Anyone know how many PSI a 1995 Mariner 150 EFI should have on each cylinder. Doing some routine maintenance right now and decided to check the other day. I'm showing 100-101 on every cylinder. I had the carbed version and it was around 125 psi on each cylinder is why i ask. Thanks.

Known accurate gauge? If so its low.............Needs hone and rings at tha very least. Rings could be wore out or jus loss of tension. Either way need replacin'

Guess I should add. It will run decent at those numbers but with a little power loss. Most average "Joe's" probably won't notice till it drops lower. Some will start dropin' kinda quick some not so quick. Ya just hafta keep monitorin' it. Once it starts dropin' ya get ina tha bigger "damage possible area"...........jmo

Ok i was told from another source that they were supposed to be at 105 psi. I will double check my gauge but im sure its accurate as its only 6 months old.

Does the entire powerhead have to be removed to pull and inspect pistons? Not much experience rebuilding outboards but im sure i can handle it once i start. Have experience with motorcycles, 4 wheelers, and trucks.

My personal deal is nothin' less than 115 on any 2.5. I start lookin' then....jmo

They can be done but it takes some experience and real know how. Can be screwed up pretty easy when ya go back with tha rod caps and ya not even know what ya did wrong. Its a lot easier for most ta pull it and break it down....again, jmo.

One thing though. If your happy with tha way it runs, run it but just keep an eye on tha readings. It may run from now on at 100 lbs.

If it is really that low I would get someone to run a leakdown on it if you are not familiar with the test. A leakdown will show more than a compression test, by far. Sounds like it is on the low side to me but if it leaks good you are good to go. A compression test has too many variables for me so that is why I bought a leakdown.

Was the motor warmed up when you tested? Makes a difference.

For sure. I had a shop tell me my engine needed a new powerhead when it was cold and hadn't been started in 6 weeks. I took it home ran it and bought my own gauge and it tested within 5 pounds.

[/quote]it tested within 5 pounds. [/quote]

'bout right. Hot, cold, throttle open closed doesn't make a whole lotta difference. We check 'em soon as get a front half on 'em sittin' on tha bench without showin' much difference after its complete and runnin'............

I believe That model has a compression relief slot cut into sleeve and static(cranking)compression will be 90-100psi and compression will be increase at about 2000 rpm's.

Gotta disagree. This is true 'bout having tha slots. Very few of tha "production" 2.5's that don't have slots but should still be 115 plus even cold, butterflies closed if its decent in tha healthy department......

Even if your gauge is thought ta be good its always a good thing ta check it against another one. We've got three that we are always comparing on "certain" motors......

Tha carb version (shoulda been an XR6/Mag III if it was a 2.5) rotatin' assy is tha same as tha one ya got now. It has tha slots also.

It may differ slightly on an efi but the service manual for my opti says check with block warm, throttle shutter wide open, all plugs removed , and fully charged battery. Normal compression on all cyl should be between 110-130 psi and should not vary more than 15psi between one another. I would say check it again and if they are all close to the same run it. May be down on power some but as long as it runs good and stays at that level should be ok.

Disagree if you want. They are called compression relief slots for a reason. They relieve compression at low rpm. Only way to accurately check compression on a motor with relief slots is to check it at 2000 rpm. If his compression is still at 100 then I will agree that the motor is a little weak.

That's outta Merc/Mariner manuals. Some manuals are 110 ta 135. That's crankin' and a big variance motor ta motor. Even they are kinda vague about compression ratio's. Guess they are just coverin' they're butts........

EFI's aren't any different as far as comp goes.

I'm not going to get a argument started on here. All I will say is that there are more variables to affect compression readings on a mercrury with relief slots than one without. To get a accurate reading the compression on a relief slot engine needs to be checked at 2000 rpm's. If not the point of the relief slots are worthless.

Not arguin' 'bout nothin'. Justa a little difference of opinion is all...........