'98 volvo penta compression test question

[myersdlm]

Hi, new here, and was hoping I could get some advice. I'm considering purchasing a '98 Chaparral with twin volvo penta 4.3 GL PYBC motors. I had a survey done, and the compression test on the starboard motor showed:
1-170, 2-180, 3-155, 4-190, 5-185, 6-180. Also, during my sea trial the starboard tach didn't work - during the survey it was working, but reporting 3700 rpm as opposed to 4600 on the port. Surveyor said he thought the problem was with the tach as the motors 'sounded' like they were in synch. I didn't notice any performance problems, in terms of speed or getting up on plane when I did the sea trial either.

The question is - with one cylinder at 155, should I expect expensive repairs in the near future or is it acceptable on an older motor to have this type of variance. Trying to figure out whether to 1) move on to another boat, 2) Counter with a lower offer due to near term expensive repairs, or 3) Buy the boat if 155 is within reasonable limits.

Any advice?

 

[Don S]

Re: '98 volvo penta compression test question

I think it's a 4.3GLPBYC not PYBC. But that's not a biggy. The BY would indicate it's a 98 engine, no such thing as a YB.
The max allowed by Volvo & Mercruiser & GM is 15% variation, but older rule of thumb was 10% variation. Even at 15%, you shouldn't be lower than 161psi on #5,
The other thing that bothers me is the low rpm. A weak cylinder may be part of the problem. Since it's a 6 cylinder, #5 is the back cylinder and I would suspect a possibility of water intrussion due to faulty manifold & riser joint or gasket.
Is this a salt or freshwater boat?

 

[myersdlm]

Re: '98 volvo penta compression test question

Hi, thanks for the quick reply, although I must admit I was hoping for better news.


Actually, it is cylinder #3 with the 155 psi.

It is a saltwater boat, located in south florida. The owner just installed a new riser and manifold on the starboard motor.

I had read older posts where some motors are allowed 25% variance, but it sounds like I need to be concerned here.

I have no clue now what order of magnitude a repair might cost to repair this in a worst case scenario. Can you provide me any idea? I need to make a decision on how to proceed with this. Thanks again for your help!

 

[Bondo]

Re: '98 volvo penta compression test question

I have no clue now what order of magnitude a repair might cost to repair this in a worst case scenario. Can you provide me any idea?

Ayuh,... Going that Deep would mean,..
That would be the price of a reman Long Block, or a complete Bobtail motor,...
Plus the local Labor rate of course...

 

[Don S]

Re: '98 volvo penta compression test question

The owner just installed a new riser and manifold on the starboard motor.

Does that mean he installed a new manifold and riser on 1 side of 1 engine? If that is the case, run away from that deal.
First, you always replace as a set, since the new one will flow more water, there may not be enough water to supply the old side.
Second. Salt water kills manifolds and risers in 5 to 10 years. If one goes bad on a twin engine boat, you replace all 4. Because if they fail, they usually destroy the engine in the process. And it sounds like you have one with some problems, and the previous owners was just keeping up by patching as needed. and Now realizes it's time to unload it on someone before it fails completely.

 

[myersdlm]

Re: '98 volvo penta compression test question

Does that mean he installed a new manifold and riser on 1 side of 1 engine? If that is the case, run away from that deal.

No, he replaced both sides on the starboard motor. Apparently the port motor is newer, so he didn't make any changes to it. The port motor had good compression numbers, all between 180-190 psi.

So I have 1 cylinder in starboard that is 155 psi, or 18% variance from the highest one. Trying to determine if I should pass on the boat due to this or if there is some way to determine how bad they cylinder is. If I throw too much money finding the problem I probably just do a rebuild.