Another compression issue

[Shadow66]

Had my boat in the shop (3.0 Mercruiser Alpha 1 gen 2 sterndrive) because it was running so-so at idle.

They cleaned the carb and did some other small things like change the filters etc...and then told me my compression was:

#1) 160 psi
#2) 160 psi
#3) 160 psi
#4) 100 psi

They looked inside with a bore scope and said there was scoring in cyl #4. No mention of rings, valves etc...

I am mechanically inclined but not to the degree where I can take apart engines and do these kinds of tests/work.

As it stands now it is running great at anything above 1,200 rpm and average at standing idle and better when hot.

If I bring it somewhere else what should I ask them to do and/or to look for? Is the only fix a new block? If so where can I go to get one?

I realize it could get worse over time and I will bring it in for this kind of thing eventually.

Oh yeah one more strange thing...when I asked about what could cause that (It's a 2002 Stingray Bowrider I just bought ijn February "Caveat Emptor") he said that with the new gasoline and the water absorbtion it can get that water gets in the cylinder and acts as a spray mist removing the oil along the walls of the cylinder.

This sounds strange to me. I have heard of techniques of spraying a small stream of water in the carb to clean deposits, and wouldn't the explosion just burn off the water? I may be missing something but...

 

[mkast]

Re: Another compression issue

Scored cylinder will need to be sleeved.

 

[Maclin]

Re: Another compression issue

The next step after finding low compression is a leakdown test. This involves getting the cylinder to tdc on compression stroke, introducing compressed air via a spark plug hile fitting, and listening to 1.) The intake manifold hole or thru the carb and 2.) Using a stethoscope and holding it to the block or oil pan.

Hissing thru the intake indicates valve sealing issues.

Hissing into the crankcase indicates a piston ring sealing issue from bad rings or compromised/scored cylinder walls, or holed or cracked piston.

 

[danond]

Re: Another compression issue

I think your mechanic was trying to say "I don't know" by blowing a bunch of smoke up your butt.

 

[Shadow66]

Re: Another compression issue

The next step after finding low compression is a leakdown test. This involves getting the cylinder to tdc on compression stroke, introducing compressed air via a spark plug hile fitting, and listening to 1.) The intake manifold hole or thru the carb and 2.) Using a stethoscope and holding it to the block or oil pan.

Hissing thru the intake indicates valve sealing issues.

Hissing into the crankcase indicates a piston ring sealing issue from bad rings or compromised/scored cylinder walls, or holed or cracked piston.

Thanks Maclin, is there any other symptom that would indicate either of these? Like spark plug condition or contaminants in the oil from blowby or anything I can diagnose?

What he "said" was that he saw the scoring on the walls with a bore scope. But I am beginning to wonder about all this. I have reason for suspicion because of some other actions by this dealer. But before I waste a whole lot more time by bringing it somewhere else maybe I can do some self diagnosis to some degree.

 

[TilliamWe]

Re: Another compression issue

Shadow, the only other thing you can possibly do to narrow it to a valve issue is to remove the valve cover, and loosen the rocker arm bolts on that #4 cylinder, a little, and recheck compression. This would allow a tulipped valve to seat better and raise the compression. Kind of a "backyard" way of doing it, though. And it's not 100% accurate.

The leakdown test is the real way of doing it.

BTW, I'll bet a nickel you have a valve issue, not a bad cylinder issue.

 

[45Auto]

Re: Another compression issue

The thing about the gas and water spray is pure crap. Take it to someone else next time.

You need to do a leak down test and isolate your problem to valves or cylinder.

Another quick check is to re-run the compression check, but squirt some oil into the spark plug hole first. The oil will provide a better seal to the cylinder walls if that's the problem, and you will see a higher compression reading. The oil WILL NOT help if it's a valve problem and the compression reading will stay the same.

If it's valves, a cylinder head rebuild isn't real bad.

A damaged cylinder is a little more involved. If you can live with the way the boat runs, it'll PROBABLY keep running like that for a long time. Friend of mine has a 1981 boat with a 2.5L Mercruiser (earlier version of your engine, just slightly less displacement) that's been running with one bad cylinder at about 90 PSI compression (other 3 are 135 - 140) for over 10 years - as long as I've known him. No idea how long before that it dropped to 90.

