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Mercruiser low compression


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dldeuce
Novice

Apr 17, 2017, 9:57 AM

Post #1 of 13 (366 views)
Mercruiser low compression Sign In

I've got a 95 Mercruiser V8 with about 600 hrs on it. A mechanic did compression and leak down tests and found the number 6 cylinder with about 60psi compression leaking into the intake. The other cylinders were good on compression. Not being able to afford the $110/hr mechanic, I took the heads off myself. The obvious thing I see is the head gasket. I'm thinking that doesn't explain the leaking into the intake. I don't see anything immediately wrong or different about the valves or cylinder head on that cylinder. I can supply pictures of that later. For now though, does this explain the symptoms? Advice/comments? What else should I check for?

[url=https://flic.kr/p/TndEzC][url=https://flic.kr/p/TndEzC]973 by [url=https://www.flickr.com/photos/61389024@N07/]ABC_photos, on Flickr


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Apr 17, 2017, 11:43 AM

Post #2 of 13 (352 views)
Re: Mercruiser low compression Sign In

Marine - Mercruiser: Owned many so let's get going. You have the heads off so at least look at an intake valve but all caution sport - saving only $110 for marine work some is easier some aggravated by harsh use, fresh or ambient water cooled issues plus improper off season storage as applicable.
Which engine? A 350 GM or what?


Same issues with cars but marine parts are tougher against corrosion to a point. Salt water use? If so do you flush it each use or just let it be? It super matters if you trailer this.


The most common problem I find with marine stern drives or inboards is the risers to wet exhaust have water jackets just like engine does. Also if you flush this out yourself do you keep side block plugs clear when flushing it or didn't know you have to do that? That mistake can cost you an engine.


I doubt this being a '95 you haven't replaced risers or water impeller yet or on a second or third! That means you are running hot and like about any engine they seriously don't like that marine too just a bit more tolerant.


If you've determined #6 intake valve isn't sealing well take that ONE out and look. Count turns of rocker arm till that one is free. Just maybe you can see the problem visually with the valve or seat. You need some help to determine sending it out or just clean it up.


The #1 question is going to be did this overheat. Next is did you make the mistake of taking the thermostat out? If so that will cause this no question as you defeated the by-pass of coolant to keep engine temp all uniform by doing that. A lobster boat gets away with it, this wouldn't,


T



dldeuce
Novice

Apr 17, 2017, 12:19 PM

Post #3 of 13 (348 views)
Re: Mercruiser low compression Sign In

It's a GM 350, and although it's a '95, it only has 600 hrs, or so the computer says. It's been run in brackish water and is flushed after each use.

What are side block plugs? There are two water plugs I guess on either side of the block. I don't open those when I flush. I was looking at that before I took the head off. I wanted to drain the heads down before I took them off. I opened those plugs, but no water came out. The heads were still full of water. That bothered me. I'm definitely concerned there's a cooling problem although I'm not getting any temp sensor indication or alarms.

The problem I've been having is that once the engine got really heat soaked, it wouldn't want to restart after sitting for 45min. It wouldn't ever do it just bringing it up to operating temperature. You could cycle the electric fuel pump for a few minutes, and eventually, it would start to stumble and then finally start up and run. I presumed that was vapor lock or some sort of fuel problem from ethanol. That had been going for several years. I managed that by not letting it sit or by starting it after 5-10 minutes and running cool water into it after shutting it down.

Last summer, it just stopped wanting to idle. It would idle, stumble, and then die. Didn't want to restart, similar to above, but eventually it would restart.

Other than those two problems, it runs perfect. I didn't see any evidence of a rough idle either so I was surprised with the low compression, but I confirmed it myself.

So, I'm taking the heads to a machine shop tomorrow, but I want to look for other problems rather than just putting it back together as is and hoping for the best.


dldeuce
Novice

Apr 17, 2017, 12:21 PM

Post #4 of 13 (346 views)
Re: Mercruiser low compression Sign In

Original risers. Impeller replaced yearly. Thermostat never touched.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Apr 17, 2017, 12:43 PM

Post #5 of 13 (342 views)
Re: Mercruiser low compression Sign In

When you say "leaking into the intake", what is leaking and how did you determine that?



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
Ultimate Carjunky / Moderator
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Apr 17, 2017, 1:15 PM

Post #6 of 13 (337 views)
Re: Mercruiser low compression Sign In

HT - I took that as valve compression leaked not known until OP says more.


OP - the plugs down low should be 9/16th hex and so full of rust and dirt with low flow you coat hanger those clear to flush or water jackets fill up.


Risers are easily removed if not too bad you can coat hanger those out too or it can't cool properly + worse if exhaust is hot on the way out it will burn up the flexible parts all along the way as it's always flooding the exhaust so it doesn't glow hot as hard as they work.


Send the heads out and get that fixed. It's a very tough engine for marine but anything has limits. Clear out what you can now with heads off and the side drains out as said a lot of junk can be removed but not all.


Hate to say with ages this was my weekend warrior thing both lakes and Ocean so at it all the time up to full blown yachts this stuff is routine and so is replacing engines from them being clogged up. You can add fresh water cooling but if you take it out I wouldn't bother just flush it with clear fresh water every time it comes out. You can put drain cocks on the side bolts but just use brass plugs and some water proof grease and do those often.


