3.0 mercruiser low compression

[welovetoboat]

I have a 3.0 140 hp mercruiser with low compression. Check read 100/75/25/0 Mechanic said has a cracked head. Will cracked head cause this???? Still ran with this problem, just not very good. Sorry If this sounds like a stupid question. Just looking for answers. Marine Motors not my specialty. I'd appreciate any thoughts. Just trying to decide how much $$ to put in older boat.

 

[Don S]

Re: 3.0 mercruiser low compression

A cracked head will not cause that. Nor will a blown head gasket.
Could be bad valves. A leakdown test will tell you. Even the 100 reading is bare minium.
What year is it. My guess is you need a complete new engine.

 

[welovetoboat]

Re: 3.0 mercruiser low compression

It's a 1987 motor. Our mechanic told us that the head could solve the poblem.. I kind of doubted that myself. The next to the last time we took the boat out last year we had a blocked water pump and the motor got steaming hot. (We fixed that and took it out one last time and it ran badly.) I figured maybe this is when we cracked the head and blew the gasket. This is a guy that has been working on boats for 15+ years.. Go figure.

 

[Bondo]

Re: 3.0 mercruiser low compression

The next to the last time we took the boat out last year we had a blocked water pump and the motor got steaming hot.

Ayuh,............

I'd say that, That incident lead up to the Issue at hand,.......

Probably Cooked the Rings,+ Cracked the Head.......
I'd say you're a canidate for a Repower with either a New or Used motor,.....
Your's Might be rebuildable,....... I guess it depends on the Money..........
Might be time to Retire the Ship................

 

[welovetoboat]

Re: 3.0 mercruiser low compression

Update: Mechanic says everything inside looks good. That cracked head/blown head gasket and a couple of bad cylinders. Found head new cylinders etc. Bought it off ebay. Mechanic said.... head looks good... old gaskets line up and are the same.... problem.... said the head looked somewhat smaller than the other one?? Supposedly this head came off a 3.0 140 hp mercruiser just like mine. Mine was 1987 new one 1990. Before I bought the new head on ebay called 2nd mechanic we use and he thought it should fit. Ordered new gaskets. Going to try it out..... Any thoughts from anyone??

 

[cheburashka]

Re: 3.0 mercruiser low compression

Update: Mechanic says everything inside looks good. That cracked head/blown head gasket and a couple of bad cylinders. Found head new cylinders etc. Bought it off ebay. Mechanic said.... head looks good... old gaskets line up and are the same.... problem.... said the head looked somewhat smaller than the other one?? Supposedly this head came off a 3.0 140 hp mercruiser just like mine. Mine was 1987 new one 1990. Before I bought the new head on ebay called 2nd mechanic we use and he thought it should fit. Ordered new gaskets. Going to try it out..... Any thoughts from anyone??

You've got to be careful with Ebay stuff. You may have bought a head that was listed incorrectly. One guy last week had a head that he claimed would work on either a 140 or a 120, but he wouldn't show pictures of the ports. I'm hoping you didn't buy from him, since he either doesn't know the difference, or does know the difference and isn't letting on. When you say the head is smaller, do you mean the ports for the intake and exhaust, or the valves? Valve size is the best way to figure out whether it's an appropriate head. It will still run with the 120 head, but if you paid a premium for the 140, that's what you should have received.

I'm still kind of chuckling over the line "everything inside looks good. Cracked head, blown head gasket and a couple of bad cylinders." I hope my engine never looks that good.

When you say "new cylinders" do you mean you bought oversized pistons and you're having the cylinders bored out?

 

[welovetoboat]

Re: 3.0 mercruiser low compression

Mechanic did not say what was smaller just that it looked smaller than the one on the motor???? I am kind of a dummy when it comes to this. Sorry. Only paid $150 for it. Not a huge amount of $$. I meant to say came with valves not cylinders. So, Let's say the person who sold it to me "got it wrong" and it is the 2.5 head and not the 3.0. Will it still run with this head?? And if so what are the consequences?? Less Power?? Or worse?? My objective in this whole situation is to get on the water. Never missed a boating season. Can't stand to be on land all season. We are planning on purchasing new boat next year tired of working on this one. Any ideas?

 

[cheburashka]

Re: 3.0 mercruiser low compression

It should run fine with that head. So you're saying the valves were bad and the cylinders were fine? That's a different story. If it was just the valves then you might be in OK shape. Have your mechanic test the manifold and riser for leaks. The overheating you experienced is one possible cause for the valves going bad and the head cracking, but you still might have other problems that can be checked out while everything is apart. Once he reassembles it, he should be able to perform a leakdown test on the manifold, but right now he can at least fill it with water and see if there's any leakage into the intake and exhaust passages.

 

[welovetoboat]

Re: 3.0 mercruiser low compression

Thanks for the info and sharing your knowledge. He is going to put it on this weekend. Wish us luck. Keep you informed.

 

[welovetoboat]

Re: 3.0 mercruiser low compression

Good News..... The head works Mechanic said we have great compression in 4 starts great runs smooth. What a relief. Although, we were sent a smaller head than what we bought. Still working that one out with the seller...
Thanks for all your help. This is a really great place to come for answers. I have learned alot from you all.