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Old 12-16-2009, 01:27 PM   #1
jackdempsey
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Default motor question - leakdown vs compression test

So the STi is ticking a little bit and the motor fails to stay running. It does turn over. I don't have the time or tools to work on it so I sent it over to subaru (this is possibly a mistake).

I asked them to perform a leakdown on it to tell me if the valves are bent or I blew the headgasket or cracked a piston.

They call me a day later and tell me that they performed the compression test and all the cylinders are down on pressure and I need a new shortblock.

I stopped them right there and said I asked for a leakdown. Got transfered over to the mechanic and he explained to me that I threw a rod and he knows this from doing the compression test since it is failing compression on all the cylinders.

Anyways - I convinced him to do the leakdown and give me the results beacuse a compression test won't tell me where the problem is like a leakdown will.

So today they call me and say it _passed_ the leakdown and they want to start tearing into the bottom of the motor (ie remove the oil pan) and see if there is internal failure in the motor.

My question is -

Why would a motor pass a leakdown test but fail a compression test???
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Old 12-16-2009, 01:53 PM   #2
the suicidal eggroll
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What were the compression test numbers? It sounds like the mechanic doesn't know what he's doing. For all you know, the numbers were low because he did the test cold or with the throttle closed, the motor is actually fine, and it's a bad injector or an intake leak that keeps it from running correctly. Is this on your dime or under warranty?
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Old 12-16-2009, 02:01 PM   #3
wrx4him
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Yeah, I'd definitely ask for the numbers. Sounds fishy to me. Theoretically, you could pass a compression test, but fail the leakdown, but afaik that wouldn't go the other way. If the car isn't making compression, then there's no way to pass a leakdown....
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Old 12-16-2009, 02:19 PM   #4
jackdempsey
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My dime. Car is out of warranty.
They are going to get back to me on the numbers :/

Little history of what happened

I had turbo failure on the vf39 - bought a vf43 to replace it... I primed the turbo before starting the car. I refilled my coolant. I also ran an accessport stage 2 tune and did not drill the restrictor valve out and overboosted the motor. I saw higher boost levels on the gauge - turned it around to head home and noticed i had a knock at part throttle only.

I threw a code for AVCS P0021 - and another code for Over boost P0244 The motor died on me before i could get it home. I tried to restart it and the knock was much more audible.

So this is why I sent it to subaru.

So do I run to subaru and get my car and tell them to get their hands off of it?

I can do simple bolt turning - but like I said earlier - I don't have all the tools, time or garage to do everything correctly. I don't have a leakdown tester which is why I sent it to them.

Last edited by jackdempsey; 12-16-2009 at 02:35 PM.
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Old 12-16-2009, 02:37 PM   #5
jackdempsey
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Quote:
Originally Posted by wrx4him View Post
Yeah, I'd definitely ask for the numbers. Sounds fishy to me. Theoretically, you could pass a compression test, but fail the leakdown, but afaik that wouldn't go the other way. If the car isn't making compression, then there's no way to pass a leakdown....

Theoretically how? Other than the motor being ok on the topside/valvetrain and the test was incorrectly performed.

http://www.wcengineering.com/articles/leakdown.html
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Old 12-16-2009, 02:44 PM   #6
the suicidal eggroll
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Quote:
Originally Posted by jackdempsey View Post
Theoretically how? Other than the motor being ok on the topside/valvetrain and the test was incorrectly performed.

http://www.wcengineering.com/articles/leakdown.html
A piston with a cracked ringland might not leak air out fast enough to fail a compression test, but it would fail a leakdown. I can't think of any scenario where you would pass a leakdown and fail a compression test.
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Old 12-17-2009, 11:42 AM   #7
jackdempsey
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Yeah I think im about to bail on this dealership. And it's frustrating cause no one wants to work on the subaru around here and they're the only dealer... but I can work through that I guess.

So I went to the dealer this morning and didn't see my car in the shop - the shop manger came out quickly to tell me they had an emergency yesterday and didn't get around to removing the oil pan off my car (its what like 10 bolts???) to get started in looking on the underside. Which is ok - I want them to take their time - but it did feel like a lame excuse as to why my car wasn't worked on.

I asked them to give me another explanation as to why the car would fail a compression test and pass a leakdown - and started to explain what was going on - but the look on their faces made me end my story short. It was one of those looks that was like "I can't believe this guy is here why is he trying to explain cars to me" looks. That lead me to end my story short and they said they will call me today with what's going on.

I also asked for the numbers from the compression test - and they kindly nodded that they had written down the numbers but never produced them or had the want to produce them to me.

I need to find another mechanic thats trustworthy (ha ha)
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Old 12-17-2009, 09:50 PM   #8
falcon_view
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Those guys sound like my uncle who owned a service station that would short the oil dip stick and sell a empty can of oil. Of course my uncle would make like he was pouring the oil in the engine. He got so cocky that one night playing poker at a river boat he got his clock cleaned.
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Old 12-18-2009, 05:20 AM   #9
jamal
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On the dozen or so motors I've pulled apart with cracked ringlands it was apparent from the compression check.

I doubt you threw a rod. pistons failure is much more likely, especially since the motor turns over and don't have a giant hole in your block. They probably won't be able to gain much by pulling the pan off either. Since it's not warranty work you should probably take the car to a shop that can put in forged pistons.
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