Compression check after survey

[MikeBravo]

I just had a survey done on a 2007 320 DA. The survey found a few minor problems but nothing you wouldn't expect on a nearly 10yo boat. After the haulout portion of the survey and the boat was splashed I had a compression check done by the mechanic from MarineMax. The boat sat in the water for about three hours before the mechanic got to it. The port engine was fine, but the starboard engine had #2 @110, #4@110 and #5@80. The others were around140-150. He did not do a wet test and said all plugs were clean. Before the haulout we did a seatrial test and both engines ran fine up on plane quickly and wot at nearly 4900rpm, no problems. Tested at 1000rpm, 2000rpm, 3000rpm,3800rpm and wot again no problems temps were good, engines started right up. I am perplexed at the compression numbers for the starboard engine. The mechanic said the engine needed to be replaced and they could do it for $12,500. Told seller and suggested he get another compression test and a leak test from a third party mechanic that another boater had used and said was very dependable. He is going to have that done Monday. Any ideas?

 

[hottoddie]

I would agree another compression test should be done by an independent mechanic. If it is verified that 2-3 cylinders are low then it will most likely need a top end overhaul (valve job). Cost should be 1500-2500 unless manifolds and risers need replacing at the same time then it could be higher.

 

[MikeBravo]

Thanks for the quick reply. Manifolds and risers were replaced in 2015 and have 13 hrs on them. I had to laugh when the mechanic said a new long block would be $12,500. Have a guy up in Tarpon Springs that does them all day long for $7500 with oem parts.

 

[Steve S]

What engines? I would get the specs from Mercruiser on the engines and the compression limits. When I the survey on my 340, One cylinder of my 7.4 Horizons seemed low until I read the specs and it was actually in factory limits.


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[Mike Blake]

How was the test done. Warm or cold?

 

[MikeBravo]

Steve the engines are 350 MAG MPI's and 80 and 110 are not in spec.
Mike the test was done after sitting with engines off for 3 hrs.

 

[Hifirush]

Correct me if I'm wrong someone, but isn't the problem when one cylinder is more than 10% less than the others? My surveyor found that to be the case on one boat I looked at and I walked away. Yes it had other issues too, I wouldn't pass on a boat that needed a valve job. But I agree, you should have an independent mechanic retest. Hope I didn't confuse the situation more.

Matt


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[fwebster]

I have seen surface rust on the valve face and seat cause screwy compression readings. I would have the owner run the boat at cruise speeds for about an hour before he does the next compression test. Given that all other indicators are good, the chances are that regular use will cure the compressions issue. Anyway, a valve job in a small block GM shouldn't be much over $1000-$1200.

 

[ttmott]

At this point seems too much unknown as to determine root cause. From the original post the risers were replaced then the boat entered the market; suspicious?
The big pressure differentials could be due to valves or pitted cylinders, or piston rings corroded or cracked or a host of other things contributing to the symptoms.
At this point only a cylinder leak down test from a qualified technician can determine where this situation will land.
Spark plug condition, oil condition are variables as these are easy to change and eliminate credible evaluation.

 

[MikeBravo]

I just threw this out there because it was so odd that the engines ran so well. With the low compression numbers on 3 cylinders in the starboard engine I would have thought there would be definite lagging The risers and manifolds have 13hrs on them since they were replaced last year. The boat is on the market because the owners are moving to New Zealand in September. Having another mechanic redo the compression test and a leak test today. We'll see how that turns out.

 

[K9Deuce]

Good, choice on another opinion.. A compression test along with a leak down test should provide you with the answers on where the problem is ..the compression test is the first step to letting you know if there is a issue, once it's determined that there is a issue a leak down test will guide to to the possible causes ..

 

[MMcCawley]

This was already asked before but was the compression test done on a cold engine or one that was up to temperature? Engine temperature can affect the numbers.

 

[MikeBravo]

The second mechanic did another compression test on cold engines with better results but did not do a leak test that I thought he would. So still no answers. Guess I'll have to wait till the owner gets back from vacation to see what he wants to do. Second mechanic said to do a valve job, but without evidence that doesn't make sense to me.

 

[ttmott]

Any qualified mechanic should have the correct equipment to do a Leak Down Test; it is a basic trouble shooting techniques. Cold or hot; it's the differential between cylinders that is the issue. If you asked for a leak down test and he did not perform it I would question his invoice.

 

[KHE]

Another way to diagnose the issue is to run the engine until it comes up to operating temperature, do the dry compression readings, then repeat with wet readings - 3-4 squirts from a pump oiler in the cylinder. If the compression in the respective cylinder comes up, it means the rings are worn. If there is little change, it points to valves.

 

[MikeBravo]

My broker is suggesting that the leak test be performed, the owner ordered the second compression test so I'm not paying that bill. I guess i'm just the kind of person who wants to get to the bottom of things as quick as possible and then solve it.

 

[nickdcj7]

I was always told that if the engines run up to the correct rpm at the right speed range for the wot test, then they are good and a compression test would be a waste of money. Call me cynical, but I'd be more inclined to trust the wot test than a mechanic you don't know personally and wants to sell you an engine to replace one that's running fine.

 

[Air O'Nautical]

I was always told that if the engines run up to the correct rpm at the right speed range for the wot test, then they are good and a compression test would be a waste of money. Call me cynical, but I'd be more inclined to trust the wot test than a mechanic you don't know personally and wants to sell you an engine to replace one that's running fine.

A friendly FYI.....

Not always true.....I've seen a boat run very nearly up to the correct RPM and had low compression numbers in a cylinders or two.
RPM was close enough to explain away as scummy bottom or a slight prop problem.
I performed the test for the seller and was surprised to see the pressure numbers all over the place because, the boat seemed to run very well.

A compression test is dead easy to do and should be relatively inexpensive, depending on access.

In my case the buyer made an offer leaving room for a total repower....and ended up doing just the heads on both motors.

 

[aerobat77]

I was always told that if the engines run up to the correct rpm at the right speed range for the wot test, then they are good and a compression test would be a waste of money.

there is some thruth in this. if the engine sounds , feels and performs right it is right .

a compression test on the other hand is a cheap method to judge the wear condition of the engine and detect problems which are waiting to develop but might also point to false assumptions .

 

[MikeBravo]

Should know something by the end of today. Mechanic is taking it out to run for an hour and then perform compression and leak down test.