2005 Mercruiser 3.0 compression

[oldjeep]

What should I expect to get when doing a compression test on a 2005 3.0L? I'm assuming somewhere around 150-160 psi? Have a potential buyer who wants a compression check done, which is fine but doesn't help a lot if I don't know what the expected range is.

 

[alldodge]

Re: 2005 Mercruiser 3.0 compression

What should I expect to get when doing a compression test on a 2005 3.0L? I'm assuming somewhere around 150-160 psi? Have a potential buyer who wants a compression check done, which is fine but doesn't help a lot if I don't know what the expected range is.

Manual states 9.25:1 and 100 PSI minimum

Also note 7 - states Minimum recorded compression in any one cylinder should not be less than 70 percent of the highest recorded
cylinder.

 

[oldjeep]

Re: 2005 Mercruiser 3.0 compression

Cool, thanks.

 

[Lager]

Re: 2005 Mercruiser 3.0 compression

Seems kind of strange for the potential buyer to ask the seller to perform the test instead of a repair facility, because you can write down anything you want to?
But either way, just in case anyone is interested?
Compression tests should be done with a fully charged battery, all spark plugs removed and ignition disabled. The throttle should be in the wide open position and allow about 5 compression cycles( which can be observed on your gauge as each cycle it goes up a bit) write this # down for each cylinder.
While Ive been an auto mechanic for well over 30 years I couldnt tell you what it exactly supposed to be because there are too many variables that determine compression pressure. BUT, Im thinking a figure of about 150 or a bit more would be normal.
What you are really looking for is the first stroke, that should be around at least 90 PSI to fire off the mixture in the cylinder. It will then pump up as you reach to target of 5 compression strokes.
A difference of 15 % difference is cause for slight concern. 25% is a major concern and loss of power or missfire might occur.
Hint, squirting engine oil down the spark plug holes before testing will increase pressure IF the problem is in the piston rings not sealing well. If there is a valve problem, the pressure will stay the same.
Tech tip of the day.

 

[Pete104]

Re: 2005 Mercruiser 3.0 compression

Seems kind of strange for the potential buyer to ask the seller to perform the test instead of a repair facility, because you can write down anything you want to?
But either way, just in case anyone is interested?
Compression tests should be done with a fully charged battery, all spark plugs removed and ignition disabled. The throttle should be in the wide open position and allow about 5 compression cycles( which can be observed on your gauge as each cycle it goes up a bit) write this # down for each cylinder.
While Ive been an auto mechanic for well over 30 years I couldnt tell you what it exactly supposed to be because there are too many variables that determine compression pressure. BUT, Im thinking a figure of about 150 or a bit more would be normal.
What you are really looking for is the first stroke, that should be around at least 90 PSI to fire off the mixture in the cylinder. It will then pump up as you reach to target of 5 compression strokes.
A difference of 15 % difference is cause for slight concern. 25% is a major concern and loss of power or missfire might occur.
Hint, squirting engine oil down the spark plug holes before testing will increase pressure IF the problem is in the piston rings not sealing well. If there is a valve problem, the pressure will stay the same.
Tech tip of the day.

And not all compression testers will give the same reading!

 

[Lager]

Re: 2005 Mercruiser 3.0 compression

Very true Pete, Thanks for bringing that up.
Ive personally seen a few very expensive gauges that wouldn't read correctly even when brand new. Ive had my local auto parts store gauge for about 30 years now and its the one everyone in shops compares theirs to for calibration or comparison.

 

[Volphin]

Re: 2005 Mercruiser 3.0 compression

And not all compression testers will give the same reading!

You are SO correct. I have 3, and they vary slightly. Compression tests are more about the relation between cylinders. It's painfully obvious when one is low, WAY low.