Bad leakdown in one cylinder...

Hello there. I am considering buying a 911 with a 3.0 engine. I had a mechanic inspect it, who found it was fine except that one of the cylinders was showing a leak of 35% though compression was normal. The mechanic diagnosed this as valve guide wear. The engine has only done about 30k miles since the last rebuild and has done some racing during that time. Do you think this will require immediate attention, or will I be able to drive the car as it is for the time being?

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This post was auto-generated based upon a question asked on our tech article page here: Pelican Technical Article: Time to Rebuild? - Part I

[Wayne 962]

Sure, you can drive the car, but power will be down, and oil consumption will be up. I wonder if they had the valve guides replace during the last "rebuild". It's definitely an issue, I would be sure to negotiate some type of price discount.

- Wayne

[Grady Clay]

Originally Posted by Wayne at Pelican Parts

Sure, you can drive the car, but power will be down, and oil consumption will be up. - Wayne

There is also a significant risk of major failure.
IF the valve is not seated correctly due to a bad valve guide, the valve can overheat and catastrophically fail.
This can effectively destroy an engine.

Another possibility is a broken head stud resulting in a leak between the cylinder and head.
Continued driving can further damage the parts until they are un-repairable.

Buy the engine as a core needing significant repair (low $).
Fix it properly.
You can do this DIY with the help of this Forum.
Take your time and do it ‘right’.
Then install it in your 911 and enjoy.


When considering the engine as a core, investigate all the other places where you may have to spend money repairing.
What was done at the prior rebuild?
What kind of ‘racing’? (The engine in my 914-6 ran all 24-hours of Daytona in ’81.)
Is the diagnosing mechanic totally independent or is he the seller?

What is the condition of the clutch?
Is there rust to address?
What is the condition of the CIS?
How long since it was running?
How well was it running?
Do you need to pass emission standards?
More.

What is your intended use?
Is your 911 currently driving ?
Why a ‘new’ engine?

Welcome.
You will find a lot of help here

Best,
Grady

[E Sully]

I would think that a leak down test would show a valve not seating well, but not directly show a worn guide. Did the mechanic do a visual check and see excess carbon build up, maybe remove the valve cover and check for play with the spring compressed or removed?
Did he think this was on the intake, exhaust or both? Where did he hear the air leaking from.
Also what were the compression numbers for the 6 cylinders.

[aston@ultrasw.c]

Working on 911 engines is rewarding but inordinately expensive.

Unless you get several grand (like 5) knocked off the purchase price, I would move on.

[DanielDudley]

Always lots of 911s for sale. The good ones are way cheaper in the short and long run.

Take your time and get a good one.

[James Brown]

Can steve even see our comments? If he is a guest, how does he find this? just wondering.

[aigel]

I would like to learn how a worn valve guide can cause high leak down. I have never heard of it.

Grady is correct, consider this a core engine. The leak down test is bad and it failing after short time after a rebuild does not speak well for the work done and / or the care it received after the rebuild.

G

[yelcab1]

Originally Posted by aigel

I would like to learn how a worn valve guide can cause high leak down. I have never heard of it.

Grady is correct, consider this a core engine. The leak down test is bad and it failing after short time after a rebuild does not speak well for the work done and / or the care it received after the rebuild.

G

If a valve guide is worn enough, the valve stem is crooked in the guide, and the valve does not seat flat on its seat in the head and therefore does not seal 100% against the head. Ergo, the leak.

[al lkosmal]

I also would not suspect the high leakdown number to be caused by valve guides, but by carbon build up....or perhaps a less than perfect valve job. Or if it's been racing...has there been an over rev incident? Although if that was the case, I'd think you would have more than one cylinder with issues. If the rest of the cylinders are showing good leakdown and compression numbers and the engine, and the car itself, appears to be well taken care of, I'd knock another grand or two off of the price and go for it.

Of course, I can have the engine out of the car and on the engine stand in an hour or two and deal with most engine problems....and it's your cash, so it's easy for me to say.

Does the engine show signs of valve guide wear....like smoke on decel?

[hcoles]

move on unless you can get the car very cheap and have the time/ability/desire to rebuild - once you get started and read on this forum you will generate a good sized list of while-you're-in-there. As mentioned above it is much cheaper to get a good car to start with.

