WHAT A MESS: crankcase pressure/oil breather question (long)

[ischmitz]

Yesterday we installed a 964 3.6 engine into Ryan''s SC(surflvr911cs). Check here for details We put an orifice of 1/4 inch into the breather line between the oil tank and the intake plenum. (I am not sure about the exact inner diameter). The crankcase breather hose is connected to the oil tank. Ryan replaced all seals on the engine including the flywheel seal. After successfully getting the engine to run and idle for about an hour total yesterday evening the disaster happened the next day. Ryan took it out for a first drive and after 4 miles the engine gushed oil out of the bell housing. We are talking several quarts here and not just slight leaks. This looks like the flywheel seal to me.

What would cause a failure this quickly assuming he installed the seal correctly and didn't break it in the process. My guess was crankcase pressure. I am wondering if excessive crankcase pressure would cause the flywheel seal to get pushed out of its seat and get destroyed within the short drive he took.

I know Wayne suggests Curil-T to mount the flywheel seal. Well, I didn't on mine and have so far completed about 5000 miles without any leak at that seal. So that should not be the make or brake issue here, right?

How much pressure is there supposed to be in the crankcase if any? What would give first in case of too much pressure when all hoses are attached with hose clamps.

What would you recommend to do to prevent this from happening again. Maybe leacve the hose clamps off to create a weaker link in case there is pressure buildup??

Ingo

[surflvr911sc]

Thanks Ingo!

The restrictor is a 1/4". I also used assembly lube to install the seal. Maybe an over-pressurized crankcase w/ the slippery assembly lube pushed the seal out. I'm sure it was installed correctly and 99% sure it was not damaged on install.

Do you think opening the restrictor to 1/2 would be enough?

[Steve@Rennsport]

If it were my engine, I'd leave that restrictor out completely,......

That way, you'll not replace that main seal again.

[surflvr911sc]

Thanks for the reply Steve. Your response is very pointed, have you seen this before or does my problem just seem very obvious? Guys that have not used a restrictor have the engine die when they check the oil. It seems to me that there should be a middle road but where?

I guess that I could make a bunch of different sized restrictors, start w/ the biggest one, and when I find a size that wont stall the engine I’m good to go. What do you think?

[Wayne 962]

That seal is not an area of super-high oil pressure (normally). Have you dropped the engine yet?

-Wayne

[surflvr911sc]

Nope, in a couple days.

[ischmitz]

Originally posted by Wayne at Pelican Parts
That seal is not an area of super-high oil pressure (normally). Have you dropped the engine yet?-Wayne

Wayne, forgive my ignorance but I think the crank case has zero oil pressure. So this seal show not see any oil pressure at all. The bottom of the crank case is just the area where the oil dumps to before getting picked up by the oil pump. The case can have positive pressure from cylinder blow-by though. This pressure gets relieved through the crankcase breather. My inital question was whether excessive air pressure in the crank case woud act on this seal and push it against the rotating flywheel where it would get chopped up.

Ingo

[ischmitz]

Originally posted by Steve@Rennsport
If it were my engine, I'd leave that restrictor out completely,......

That way, you'll not replace that main seal again.

Steve, I think the orignial setup in a 964/993 prevents the engine from stalling when you remove the oil filler cap with a restrictor in the breather pipe. I didn't put in a restrictor on my setup and my engine dies immediately, when I remove my oil cap. Not that I care. I am quick with the dip stick...

Do you think the pressure in the crank case has pushed the seal out? Is there a spec for what pressure the crankcase has in a Porsche engine (positive - neutral - negative)?
Ingo

[HawgRyder]

In my case with the 2.0L I do not believe any pressure can build up.
The breather hose is about 1" inside diam. and it leads to the top of the oil tank for venting.
I have never heard of any engine having much crankcase pressure that is not taken care of by the venting system.
Bob

[ischmitz]

Remember that the oil tank is hermetically sealed when the oil cap is on. There is a vent tube from the oil tank that leads into the intake. On a CIS engine it just goes into the air filter housing. I am not sure where it goes on a carbed engine.

On the 964 (Motronic EFI) it goes into the intake plenum after the barn door but before the throttle housing. Vaccum there is dependant on the throttle position. If you remove the oil filler cap on my 3.6 conversion the excessive air that is introduced through the oil tank filler neck into the intake after the barn door AFM (unmetered air) causes the engine to die. The motronic does not "get to know" about this extra air and hence the mixure will be screwed up.

Ingo

[pete911]

On my car there are 2 lines from the engine to the oil tank. One is the crankcase breather. There is no restrictor in this line. The normal flow would be air from the crankcase to the oil tank. The other line is from the back of the intake manifold to the oil tank. This line has the restrictor. The flow of air here is from the tank to the engine. Basically its a way for the engine to burn off oil fumes. Without the restrictor the engine dies because it is like having a huge vacum leak when you take the oil cap off. This is also the path oil takes into the combustion chamber when people overfill their oil tanks.

