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Valve job, or???

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GS455

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Selling my boat, as a condition of the offer, a cold compression test needed to be done. Many people have said that cold tests are no good, but nevertheless it’s done. These are Mercruiser 350 MPI inboards with roughly 1300 hours, mostly trolling. One engine has one low cylinder and the other has two. They said most of the cylinders are in the 180 range which seems really high. The lowest one is 90 and the other low ones were 115 and 135 I believe. They squirted oil in the cylinders and rechecked the low pots. Numbers came up a little, but not much. I/we would like to retest on a warm engine but the boat is stored and not easily run at this point.

From this data, I can imagine a few scenarios. One, the engines need valve jobs, seems pretty common. The next scenario is stuck or broken rings, this seems unlikely. Or finally, the test is just wrong. I want to note that these engines run perfectly and never consume oil / smoke / misfire or anything.

I know that whatever the result I’m going to have to take a bath on this deal, I want the buyer to have everything right when he takes delivery, and I’m okay to pay for that. Just wondering about what I should expect and what you may think about the tests results thus far. Any insights?
 
Just wondering about what I should expect and what you may think about the tests results thus far. Any insights?
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Ayuh,...... My compression tester is of the sort, that the gauge can be disconnected from the plug hole fittin' hose, with a quick connector,......

With the gauge off the hose, I snap on the shop air hose, with the cylinder held at Tdc, 'n Listen,...... then follow the sound,.....
An exhaust valve leak, ya hear through the exhaust,....
An intake valve leak, ya hear through the induction,.....
Ring leaks, ya hear through the oil fill cap,.....
 
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GS455 said:
Selling my boat, as a condition of the offer, a cold compression test needed to be done. Many people have said that cold tests are no good, but nevertheless it’s done.
I would rely more so on a warm cylinder pressure test. As the engine is performing under normal operation, it will be warm!

These are Mercruiser 350 MPI inboards with roughly 1300 hours, mostly trolling. One engine has one low cylinder and the other has two. They said most of the cylinders are in the 180 range which seems really high.
Yes, the way in which these SBC Marine version engines are built (GM full dished pistons and 64-65 cc chambers), 180 psi would be on the high side.

The lowest one is 90 and the other low ones were 115 and 135 I believe.
90 is extremely low, 115 is very low, and 135 is also on the low side.

They squirted oil in the cylinders and rechecked the low pots. Numbers came up a little, but not much.
On any V engine, the oil tends to pool at the low side of the ring landings. This does zero for aiding in sealing piston rings at the high side.
However, any oil volume that is added to a cylinder will consume volume, and will raise the cylinder pressure.

I/we would like to retest on a warm engine but the boat is stored and not easily run at this point.
I would want to perform cylinder pressure tests on warm engines. You will need to have it pulled from storage so that you can run them!

From this data, I can imagine a few scenarios. One, the engines need valve jobs, seems pretty common. The next scenario is stuck or broken rings, this seems unlikely. Or finally, the test is just wrong. I want to note that these engines run perfectly and never consume oil / smoke / misfire or anything.
Before you pull the cylinder heads, I would perform a cylinder leak-down test (see Bill's explanation above). Best to use a specific cylinder leak-down test kit.
cylinder leak down test kit 2 .jpg
This will allow you to discover or eliminate failing piston rings.

Once the heads have been removed, this test is no longer available!


I know that whatever the result I’m going to have to take a bath on this deal, I want the buyer to have everything right when he takes delivery, and I’m okay to pay for that. Just wondering about what I should expect and what you may think about the tests results thus far. Any insights?
See above!


As for the cylinder pressure test, due to the volume within the gauge's attaching hose, each cylinder should be cycled through at least 2 or 3 compression cycles. To be safe, this may mean 5 to 7 crankshaft revolutions per cylinder check.
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Thank you both for your insights. I will run these things by the storage facility’s mechanic and see what he has to say. Leakdown test was not mentioned in my phone call with them so I don’t think that was performed, but should be, if the warm results are not sufficient. I’m just hoping that after having been sitting for almost 6 months, maybe it’s just a few sticky rings or something that will loosen up after running.

