Compression Check

[jbarry2]

I just bought a 2000 Bayliner Capri 1950 with a Mercruiser 3.0L 130 hp with an Alpha One. I was told the motor has around 250 hours on it. I checked all the fluid levels, ran it on the hose and then checked the compression. I did not have time to get a book yet and was just wondering what the specs are for the readings. All the cylinders checked at 115 to 118 psi. Thanks you in advance.

 

[Don S]

Re: Compression Check

Here is info on the compression test.
http://www.boatfix.com/merc/Bullet/97/97_25.pdf

For a service manual for you engine and drive go here.
http://www-alt.mercurymarine.com/mnetdata/sisdata/srvcovr.pdf

You need manual #26 for your engine and #14 for your outdrive.

 

[ron7000]

Re: Compression Check

that's really crappy, you should get 170 psi.

 

[Haut Medoc]

Re: Compression Check

Welcome to iboats.....
If you did the test right, it's low......​

 

[Alumarine]

Re: Compression Check

Engine should be warm, all plugs removed, throttle wide open, crank engine same amount for all cylinders (4-6 turns). I don't think you'll get 170.

 

[ron7000]

Re: Compression Check

I don't think you'll get 170.

why do you say that?
what do you think he should get?

 

[Alumarine]

Re: Compression Check

why do you say that?
what do you think he should get?

The service manual indicates a minimum of 100psi is within spec, although I'd be happier with more than that.

I think 145-155 would be good for an engine with some hours on it.
Don't get me wrong, 170 would be great.

 

[Haut Medoc]

Re: Compression Check

I'll amend my statement to say that the #'s are low but the engine is still servicable......
Anything below 100# is unacceptable......​

 

[jbarry2]

Re: Compression Check

Apparently I did not check them correctly. I was not at full throttle and I only pulled one plug at a time. The plugs looked good and she doesn't smoke, so hopefully I will get better readings. I will try again in a few days. Thanks for the input. I enjoy reading the forums.

 

[Alumarine]

Re: Compression Check

Oops, when throttle is wide open make sure choke is wide open as well (it should be if engine is warm).

good luck and keep us posted.

 

[Don S]

Re: Compression Check

As you can see, if you don't do test properly, the numbers mean nothing. Then the replies you recieve are useless.
That is why manuals were made............. Use them.

 

[ron7000]

Re: Compression Check

not sure what kind of crap you guys are checking compression on but normal static compression readings on engines are 170-180 psi- from my 3.0L mercruiser, a 351w in my old boat with over 600 hrs on it, and the ls1 in my car. But that might be cause I'm doing it correctly, to me < 150 psi and you have either crappy ring seal, valve seating, or both. 100 psi is shhitty, I think my 3hp b&s does that and it has compression relief because it's a pull start.

 

[a70eliminator]

Re: Compression Check

My take on static compression is that you are looking for consistancy, the actual pressures are effected by the cam events and valve timing. My freshly rebuilt 428scj has a static compression pressure of 130, it builds it's compression at the higher RPM's when the valve overlap sorta catches up with itself. Granted this is not a normal engine but the static compression really doesn't mean too much as long as the pressures are consistant you'll have a good running engine.

 

[ron7000]

Re: Compression Check

yes you are looking for consistency but at some point I would hope you recognize the numbers are low. if they are all 100 psi vs. being normally > 150 psi then I would hope that would make you go hmmm, 30% loss.

yeah numbers can vary because of valve timing but it's only a factor of how long the intake stays open past bdc on a compression test, but on a normal motor not holding the intake open it should push at least 150 psi.

 

[redjmp]

Re: Compression Check

Keep in mind that the volume of the hose and brass adapter on your pressure gauge contributes significantly to lowering your readings.
If you are using a brass extention tube, try bypassing it and see what it does to your readings.

 

[JustJason]

Re: Compression Check

the numbers are a little low... but not aweful. 125-150 would be more along the lines of average. adjust the valves and recheck it. bet you get a few pounds back.