So a little history. I am struggling with a rear main seal that doesn't want to stop leaking oil (sounds like I am not the only one).

I am down to two possibilities (that I know of)
1. excessive wear on the flywheel hub
2. excessive case pressure

Focusing on #2, how can you determine if there is too much pressure?

One mechanic told me that one test is to simply place the oil filler cap on the oil fill neck (without tighting) while the car is idling. If the cap blows off = too much pressure. Anyone heard of this?

Ultimately, what is the cause of excessive case pressure? and how do you fix it?

sorry for all the newbie questions, but these things keep me awake at nite......

Doesn't crank case pressure increase with RPMs? I would guess maybe there is too much oil in engine?

Don't know how to test though.

I'm far from the resident engine guru around here (we've got plenty of those), but since noone else is chiming in, I'll offer my advice.

Crankcase pressure is a by-product of blow-by; that is, pressure from the combustion chamber "blowing by" the piston rings and pressurizing the area under the pistons - the crankcase.

If you have oil spewing from many different gasketed surfaces in the region of the crankcase, that's an indication of excessive crankcase pressure and thereby blow-by. It's usually a bad juju type of sign, since the only fix (outside of motor honey and quick sale) is an engine rebuild.

A normally running engine shouldn't have a lot of discernable flow of "air" out of the oil fill cap, but I'm not familiar with test methods or the specifics of the type IV motors.

P.S. Some crankcase pressure (i.e. blowby) is normal - a compression test will tell you how bad your blowby is, assuming your valve sealing is good.

Something as simple as a stuck PCV valve can cause excessive crankcase pressure, so check that.

QUOTE (bondo @ Jul 8 2005, 01:34 PM)

Something as simple as a stuck PCV valve can cause excessive crankcase pressure, so check that.

can this be checked with engine in the car?

sure, take off the oil breather/filler and inspect the PCV valve. Heck, even replace it as its available new from Porsche.

QUOTE (Bleyseng @ Jul 9 2005, 08:09 AM)

sure, take off the oil breather/filler and inspect the PCV valve. Heck, even replace it as its available new from Porsche.

Are we talking about the lateral plug/spring/piston thing located just below the engine tin, next to the dip stick?

If yes, then you are saying that by taking off the oil filler neck/breather that i can inspect the this valve from the the inside?

I maybe confused.

duh.....

I don't have a PCV valve, my car was converted to carbs and the PCV valve was apparently removed.

I was confusing this with the oil pressure valve that is part of the engine case.....

the search for answers continues............

all motors are going to have crankcase pressure, figure you have the pistons going back and forth moving tons of air and you have the crank and rods acting like fans inside the crankcase, this normal, as is a little bit of blow-by from the piston rings, too much blow-by and that is bad, signs would be your oil getting dirty faster than normal.

the crankcase must have ventilation, adding vents to the valve covers/heads is a good idea as well (factory did this on some models, I'm guessing for smog reasons they stopped it)

do a search and you'll solutions for adding "puke" bottles and the like.

for venting, you can vent to atmosphere, and/or vent to the aircleaner to help "suck" the air out of the crankcase....only problem with the second method is that if too much oil vapor gets ingested, it has an effect that is similar to lowering the octane of you fuel (the oil vapor displaces air)

Hey JR,

Here's the quick and easy way to see is it's excessive "blow-by" pressures causing your oil leak.

Block wheels so car doesn't move.
Remove the dipstick.
Start the engine.
If oil doesn't start spewing out of dipstick, then note if air is coming out of dipstick hole. {I, VERY CAREFULLY, use my finger.}
If there is a lot of air coming out of the dipstick hole then you probably have a "blow-by" problem.

Blow by can come from either the rings or the valves.
Note: excessive crankcasr pressures can also be caused my a blown head gasket on some engines.

As Flat 4 engines do not have valve cover access openings there is no quick way to tell which might be your problem.

You need to do a leak down test for that.
"What's that" you may ask?