[1fastredsc]

Why not just relocate the oil tank breather hose from after the barn door to before the barn door? This way the car won't die when you open the oil filler because the air coming from the oil tank will be metered like the rest.

[HawgRyder]

Or...add a crankcase breather up high on the firewall with an oil separator in it (so that any oil drains back into the engine).
I have added breathers on both banks of large V8's to make sure of neutral pressure.
Bob

[surflvr911sc]

Pete, what is the size of the opening in your restrictor?

[john walker's workshop]

i like to install the flywheel seals dry. they go in fine, and get a decent grip. i agree that the assembly lube may be the problem. the breather restrictor orifice on carreras is only about a 1/4 inch, and they don't pop the seals out.

[pete911]

The restrictor in mine is bigger than 1/4 inch. Probably about 3/8 inch or so. I really don't think this is your problem though.

[cstreit]

Was the engine just rebuilt, or already broken in? 1/4" orifice seems awfully small and I would bet a newer engine could generate more crankcase presure than that, causing oil to blow out the flywheel seal.. (But then, that's kinda what you're suspecting isn't it? )

[William Miller]

JW, on my just rebuilt engine I had the same problem with the main seal. I installed using Wayne's method in the book. I put it on the crank and put the crank in the case and ultimately bolted the case together.
It's a lot to keep an eye on when sealing the case. Mine got pinched some how because it was leaking thru the seal lips. The new seal went in using the normal method. Leak free!

Before I pulled the engine out to replace it , I was worried about the additional oil pressure created by the new cam line restrictors that Wane recommended. The oil pressure is above 5bar in most driving senerios. (I'm sure it get's higher but that's as far as my gauge goes.) I was worried that this additional pressure may have caused the leak. When I replaced the seal everything was fine. (Except the next week an o-ring on the oil return tube split. That's another story.) So the seal is holding at least 5 bar probably more.

I don't recall having any problem while putting the case together, and it is possible the seal was defective.

In regards to crank case pressure. The rear main seal is seperated from the crankcase by the rear main bearing. I think the only passages that lead to it are from the oil pump.

On the oil vent restrictor, mine is in the line between the oil tank and the CIS boot. The hole in it is about 1/4". It is down stream of the metering plate, but dumps the oil vapors on top of the metering plate.
One question: Why is the hose so big if they restrict it to 1/4".
Is it because it was a retrofit on later cars and they didn't feel like canging all the connections? Is is because the air velocity is slower in a bigger pipe?

Before I rebuilt my engine the #4 piston had a broken ring. (I was only getting 50% leak down so there must have been substantial blow by.
Asside from the miss the engine started leaking, I think from the additional crank case pressure. I'm pretty sure it was leaking wher the breather hose connected at the top of the engine.

I agree with JW's statement. The lube may have allowed it to slip out.

JW, another question: how far is the seal supposed to go into the case?
My first one was all the way in. Carlie installed his just inside the outter lip of the case. My second one when in about 1/8' before I was afraid to push it any farther.

[Wayne 962]

Originally posted by ischmitz
Wayne, forgive my ignorance but I think the crank case has zero oil pressure. So this seal show not see any oil pressure at all. The bottom of the crank case is just the area where the oil dumps to before getting picked up by the oil pump. The case can have positive pressure from cylinder blow-by though. This pressure gets relieved through the crankcase breather. My inital question was whether excessive air pressure in the crank case woud act on this seal and push it against the rotating flywheel where it would get chopped up.

Ingo

Yes and no. If there were zero pressure, then you wouldn't technically need a seal at all. Of course there is going to be oil sloshing around and pressure in there, from the air pressure inside the case.

I haven't run a car with this seal out of the engine, but I would suspect that it would be difficult to get a lot of oil out through there, unless the seal was completely missing. You mentioned multiple quarts of oil coming out - that's a heck of a lot of oil. I would think that something else would be seriously wrong to hvae the engine leak like that...

-Wayne

[surflvr911sc]

I updated on the other thread but I do it here also.

The old seal was pushed right out of the case. It was still in there but the flywheel was the only thing actually holding it in. I’m sure that the oil was mostly coming out of the tab in the case (at the top) because that was the only part that the seal wasn’t sealing at least a little. The seal looked like brand new.

The new seal was very different then the old seal. I bought the 12mm seal, which is actually the seal for the earlier 911 and the later 993 engines, why I don’t know. It fit much tighter, even though the sealing areas are supposed to be the exact same dimensions. It was black w/ a little brown in it, the old seal was all black. I used a little CurilT on the outer edge and I felt much more confident about the fit, so far no leaks.

I also opened the restrictor up to 3/8” from 1/4". It was probably unnecessary but I figured it couldn’t hurt. I now have a slight rpm change when I remove the oil cap but it will still idle fine.

I didn’t get the original seal from PP and I’m not sure what the actual part number was so I can’t say for sure if it was the right one or not, but I assume it was the correct 964 seal.

Anyway, I’m back up and running and that all I care about at this point.