Compression test

[Freeman1982]

Hello friends, I replaced the head gasket to my engine it’s Volvo penta GXI 5.0 because I have 115 psi in one cylinder and the others have between 130-135 psi
After I changed the head gasket and changed seals valve re machine the head , now I have my compression test in all cylinders 137 except one of them 125 . Is that mean the lower engine weak like having bad oil rings or bad cylinder ?

Note : after replaced the head gasket I did not took the boat to the lake yet I just run it for 5 minutes in the garage . So do you think the compression will go up after run the boat in the lake ?

I checked online it’s says compression test for healthy Volvo penta GXI 5.0 engine should between 120-175 psi . I need your opinion ?

I did all this work because the boat facing hard time to getting on the plane .

Thank you

 

[stouchton]

Basically, compression readings should be within 10% of each other on a "healthy" engine. Some folks say within 10 psi. So, you are right there on percentage and just outside the 10psi rule - which is a rule that does not make much sense since it does not take into account actual compression ratio. To further learn why, you would need to do a leakdown test. There can be some very minor reasons contributing to this (air flow, carbon, slightly leaky valve seat, etc). In my book, close enough to run without second thoughts.

 

[Horigan]

A simple test is to squirt oil in the cylinders and repeat the compression test. If the pressure is higher, you have bad piston rings.

 

[ESGWheel]

I did all this work because the boat facing hard time to getting on the plane .

As the others indicated, you should be good to go and if you desire to investigate more, that can be done. However, I am concerned about your symptoms to get up on plane. A V-8 5.0 with low compression (but not that low) on only one cylinder I would not expect to significantly degrade the performance of the boat.

My suggestion: get the boat on the water and see if any change. Post your findings and if no change also include as much info on the boat / engine history, etc. Thanks.

 

[Donald0039]

It's not clear if you had the heads machined (valve job?) or just new valve stem seals.

Did the boat ever get on a plane quickly or boat new to you?

Not getting on a plain can be many things from old gas to growth on bottom of the boat, to wrong prop.

 

[Freeman1982]

A simple test is to squirt oil in the cylinders and repeat the compression test. If the pressure is higher, you have bad piston rings.

Will do thank you

 

[Freeman1982]

As the others indicated, you should be good to go and if you desire to investigate more, that can be done. However, I am concerned about your symptoms to get up on plane. A V-8 5.0 with low compression (but not that low) on only one cylinder I would not expect to significantly degrade the performance of the boat.

My suggestion: get the boat on the water and see if any change. Post your findings and if no change also include as much info on the boat / engine history, etc. Thanks.

Will do thank you

 

[Freeman1982]

It's not clear if you had the heads machined (valve job?) or just new valve stem seals.

Did the boat ever get on a plane quickly or boat new to you?

Not getting on a plain can be many things from old gas to growth on bottom of the boat, to wrong prop.

I re machined both heads and changed the seals

 

[Freeman1982]

I re machined both heads and changed the seals

I did not check it on the water yet . But my concern is the psi reading. 137 is not close to 175 there is 35 psi difference. So that what makes me think I may have bad oil rings ???

 

[Freeman1982]

Thank you guys and appreciate your comment.
my concern is the psi reading. 137 is not close to 175 there is 35 psi difference. So that what makes me think I may have bad oil rings ???
As I check it should be between 120 -175 psi i know I am in the range but close to 120 psi

I re machined both heads change the seals re do the valve changed the gasket and I am not satisfied with the psi reading
I will check it on the lake and get back to you with the results
Thank you

 

[Donald0039]

I did not check it on the water yet . But my concern is the psi reading. 137 is not close to 175 there is 35 psi difference. So that what makes me think I may have bad oil rings ???

The oil control rings scrape oil and prevent excessive oil burning. Above the oil control ring are the two compression rings. They are what control compression along with the cylinder walls.

Dump a little oil into each cylinder and rerun the compression test. If it gets better your rings are worn. If it does not get better you have a valve problem.

 

[Scott Danforth]

Check your gauge

New motor would be 150 psi on cranking compression

If adding oil brings the numbers up, you have ring issues requiring a rebuild

 

[tpenfield]

A cylinder leak test may be more revealing than the compression numbers.

In general, your engine is OK, but not great.

Did you do the compression test with throttle wide open? I like to do the compression tests with the outdrive off (can't always do this) and battery being charged throughout the test.

 

[Horigan]

Also have all the spark plugs out when doing a compression test.