If it is the cylinder, repairing it will probably require boring it out and an oversize piston at a minimum. If it's real bad it'll have to be sleeved. Either option is about as serious as you can get and requires total engine removal and teardown.

Usual cause of a bad cylinder I see down here on a 7 year old engine, especially if the boat has been in salt water, is a rusted and slightly leaking water passage/head gasket or exhaust manifold that will allow water into the cylinder to rust it up while the boat's sitting. Won't affect it (except for the low compression) when the boat is running. If it sits there without running long enough it'll rust bad enough to lock the piston to the cylinder and lock up the engine.

Good luck!

 

[Shadow66]

Re: Another compression issue

Thanks 45 & Tilliam I appreciate that. I called another dealer yesterday and told him the story. He led me also to some suspicions about the original mechanic. He told me that he wonders why my boat still revs smoothly and gets to 4800rpm's with a bad cylinder. He also said in his over 3 decades of being a boat mechnic he has never seen a bad cylinder wall on my type on the 3.0. (That doesn't mean it cannot happen, it just means it may be rarer than I was led to believe). He said it just doesn't add up to him.

I called the original dealer also and inquired more. They said that they just looked at the scope and the mechanic was asked: "Do you feel we should do more tests?" He said "No, the scoring seen on the scope is going to be the leak issue." From what I am seeing here and the 2 other places I've called this is not necessarily the case.

So I will bring it to this other guy (who actually spent about 15 minutes on the phone with me and showed some respect) to do the leakdown test. But I am going to have fun with it for a month or so since the boat has been in the shop for more time than I've owned it!

Thanks again!

The thing about the gas and water spray is pure crap. Take it to someone else next time.

You need to do a leak down test and isolate your problem to valves or cylinder.

Another quick check is to re-run the compression check, but squirt some oil into the spark plug hole first. The oil will provide a better seal to the cylinder walls if that's the problem, and you will see a higher compression reading. The oil WILL NOT help if it's a valve problem and the compression reading will stay the same.

If it's valves, a cylinder head rebuild isn't real bad.

A damaged cylinder is a little more involved. If you can live with the way the boat runs, it'll PROBABLY keep running like that for a long time. Friend of mine has a 1981 boat with a 2.5L Mercruiser (earlier version of your engine, just slightly less displacement) that's been running with one bad cylinder at about 90 PSI compression (other 3 are 135 - 140) for over 10 years - as long as I've known him. No idea how long before that it dropped to 90.

If it is the cylinder, repairing it will probably require boring it out and an oversize piston at a minimum. If it's real bad it'll have to be sleeved. Either option is about as serious as you can get and requires total engine removal and teardown.

Usual cause of a bad cylinder I see down here on a 7 year old engine, especially if the boat has been in salt water, is a rusted and slightly leaking water passage/head gasket or exhaust manifold that will allow water into the cylinder to rust it up while the boat's sitting. Won't affect it (except for the low compression) when the boat is running. If it sits there without running long enough it'll rust bad enough to lock the piston to the cylinder and lock up the engine.

Good luck!

 

[45Auto]

Re: Another compression issue

Hope you have better luck with the new mechanic. Those are nice little motors, almost bulletproof. The picture below is of one that was locked up from a leaking head gasket in the #4 cylinder. The arrow points to the rust trail down the side of the cylinder. Rings were rusted solid to the side of the cylinder, had to break up the piston to get it out. Honed the cylinder and replaced the piston and rings (wasn't perfect, had some scratches and pits and would only pull 135 PSI or so on a compression check), it's been running perfectly for about 2 years now. Hopefully yours isn't anywhere near as bad as this one was.

 

[TilliamWe]

Re: Another compression issue

Shadow, I had a 5.7 with Zero compression on one cylinder, and it still ran great at WOT. Or seemingly great. After I fixed the valves, it quit smoking at idle and ran even better on the top end. So it is possible that your engine is still running good. It'll just run better when fixed.

I'd do exactly what you are doing, use it, then fix it later. Good luck