# What causes bad valve stem seals?



## Hugh Gordon (Oct 2, 2003)

I had been having some smoking problems with my car, and on Saturday, it got a lot worse, tons of smoke from the oil cap at idle, and smoke from the tailpipe when driving. I did a compression test, found 150 psi on cyls. 1 and 3, 145 on 2, and 40 on number 4. Wet compression test had no effect. Did a leakdown test and found over 60% leakdown on all cyls. Air only came out of the top end of the engine during the leakdown test. So I'm thinking that my valve stem seals have gone bad, really bad. So, I'm trying to figure out what could have caused this, so it doesn't happen again. Oil temp? Too lean? Too much timing advance? Help me out! Thanks.


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## TBT-Syncro (Apr 28, 2001)

*Re: What causes bad valve stem seals? (Hugh Gordon)*

40 on #4 equals dead headgasket.


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## Hugh Gordon (Oct 2, 2003)

*Re: What causes bad valve stem seals? (TBT-PassatG60)*

Wouldn't I be leaking coolant though? Gonna pull the head tonight and see whats up.


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## Stephen Webb (Apr 12, 2001)

*Re: What causes bad valve stem seals? (Hugh Gordon)*

Head gaskets can blow in different ways. Often you will have coolant leakage / burning problems with a blown head gasket, but it can also blow between two cylinders, or somewhere else that doesn't necessarily affect the coolant passages. 
40 psi is definitely not good. Maybe a burned exhaust valve? Just a guess.
Improperly installed valve stem seals can lead to early failure. Worn valve guides will definitely reduce the seal life too. 
How many miles does the engine have on it? What's the history, etc...
You did the leakdown test and all cylinders got 60% leakage, but with the compression test 3 were about the same at 150, and one was 40? Something doesn't sound right to me.
You said that all of the leakage was coming from the top end -- what do you mean by this? If it's coming out of the oil cap, then you can't really say where it's coming from because that is connected to the crankcase via oil drains. You need to put a stethoscope in the intake mainfold and the exaust manifold to test for intake/exaust valve leaks.
Also, the valve stem seals only seal the tops of the valve stems from the oil in the head, and aren't responsible for high pressure the like the rings and valves are...Replacing the valve stem seals won't fix any compression related problems.
From the info I have, my guess is piston/rings. Just a guess, of course.
-Steve


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## Hugh Gordon (Oct 2, 2003)

*Re: What causes bad valve stem seals? (Stephen Webb)*

Yeah, I pulled the head, and I found piston # 4 burned through at the back. I'm not sure what caused it, maybe detonation, or the head gasket leaked oil into that spot? Time to build an engine I guess.


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## Stephen Webb (Apr 12, 2001)

*Re: What causes bad valve stem seals? (Hugh Gordon)*


_Quote, originally posted by *Hugh Gordon* »_Yeah, I pulled the head, and I found piston # 4 burned through at the back. I'm not sure what caused it, maybe detonation, or the head gasket leaked oil into that spot? Time to build an engine I guess.

Leaking oil will not burn a piston. Detonation an/or running lean.
If you put the money into building an engine, don't stop there. Make sure you get your tuning right -- else you will burn a hole in your new piston too.
-Steve


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## Hugh Gordon (Oct 2, 2003)

*Re: What causes bad valve stem seals? (Stephen Webb)*

Yeah, I made the mistake of advancing the ignition timing without my knock meter hooked up. I thought it was ok, because I couldn't hear any detonation. It's probably kind of hard to hear with an open dump tube though........... Oh well, it was fun for a little while.


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## fast_a2_20v (Jun 25, 2004)

:-/ 
i'm staying conservative on timing until i can get to the dyno... 
I was however going to say that smoke out the oil filler cap 99% means you have a piston / ring problem...


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## Hugh Gordon (Oct 2, 2003)

*Re: (fast_a2_20v)*

yeah, I was going to go to the dyno this week..........







Idiot!


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## mechsoldier (Aug 14, 2004)

*Re: What causes bad valve stem seals? (Hugh Gordon)*

Yeah for future reference you have to get a stethoscope and check the dipstick tube, in the valvecover, intake manifold, and exhaust manifold. also look for excessive pressure in the radiator hoses when you're pushing air into the cylinders....
The one way you'd know that you had bad stem seals and it WASN'T the pistons or rings is if you had air coming out the valve cover, and either the intake or exhaust manifold, but none (or I should say not as much) coming out of the dipstick tube.


_Modified by mechsoldier at 3:47 AM 7-20-2005_


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## Hugh Gordon (Oct 2, 2003)

*Re: What causes bad valve stem seals? (mechsoldier)*

Yeah, I don't think I did the test right as far as finding where the air was coming out. Hopefully there won't be a next time, on my car anyway.







Also, I am wondering why only one piston would have been destroyed? It looks like the head gasket could have been leaking, would this destroy a piston over about 2000 miles? or was this caused by detonation? If it was detonation, why only on one cyl? Wouldn't I see some damage on the other pistons?


_Modified by Hugh Gordon at 2:46 PM 7-20-2005_


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## mechsoldier (Aug 14, 2004)

*Re: What causes bad valve stem seals? (Hugh Gordon)*

Because all engines have a piston or two that run more lean than the others. On aircooled engines it's number 3....It's just the way engines are, usually the last runner furthest from the throttle body get's more air, that's the reason that manifolds usually have bumps between the runners that vary in size.


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## Hugh Gordon (Oct 2, 2003)

*Re: What causes bad valve stem seals? (mechsoldier)*

Ok, makes sense.


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