hottest part of the day and with one driver that did end up with the fastest lap in the car.

The thing I don't get is that we added oil and the issue didn't immediately go away. I think we added 2 quarts at the first stop, then another 2 quarts when he came back in to check then another at the end of his stint.

I am torn right now because I have a rebuilt head sitting on my bench but I don't want to spend the time removing the old/installing new if that wasn't the issue in the first place.