Seems like a lot of work and understand last thing you feel like doing all the time but it really helps. If daily use and out put drain cocks in those side plugs then ears on the outdrive and plug out of rear transom and just let it flush out.


Always use marine grade parts despite automotive stuff can fit. It's much more corrosion resistant and spark arresting for electrical even the silly solenoids that run the outdrive!


Good luck. It may work out for you and I don't think you can really use and keep a boat engine indefinitely without surreal care with every use,


T



dldeuce
Novice

Apr 17, 2017, 3:17 PM

Post #7 of 13 (325 views)
Re: Mercruiser low compression Sign In


In Reply To
When you say "leaking into the intake", what is leaking and how did you determine that?


The mechanic did a compression test, 60psi, and then a leak down test. He concluded it was leaking into the intake from the leak down test.


(This post was edited by dldeuce on Apr 17, 2017, 3:39 PM)


dldeuce
Novice

Apr 17, 2017, 3:35 PM

Post #8 of 13 (319 views)
Re: Mercruiser low compression Sign In

Here's a picture of the head. The close up is the bad cylinder.

[url=https://flic.kr/p/TUcPFq][url=https://flic.kr/p/TUcPFq]5D3_1740 by [url=https://www.flickr.com/photos/61389024@N07/]ABC_photos, on Flickr

[url=https://flic.kr/p/SJ7C3a][url=https://flic.kr/p/SJ7C3a]5D3_1747 by [url=https://www.flickr.com/photos/61389024@N07/]ABC_photos, on Flickr


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Apr 17, 2017, 3:50 PM

Post #9 of 13 (309 views)
Re: Mercruiser low compression Sign In

OK, so it was compression leaking, not coolant. You did have coolant/water leaking into the cylinder through that head gasket. That may have contributed to the valve problems causing the blowby through the intake valve. He should have also done a wet compression test while the engine was still together.

Either way, you have to send those heads out to the machine shop. Ypu will likely need new head bolts also.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Apr 18, 2017, 12:35 AM

Post #10 of 13 (290 views)
Re: Mercruiser low compression Sign In

! Middle two are plain water involved via gasket failed either corroded or ran to hot there? Redo of that and match other head would or should fix. this.


BTW - crusty rust inside like that is a really bad sign suggesting water (raw water cooled no doubt) had been doing that for some time. Said in that it had some age to it, it might have been thru problems before. Know that marine or OE marine gaskets, freeze plugs, water pump parts are made not to corrode vs automotive stuff that will work but not last long at all,


T



(This post was edited by Tom Greenleaf on Apr 18, 2017, 12:41 AM)


dldeuce
Novice

Apr 19, 2017, 7:48 AM

Post #11 of 13 (264 views)
Re: Mercruiser low compression Sign In

This is all factory equipment on a 21 year old inboard ski boat. Water penetrates. That's the bottom line. In coastal Texas, you don't really need to drain the block in the winter unless you get a really extended freeze, which rarely ever happens. For 21 years, the head has been full of untreated water, and it found it's way through the gasket. Such is the life of a marine engine operated in salt and brackish water. It is what it is. There's lots of rust all around, and at some point, I either get rid of the boat or replace the engine and trans entirely, maybe this time with a closed cooling system.

I'll let you guys know how it turns out. Heads are into the machine shop.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Apr 19, 2017, 8:19 AM

Post #12 of 13 (258 views)
Re: Mercruiser low compression Sign In

Those ski boats are built pretty well. I had a Correct Craft for 20 years and never had an engine problem but I ran exclusively fresh water.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Apr 19, 2017, 8:28 AM

Post #13 of 13 (256 views)
Re: Mercruiser low compression Sign In

Yes - 21 years without a disaster is still good. It may run and be fine with heads done up hoping rings were ruled out by compression test.


The whole help of draining the block on the sides is while it's freshly dumped assorted debris there you get that much out at least. If water jackets filled with muck of dirt or rust bits slowly cooling thru water jackets is compromised. IDK - from MA - mega hours of both day boating and yachting too much of a pest to pay to haul out if avoidable.


Said earlier it will take a literal coat hanger to clear out the sticky whatever when those side drain bolts are removed. I doubt it's perfect just helps perhaps a lot?
Other is check out now after knowing it's back in proper order with a steady temp gauge always when warmed up under any load (it should stay put) to just add closed cooling which is just a heat exchanger like a small scuba tank and radiator cap on it such that engine itself is with antifreeze not for the freeze protection but save the corrosion. Raw water still goes thru heat exchanger and out exhaust as much as before.


Those are not regularly taken off and water jackets of exhaust manifolds and the riser which also prevents raw water from coming backwards some should never be too hot to touch.


Your call on this for this or your next boat if you do that. Same old saying about boats definition "THEY ARE A HOLE IN THE WATER INTO WHICH YOU POUR MONEY" stands true!


This small block Chevy will never go out of date! Virtually all marine engine makers use this and the 454 as well as they are all iron and take a wild beating, long runs expected at full throttle against what would be full force brakes on, land you don't have or need for this.


Trivia: You can check but this should be the block that matches around 1969-1972 or so truck blocks - everything possible put on it to marine grade - gaskets or anything that would corrode much less likely.


Hope the heads alone fixed does the trick. You shouldn't have a problem with those redone selling those as good guess if engine has had it,


T







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