Mercruiser 7.4 MPI compression question

[Vadave]

Hello,

I am considering a new to me boat. I know just enough to be dangerous and did a compression test with a friend who knows a little more. The numbers have me confused. After searching a bit I think we made some mistakes that may or may not explain the numbers. It is a 1999 Mercruiser 7.4L MPI motor.

The engine was cold and the throttle was at idle(in the neutral position). The numbers are:

1: 90dry/90wet 2: 90dry/90wet
3: 120dry 4: 90dry/105wet
5: 100 6: 120
7: 90dry/110wet 8: 80dry/80wet

Can anyone explain what may or may notbe going on and/or whether the test is useless due to the procedure used? Service records show some previous overheating issues.

Thanks!

 

[achris]

Re: Mercruiser 7.4 MPI compression question

Hello,

I am considering a new to me boat. I know just enough to be dangerous and did a compression test with a friend who knows a little more. The numbers have me confused. After searching a bit I think we made some mistakes that may or may not explain the numbers. It is a 1999 Mercruiser 7.4L MPI motor.

The engine was cold and the throttle was at idle(in the neutral position). The numbers are:

1: 90dry/90wet 2: 90dry/90wet
3: 120dry 4: 90dry/105wet
5: 100 6: 120
7: 90dry/110wet 8: 80dry/80wet

Can anyone explain what may or may notbe going on and/or whether the test is useless due to the procedure used? Service records show some previous overheating issues.

Thanks!

Engine should be warm and the throttle fully open....

 

[Vadave]

Re: Mercruiser 7.4 MPI compression question

Any idea what kind of difference that would make in the numbers?

 

[achris]

Re: Mercruiser 7.4 MPI compression question

Lots....

 

[26aftcab454]

Re: Mercruiser 7.4 MPI compression question

the #s should be within 10% of each other- the original #'s were not good.

 

[searay3]

Re: Mercruiser 7.4 MPI compression question

Never to take away from Achris, very reliable source...any motor that pulls those kind of numbers, warm or cold, is well used. Not to say it won't run, but don't expect any real performance, it's tired....you have to weigh what a long block will cost against what a rebuild will cost..my bet is a long block.

 

[Bondo]

Re: Mercruiser 7.4 MPI compression question

it's tired....

Ayuh,.... I can agree with That....

 

[can-amsledder]

Re: Mercruiser 7.4 MPI compression question

Those numbers don?t sound good.

What brand of gauge are you using? A Snap-on, or something from Harbor Freight? I would verify those numbers before I when any further.

 

[6meter]

Re: Mercruiser 7.4 MPI compression question

Numbers look low. A wet test will increase if your rings are losing it. It didn't so atleast you know the valves are going. But when you do a valve job, the increased compression puts alot of strain on the bottom end, then that goes. Yep basicly tired.