It is where by pressurizing the combustion chamber when a cylinder is at TDC and noting is air gets by the rings or not. AKA, listening at the oil filler opening with your ear.

Valve bypass is heard at the opening in your valve covers. If you had them. Whick you don't. =-)

If you find little air coming from the dipstick or oil filler openings, then it's your rear seal.

Best of luck.

I really seal for you man. that is a bad problem, hope it is not destroying your clutch each time.

The advice on checking for blow-by either from rings, or valve guide wear is good advice. Hope that is not the problem.

The flywheel surface that the seal rides on must be mirror smooth, no nicks, scratches, dings, or even deep grooves (from the seal wearing a groove in the fly wheel. Now if you have one small bad spot on the flywheel, you could maybe get away with installing the oil seal not as deep, but it really is best to seat it all the way down, so it is squared up with the crank (I have done this on wheel bearing seals, not on an engine seal)

I also use sealant on the seal, add some crankcase half sealant around the oil seal to case joint when you assemble it. you maybe have the leak from the seal / case joint, than from the seal / crank area???


You need to have the case ventalation set up. If you are running stock, you should have a hose from near the oil fil tower that goes to the air cleaner. this set up will actually help pull a vaccum on the case (unless blowby is too excessive). if yo have aftermarket carbs, you will need to rig a ventalatin system hooked to the intake.


good luck, and seal you later!

Thanks for all the replies people... I really appreciated it.

I performed the dipstick test and only found a "little air". In fact it almost felt like a slight vaccum. That's a relief cause I really felt my engine has been running good and strong.

BTW, I have aftermarket carbs and the outlet of the oil filler neck has a hose connected to a small air cleaner

So, I guess I will move on to the flywheel scenario. While I have replaced this seal twice, I have never bothered to "polish" the flywheel hub. So, live and learn I guess.

I am going to let it leak for a week or so before pulling the tranny......again. So far, my clutch disk doesn't seem any worse for it. Gonna need a new one soon anyway.

later folks, you guys rock

Oh, and if I may add another $.01 here....

When your crank seal surface is exposed, measure the diameter of where the seal comes in contact with the crank. Compare with anothers diameter to ensure everything is apples to apples. Check the seal itself also.

I almost made this mistake with small block chevy cranks.. but caught it.

JR,
Could you explain exactly the findings that you are finding and what has lead you to believe it is the rear main seal leaking?

I am having trouble with an oil leak that may or may not be coming from the rear main seal, I too have recently changed this seal twice trying to stop the leak.

I am now not too sure that the rear main seal was the source of my leak for the following reasons (which may be totally wrong):
No oil or oil residue on the clutch and just a tiny amount of near the seal before the first seal change, none on the following .
There is however gobs of oil in the bellhousing and after driving the car it appeared to be leaking out of the trans "window" on the underside of the bellhousing.
There was no other apparent large source on the top/frontside of the engine.

So drained and gave the trans oil the sniff test and it really didnt smell like trans oil so I changed the mainshaft seal on the trans.

I still havent started iti after this replacing this seal becasue I now have a stripped exhaust stud from installing and removing the H/E's

Hopefully you are able to remedy this problem, if you do please post your methods and experiences.

Basically, the oil is localized to the bell housing area only. Everywhere else is dry. It is definately NOT gear oil.

Pulled the trans and flywheel (bout a month ago), all the galley plugs are fine. Now I won't completely garantee that it is NOT the seal/case mating surface, but it looks fine. So my bet is the seal / flywheel surface. But I will have to double check that the next time the tranny is out.

Yesterday, I was thumbing through Tom Wilson's book on "how to rebuild a VW engine". He clearly recommends polishing the flywheel hub with some 220 grit paper everytime this seal is replaced. wish I had read that a month ago

It will be a week or two before I get to it, but I will definately post my results.

good luck yourself.

While you're in there check the mating surface of the case halves to make certain someone in the past didn't booger them up trying to seperate the case. Good luck