Crancase pressure, oil mist in air filter.
12-13-2001, 09:54 PM
#1
Developer
Thread Starter
Crancase pressure, oil mist in air filter.
I autocross my 78 928. The engine is very healthy, good compression all around, no blue smoke. I removed the EGR rail and air pump off of this motor and clamped shut those ports where they enterred the back of the heads. That was two years ago. This year, I started noticing more and more oil leaks, and oil in my air filter box after a hard driving. I have installed the DEVEK oil filler neck baffle, and a 928 GTS oil pan baffle. I have replaced the oil pan gasket, the oil sender switch, the oil lines, and all the other oil-related gaskets I can get my hands on. Still she pressureizes that crankcase and pushes mist up out into the air filter tubing. I have started venting the system to the atmosphere just to try and keep my air filter from swimming. Actually, I worry that the injestion of oil into the intake will lower my octane rating/cause detonation, foul my plugs, make blue smoke. Has anyone else had this experience? What does that pipe that goes from the rear of one head to the rear of the other do?
12-14-2001, 02:17 AM
#2
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
QC:
Excessive crankcase pressure with good compression is a common sign of a partially plugged exhaust. Might be a cat that's melting down, a good possibility with bursts of high load and no air pump.
The guts of the cat look like a honeycomb, a ceramic substrate with lots of tiny holes for the exhaust gas to pass through. The contamination from high load, maybe a rich mixture, and the lack of a pump supplying cooling air will actually melt the ceramic so that the tiny holes close up. With lower gas flow (caused by some of the holes plugging) the melting is slowed, so it may not melt all the way across. Kinda self-limiting in a way.
There's a diagnosis that involves a vacuum gauge, with a hose routed to inside the car and some driving instructions for steady throttle. Basically, the problem shows up as rising intake pressure under constant load. I wrote this up a long while ago and I know that Wally P has posted it a couple times since. It is probably archived at the Nichols Tips site.
If you notice that the higher-RPM performnace has fallen off some, this confirms the fading catalyst flow diagnosis.
The cure? New cats, and reinstall the air pump so the new cats will stay healthy. If it's a non-street car, then the cat bypass pipe may be an option for you.
Hope this helps! I chased the same problem on a Saab Turbo a long while back, where the symptom showed up as poor boost, as displayed on the dash-mounted gauge. I rebuilt the turbocharger a couple times before I decided to read the manual and check all the other possible causes listed. Darn cat was plugged from high load operation I guess, though it certainly couldn't have been me, officer...
Good luck!
Excessive crankcase pressure with good compression is a common sign of a partially plugged exhaust. Might be a cat that's melting down, a good possibility with bursts of high load and no air pump.
The guts of the cat look like a honeycomb, a ceramic substrate with lots of tiny holes for the exhaust gas to pass through. The contamination from high load, maybe a rich mixture, and the lack of a pump supplying cooling air will actually melt the ceramic so that the tiny holes close up. With lower gas flow (caused by some of the holes plugging) the melting is slowed, so it may not melt all the way across. Kinda self-limiting in a way.
There's a diagnosis that involves a vacuum gauge, with a hose routed to inside the car and some driving instructions for steady throttle. Basically, the problem shows up as rising intake pressure under constant load. I wrote this up a long while ago and I know that Wally P has posted it a couple times since. It is probably archived at the Nichols Tips site.
If you notice that the higher-RPM performnace has fallen off some, this confirms the fading catalyst flow diagnosis.
The cure? New cats, and reinstall the air pump so the new cats will stay healthy. If it's a non-street car, then the cat bypass pipe may be an option for you.
Hope this helps! I chased the same problem on a Saab Turbo a long while back, where the symptom showed up as poor boost, as displayed on the dash-mounted gauge. I rebuilt the turbocharger a couple times before I decided to read the manual and check all the other possible causes listed. Darn cat was plugged from high load operation I guess, though it certainly couldn't have been me, officer...
Good luck!
12-14-2001, 10:57 PM
#3
Addict
Rennlist Member
Rennlist Member
Join Date: Nov 2001
Location: Endwell, New York
Posts: 620
Likes: 0
Received 0 Likes
on
0 Posts
If it's a true Euro there should be no "catastrophic perverter". There should only be a front muffler, silencer, and rear muffler (unless the ehaust was americanized). Could a muffler plug up?
12-16-2001, 09:27 AM
#5
Developer
Thread Starter
I am using a K&N air filter. I have a coatalytic converter test pipe, and a Borla stainless exhaust behind that. I do not think I will be able to get prtessure readings off of the intake plenum as suggested because they will be vacuum. But I do like the idea of a tee off of the oil filler cap, I may be able to watch pressure rise there.
12-27-2001, 10:14 PM
#6
Carl,
While looking for Data on the K&N awhile back easily 1/3 of the reply posts had the K&N passing debris, about 1/2 the remaining mentioined no problems and/or thought we owners were introducing the debris while servicing the filter/airbox.
One of the questions I was asking was: DO you have a light coat of oil in the airbox/throat or parts even further on?
There were a few positive responses to that query.
Possibility of Hi RPM Vac. sucking some of the oil out of the filter, at this time unsure. As an experiment If you still have that paper element lying around clean up the misted area and install the filter. Trash the sHARk for a bit on and off for a couple of days - maybe an AutoCross or two and inspect the box again. If mist is eliminated you may have found part or all of your problem.
HTH/LOL
John S. 82' Weissach, Auto., 'Pattycakes.
While looking for Data on the K&N awhile back easily 1/3 of the reply posts had the K&N passing debris, about 1/2 the remaining mentioined no problems and/or thought we owners were introducing the debris while servicing the filter/airbox.
One of the questions I was asking was: DO you have a light coat of oil in the airbox/throat or parts even further on?
There were a few positive responses to that query.
Possibility of Hi RPM Vac. sucking some of the oil out of the filter, at this time unsure. As an experiment If you still have that paper element lying around clean up the misted area and install the filter. Trash the sHARk for a bit on and off for a couple of days - maybe an AutoCross or two and inspect the box again. If mist is eliminated you may have found part or all of your problem.
HTH/LOL
John S. 82' Weissach, Auto., 'Pattycakes.