Unfortunately the boat is buried at the back of the indoor storage building, so myself and the buyer are playing the waiting game until it can be retrieved or perhaps a hose could be run to it.
 
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GS455 said:
Thank you both for your insights. I will run these things by the storage facility’s mechanic and see what he has to say. Leak down test was not mentioned in my phone call with them so I don’t think that was performed,
It requires some time do..... if they did perform it, you would have been billed for it.
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As Chris says..... at 1,300 hours, these engines are likely getting tired.



 
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Docksidemarineservices said:
1300 hrs, they are tired.
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Maybe, maybe not. We are in freshwater here and most of the hours are trolling, so things tend to live a lot longer. Boat has also been meticulously maintained so I would hope to be safe from major issues. Personally never had any doubts or reservations about the engines. They rev right out and run great. Guess we’ll see.
 
1300 hours at the rough equivalent of 50 mph in a car would be about 65000 miles. engine in a boat probably gets a bit less wear compared to car. So doubt the engines are anywhere near death. I think you are getting false readings on a cold engine that has been sitting.
 
1300 on a 5.7 is a lot of hours. Close to the end of its life.
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Ayuh,..... Tell that to my 4.3LX with well over 2,000 hours on it, 'n still carries 'bout 150 psi across the board,.....
 
You need to launch that boat and run (the hell out of it) for a while. Then take your readings.


1300 hours is nothing on a good small block Chevy, properly maintained, especially since it had an easy life.

Jeff

PS: No one should buy a boat without a sea trial in any case, so...
 
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fastjeff said:
You need to launch that boat and run (the hell out of it) for a while. Then take your readings.
I would agree.
If the pressure readings do not come up to within healthy specs, I would then perform the cylinder leak-down test.


1300 hours is nothing on a good small block Chevy, properly maintained, especially since it had an easy life.

Jeff

PS: No one should buy a boat without a sea trial in any case, so...
Also agree!
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Pull it out of storage, run the hell out of it and retest. Maybe the numbers will come back. If not, you just proved my point.

I am not trying to disrespect anyone by posting this so don't take it personal.

Sorry, 1300 hours on a SBC marine engine puts it close to the end of its life. Trolling hours, cruising hours, or wot, it's a lot of hours. The valve train is only going to take so much.

You cannot compare your marine engine to a car and apply a mileage to it. Your boat is always driving up hill with a max load on the engine. Boats do not coast. How long would your 350 powered Chevy Pick up last pulling a loaded trailer through the Rockies?

The heat cycle in the cylinder heads will fatigue the valve train way before the pistons and cylinders give up their ability to make vacuum and compression. If your Gasoline engines are still performing like new @2000 hrs you are a marine mechanic, have a good marine mechanic or very lucky. Once something fails in the valve train the water is coming in and it will be all over. That is what causes the death of most gasoline powered boat engines.

A 1300hour SBC boat is a hard sell here at the Jersey shore.
 
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fastjeff said:
PS: No one should buy a boat without a sea trial in any case, so...
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Indeed. On top of the compression test, as a condition of the sale, there was going to be a sizable amount of money held in escrow until after completion of seatrial. When you sell a boat at the tailend of winter this is the best you can do.

I’m not in denial, or offended by anyone’s opinions on the motors. That’s why I started the thread in the first place. Ultimately I want to make the boat right for the new owner, but I don’t want to do a whole mess of unnecessary stuff to a good running boat.

Even if the engines are starting to wear, we all know that they could last 10 minutes, or another 10 years and run fine, especially in this environment.
 
You can do the compression test cold....its been published in several service bulletins since the 90's.

I'd agree with Chris that, on average, the engines are tired....likely to need the valves reworked....the short block should go ~ 2000 hours with decent maintenance. the leakdown test will tell you for sure.

Averages are just that....our BBC's saw over 3000 hours before major overhaul. Lots of variables involved.....lots.
 
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