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Discussion Starter · #1 ·
I am planning a compression test tomorrow on my GT 3.2 and have always tested on a warm engine.

However, I know a lot of people prefer to do this on a cold engine, the reason being is - apparently - that it tends to highlight any problems as the pistons/rings etc are cold and the resulting figures will be lower.

Pondering on cold air density, thicker oil etc, I ended up thoroughly confusing myself as to whether the resulting figures would be higher or lower on a cold engine.

Having Googled extensively, I am none the wiser. Opinion seems divided? I watched a YouTube video where a guy tested both warm and cold and on the cold test, 2 pots were higher than the warm result and 2 lower??

Does anyone have a definitive answer and simple explanation which won't fly over my head?

Thanks:happy:
 

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I wouldn't be overly concerned about hot engine / cold engine. If you have a compression issue it will be apparent in either case. Usually a good 3.2 will make 170 to 200 psi on each cylinder.
 

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I've never heard of the debate between hot and cold engines. Usually the problem is variation between cylinders which will show up on both tests.
A hot engine will burn your hands as you try and get the plugs out (especially on a V6!!) and by the time you've got the plugs out and done all six cylinders it'll have cooled down anyhow.
So do it cold then it'll be a constant temp between cylinders.
 

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Discussion Starter · #4 ·
I wouldn't be overly concerned about hot engine / cold engine. If you have a compression issue it will be apparent in either case. Usually a good 3.2 will make 170 to 200 psi on each cylinder.
Thanks for that - hopefully, mine will fall within that range, otherwise..........:rolleyes:
 

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If they were all 160psi for example then it wouldn't necessarily indicate a problem, it might indicate a badly calibrated pressure gauge. But if they were all 160psi except one which was 40psi then you know which cylinder you have an issue on.

When these engines are down on compression on one cylinder, I can tell without looking at the gauge which cylinder is down as you can hear it in how much easier the engine spins.

You can leave the engine ECU bolted to the plenum and then fully remove the plenum from the car for the purposes of the test, it'll still crank on the starter but you won't have any fuel injection.
 

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Discussion Starter · #6 ·
Thanks! I have now done the test .....cold.

I left the ECU connected, as per your instructions

I had 5 pots between 195 and 201 - the sixth (no 2 ) was 187. I am hoping this difference is not enough to worry about? Thanks again for your advice. It is appreciated.
 

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Thats a good result. Its not surprising number 2 is the worst as thats the middle cylinder of the back bank, presumably it gets the hottest.
 

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Discussion Starter · #8 ·
Thats a good result. Its not surprising number 2 is the worst as thats the middle cylinder of the back bank, presumably it gets the hottest.
Do you think I should be concerned about no 2? Am I looking at booking it in with you yet......or is it just paranoia setting in?

Maybe I should focus on the acceptable results with the other 5 ;)

Thanks for your input.
 

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Do you think I should be concerned about no 2? Am I looking at booking it in with you yet......or is it just paranoia setting in?

Maybe I should focus on the acceptable results with the other 5 ;)

Thanks for your input.
No, no need with that sort of result. Compression test again next year, see how you go on. When the gasket goes or a valve starts to seal badly it'll show something like 60 or 80psi on that cylinder.
 

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Discussion Starter · #10 ·
No, no need with that sort of result. Compression test again next year, see how you go on. When the gasket goes or a valve starts to seal badly it'll show something like 60 or 80psi on that cylinder.
Thanks for the reassurance. If it starts to drop lower, I will book it in:thumbu p:
 

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Squirt some oil in the plug holes and repeat the test. If the results go higher then you have worn bores/rings. If they stay the same then it's valves.
 
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