Very Low Compression

[cdre]

I ended up getting water in my engine courtesy of the manifolds, and now I have very little compression. I have anywhere from 35 to 50 psi on 7 cylinders, and 85 on one. From what I understand, the pressure should be around 150 psi ('91 mercruiser 5.7). This is a relatively fresh rebuilt engine. I pulled the heads today and found all of the cylinders had water in them. None of the valves are beat up, but I am sure they are not sealing well. No significant scoring on the cylinder walls. <br /><br />Is it likely that my piston rings are bad as well? Is there any way to tell without pulling the pistons?

 

[Don S]

Re: Very Low Compression

At this point, all you can do is pull the pistons and look at 'em. Also check the rods to be sure they aren't bent (Twisted). Since the cylinders aren't scored, no since in taking a chance of a broken ring digging a groove in a cylinder.

 

[qystan]

Re: Very Low Compression

cdrec<br /><br />Too many cylinders. <br /><br />Was your throttle in wide open position during the test? <br /><br />qystan

 

[Don S]

Re: Very Low Compression

Eight is Enough........Wasn't there a TV show by that name Anyway, even with the throttle closed, it wouldn/t get consistantly low readings like that.

 

[cdre]

Re: Very Low Compression

There was evidence of water intrusion in all cylinders (rust on the head and crusty stuff on the top of the pistons). Both of the manifolds failed on me. Engine turns over fine with heads off, and it doesnt look like anything is obviously wrong when just looking at it. I will measure the piston travel to check for bent rods.<br /><br />No, I did not have my throttle all the way open, and the choke was still on (engine not warm). Makes sense that that would be important for consistent readings. Even so, with one cylinder at 85psi, the others should have been at least close to that, rather than at 35 or 40. <br /><br />Is there anyway for me to grind these valves myself, or is this best left to a pro?

 

[Bondo]

Re: Very Low Compression

Is there anyway for me to grind these valves myself, or is this best left to a pro?

The only way that motor is going to be a Dependable Runner,<br />Is with a trip to a MachineShop...........<br />Same with the Rods... I wouldn't even think of running those Rings again......<br />You,ve got Serious Problems......<br />Sorry............

 

[cdre]

Re: Very Low Compression

I really only need the boat to get me through striper season this year... I am being transfered to Japan in June, and I will probably sell the boat back to the boat junkyard I bought it from. If there is a way i can get enough compression to run the engine for one more season, I would be happy with that.

 

[CTD]

Re: Very Low Compression

With a cold dry engine compression readings are usually lower than they should be but not that low. If you're sure the guage is accurate and is not misreading from dirt in the check valve the rings are probably the cause. For future reference its better to finish checking before tearing down. You should have added some oil to each cylinder cranked it over a little to spread it around and then re checked the compression. A big increase would indicate rings a modest increase would indicate valves. In your case with almost all reading low its probably rings. Was the high cyl the last one you checked? You could just replace the rings, plasti guage the bearings and put it back together if everything else looks ok.

 

[Don S]

Re: Very Low Compression

I really only need the boat to get me through striper season this year...

Ahhhhhh.....another statistic, when the engine fails at the worst possible moment.

 

[cdre]

Re: Very Low Compression

The high cylinder was the second checked, and after I got done, I checked the guage with my compressor... checked out ok at 40psi.<br /><br />So chances are, the heads should be ok? There is a little rust in the exhaust ports and on the shafts of the valves, but the valves dont appear to be broken or beat up in any way. I am thinking I will get a rering and gasket kit and take my chances. Just have to pull a piston and have it measured to see if it is oversized.

 

[waterone1@aol.com]

Re: Very Low Compression

Just my opinion, but if you have "crusty stuff" on top of the pistons and had water in the cylinders, I don't think there is much chance the valves and seats survived.<br />Your original post said that you had water in the cylinders, but you did not mention the circumstances that this occurred in ?<br />It has been my experience that with minor water intrusion, there is more likely damage to the valves than to the piston rings. Fill a cylinder with water while the engine is running and just about everything can be damaged. On the other hand if you have a small source of water that over time causes the engine to run worse and worse, the pistons and rings can be fine, but the valve seats will have(microscopic) pits, causing severe loss of compression.<br />I know I will catch some flack for this...but...If the engine turns over ok, there is not water in the oil and the cylinder walls do not have any scoring, and you do not have any bent push rods. I would have the heads tested by a machine shop. If they find pitted valves and seats (no bent valves) I would have that fixed and re-asemble the heads onto the engine and do another compression test and see what happens. If compression is still low, then by all means proceed with "deeper" repairs. By the way, did you find the cause of the water intrusion and a plan to fix it ?

 

[cdre]

Re: Very Low Compression

The cause of the water intrusion was bad exhaust manifolds. New manifolds will definitely be a must once I get the thing running.<br /><br />I pulled a piston last night, and all of the piston rings were stuck in their grooves. I also found water in the bottom of the oilpan when I pulled it, but it was not emulsified in the oil, so I think it got in there when I was troubleshooting. <br /><br />I think I am going to start out with new piston rings, and like you said, having the heads tested. Or, I might just bolt the heads back on and check the compression to see what happens.

 

[rodbolt]

Re: Very Low Compression

hello<br /> are you by chance in the navy. if so where are you located?<br /> most bases have a base hobby shop.

 

[cdre]

Re: Very Low Compression

Yes, I am in the navy. I have used the hobby shops before, but I have all of the tools I need to do the piston swap. Not sure if the hobby shop can test heads though...

 

[rodbolt]

Re: Very Low Compression

maybe not but they usually have the solvent tanks and tools to R&R the valves. take them out clean everything and use lapping compound to see if the seats or faces are distorted.pull the rest of the pistons and clean all the parts and oil passages with solvent. hone the cyl's and check them, usually the hobby shop has the tools for this. if not get in good with someone at sima that at least has some telescope gauges and the proper sized micrometer. plasigauge all bearings and if all is still within spec reassemble with new gaskets seals and rings.most marine engines dont get used enough to wear the bores to bad. most die an untimly death due to neglegence. keep all the bearings and pistons marked and labeled to insure they go back in the same spot they came out of.<br /> if the lifters have had water in them it would be wise to remove,dissasemble and clean them. the plunger and valve body will come out of the lifters. do them one at a time as the parts are normally matched.check the cam lobe faces for wear. check the lifter bases for wear. if all look good you may get a decent engine if they dont you will have more problems.

 

[butch290]

Re: Very Low Compression

Just nosy; salt water or fresh.

 

[cdre]

Re: Very Low Compression

Salt. Pulling the engine today just to gain a little more access without covering my arms with fiberglass. I think I should be able to clean it up a little better with the engine out as well.

 

[cdre]

Re: Very Low Compression

Any hints as to how to remove piston rings that are stuck in the grooves? I have tried WD-40, PB Blaster, a very small drill bit, ect... wont budge. Has my engine officially given up the ghost, or is it savable? Bearings look decent.

 

[Don S]

Re: Very Low Compression

No significant scoring on the cylinder walls.

I'm sorry, but if the rings are rusted into the pistons that bad, then the statement about scoring can't possibly be correct. Are the cylinders smooth? If they are rusty and pitted, then you need to have it bored and new pistons installed. Also, if you were trying to do this with the engine in the boat, I really have to question your knowledge of what you are trying to do here.

 

[cdre]

Re: Very Low Compression

I pulled the engine. I was thinking about just doing the piston rings in the boat, as I have decent access, but I ended up figuring out a way to pull the engine (a tree and a chain fall) so I could get it on the stand. The cylinders really arent bad. No significant scoring...