[aigel]

Originally Posted by yelcab1

If a valve guide is worn enough, the valve stem is crooked in the guide, and the valve does not seat flat on its seat in the head and therefore does not seal 100% against the head. Ergo, the leak.

Thank you. So it is possible. Likely in this case? I have seen a ton of flat six engines age and they would drink oil and blow smoke but none of them had high leak downs past the exhaust valve due to valve guide wear. Also, valve guide wear from high miles is even across cylinders.

Just buy it as a core and go through it. It may have a lot of good parts and only need a refresh on the heads!

G

[yelcab1]

You know, the Porsche 911 is one of those cars where you can actually fix one (of six) heads if that is all the money you want to spend. Can't do that other cars...

So if you want to fix it cheap, this is the one to buy.

[al lkosmal]

"do you feel lucky punk.....well do you?"

[yelcab1]

It is actually "well, do you punk?"

I also like this "A man's got to know his limitations !"

[Wil Ferch]

Yelcab1 answers a question by saying...." If a valve guide is worn enough, the valve stem is crooked in the guide, and the valve does not seat flat on its seat in the head and therefore does not seal 100% against the head. Ergo, the leak..."

But....the valve is closed during leak down. Please explain further. Leakdown will tell you if you have bad rings, bad valves/valve seats, but not bad "guides".....right?. I doubt with the length of the valve stem that bad guides control the seating of otherwise-good valves or seats.... enough to matter. Bad valves or valve seats themselves are another matter and can be a cause for bad leak-down.

...right?....wrong?

[BURN-BROS]

Wil,

The missalignment can prevent 100% of the valve face from touching the seat. It can be hit or miss as to wether it seats properly during each lift cycle.

[yelcab1]

Originally Posted by Wil Ferch

Yelcab1 answers a question by saying...." If a valve guide is worn enough, the valve stem is crooked in the guide, and the valve does not seat flat on its seat in the head and therefore does not seal 100% against the head. Ergo, the leak..."

But....the valve is closed during leak down. Please explain further. Leakdown will tell you if you have bad rings, bad valves/valve seats, but not bad "guides".....right?. I doubt with the length of the valve stem that bad guides control the seating of otherwise-good valves or seats.... enough to matter. Bad valves or valve seats themselves are another matter and can be a cause for bad leak-down.

...right?....wrong?

This is not a big problem with porsches other than the Carrera. But, this is a very well known problem with most Ferrari 355s. The method of failure is the same, only more severe such that 30% leak down via the exhaust valve is highly possible. The solution is a full valve job with different valve guide material.

[Flat Six]

Assuming the PPI mechanic did more than measure just leakdown (e.g., 35% loss), listening for where the air is escaping provides clues as to what the source(s) of the leak(s) is/are.

For example, if the suspect cylinder is pressurized and you hear (using, say mechanic's stethoscope or clean-tube-in-ear method) air escaping from the crankcase, then likely suspect is worn rings/cylinders. If escaping thru exhaust, then exhaust valve; if escaping thru intake, then intake valve.

But I'm with Wil -- you might be able to isolate intake vs. exhaust vs. rings, but how can you isolate guide rather than valve seat? Seems to me the valve seat would have to leak first in order for air to then escape via guide, but pressure -- being what it is -- will escape via path of least resistance (intake manifold or HE, not valve guides).

If it were me, I'd ask for a re-test and make sure the PPI mechanic didn't inadvertently let a bit of crap from around the spark plug hole fall into the combustion chamber and get wedged between a valve and its seat, causing a possible false reading.

Aaron, am I just FOS on this one?

Dale

[BURN-BROS]

Originally Posted by Flat Six

But I'm with Wil -- you might be able to isolate intake vs. exhaust vs. rings, but how can you isolate guide rather than valve seat? Seems to me the valve seat would have to leak first in order for air to then escape via guide, but pressure -- being what it is -- will escape via path of least resistance (intake manifold or HE, not valve guides).

Aaron, am I just FOS on this one?

Dale

I see what you are saying,

The test will not tell you the cause, rather the area to look. Since we do not have an entire report, I am assuming that the mechanic has knowledge of high oil consumption and smoking.

You simply cannot rule out guides as the problem.