Mercruiser 5.7 350 Where to Start??

[LiquidC]

I know I'm probably looking at a rebuild / new block ... but wanted to get some opinions first.

I've got a 1994-ish Mercruiser 5.7L V8 (350) and the compression is good in 6 of the cylinders (130psi - 140psi), but the 2 cylinders on the coupler end (I forget which cyl# they are) are about 60psi

So I have the motor out and mounted on a stand, what are some simple things that would cause these two cylinders to have low compression?? Bad gasket? Stuck valves?

Thanks for the help!

 

[fishinpa]

Re: Mercruiser 5.7 350 Where to Start??

Are you talking about 2 cylinders - next to each other or 2 cyl on opposite sides of the engine ?


fish

 

[LiquidC]

Re: Mercruiser 5.7 350 Where to Start??

Fish - The 2 Cyl on opposite sides of the engine.

 

[fishinpa]

Re: Mercruiser 5.7 350 Where to Start??

the thing now is engine out and on stand makes it somewhat harder to determine your actual problem area.

you coulfd have done a comp check ,followed by a comp check with a little oil added to the low cylinders (looking for possible ring or cylinder wear ) in the low cylinders .after and depending on the results of that you cld perform a leakdown test to further narrow down the low comp on the 2 cyl.

you need to narrow down your problem before tearing down so you know just what you are getting into !


fish

 

[littlebookworm]

Re: Mercruiser 5.7 350 Where to Start??

As Fish said, it's a little hard to diagnose without tearing things apart now that you have the engine out. I can't see any way around it - you're going to have to pull things apart. First pull both valve covers and remove all the spark plugs. Check your rocker arms and push rods. If you can, turn the crankshaft a few times and see how the valves work, especially the ones in the cylinders with low compression. If they seem to be working fine, you're going to have to go into the engine. You'll need a good manual - preferably the Mercruiser one. If you do not have the mechanical background, you're going to have to bring the engine to an engine shop. If you have the background and the tools, tear into it. Remove your distributor, intake manifold, push rods (keep them in order so you can return them properly), and cylinder heads. Then check everything. Find the problem before you put it back together. As you take it apart, check intake and cylinder head gaskets from breaks. Check heads and block for flatness. Good luck. Hy

 

[mciaio]

Re: Mercruiser 5.7 350 Where to Start??

Since it's now out of the boat I would do this;

Take the heads off and check the cylinder walls for scores. Since it is cylinders 7 and 8, they are the farthest ones from the cold water. Scores would indicate overheating. If that is the case, I would strip it down, send out the heads to check for cracks. Send out the block and have it punched .030 over. Buy some pistons, a nice agressive cam and rebuild it.

When taking the heads off, look for indications in the gaskets of tears, leaks, burn through, etc, etc. That can be the cause too. I think scored walls are going to be what you find. Having 2 heads crack or 2 gaskets fail at the same time is highly unlikely.

I'll tell you what, I'll do your engine and you replace the wet/rotted wood in my transom? Sound like a deal?

 

[TilliamWe]

Re: Mercruiser 5.7 350 Where to Start??

... 2 cylinders are about 60psi

So I have the motor out and mounted on a stand, what are some simple things that would cause these two cylinders to have low compression?? Bad gasket? Stuck valves?

Thanks for the help!

Tulipped valves is the most likely culprit.

 

[mciaio]

Re: Mercruiser 5.7 350 Where to Start??

Tulipped valves is the most likely culprit.

I beg the question, why on 2 cylinders on oposite sides on the engine at the same time?

 

[LiquidC]

Re: Mercruiser 5.7 350 Where to Start??

Interesting offer mciaio. How about we swap motors and I replace your rotted floor?

It does seem weird or maybe a concidence that the 2 cylinders on the coupler end are reading the same pressure. I'll do the compression check again tomorrow to make sure I'm not a complete idiot.

Any recommendations on where to send my heads & block to have them done? How much would punching out the cylinders typically cost?

And btw ... I needed to take the motor out of the boat because the oil pan was leaking.

 

[TilliamWe]

Re: Mercruiser 5.7 350 Where to Start??

I beg the question, why on 2 cylinders on oposite sides on the engine at the same time?

SBCs in boat are very suseptible (is that spelled right?) to tulipped valves, especially if the engine is lugged for any length of time. While he has very low compression on only two cylinders, I'll bet he has valve issues on others too. Where the cylinders are at makes no difference. For example, my low cylinders were #2 at 0psi, and #5 at about 90psi. When the heads were removed, I had deformed valves all over the place.

 

[zbnutcase]

Re: Mercruiser 5.7 350 Where to Start??

Usually in the rear cylinders its from being hydro-locked due to bad shutters in the exhaust-especially in larger boats where the riser is dangerously close to the waterline and boatbuilder too cheap to install riser blocks or boat so ****-poorly designed (read Bayliner) there isn't room for them. 'nutcase

 

[RicardoMarine]

Re: Mercruiser 5.7 350 Where to Start??

You have it out, you may as well go completely though it. Not sure which size hull this is, but now is your prime oportunity to build one using a Q/E piston/head combination and move away from the full dished piston! JMO.

 

[BRG25]

Re: Mercruiser 5.7 350 Where to Start??

How are you going to do a compression test with the engine on a stand? Can you hook a battery up and crank it while on the stand?

 

[fishinpa]

Re: Mercruiser 5.7 350 Where to Start??

How are you going to do a compression test with the engine on a stand? Can you hook a battery up and crank it while on the stand?

Easy enuff put the flywheel ,rear cover and starter back on sit it on the floor , a skid or what have ya .
hook up to a good battery and spin it over ! lol


fish

 

[TilliamWe]

Re: Mercruiser 5.7 350 Where to Start??

Usually in the rear cylinders its from being hydro-locked due to bad shutters in...

I would agree with you on this, but he did do a compression test already, (prior to removing the engine), so it can't be hydrolocked.

 

[LiquidC]

Re: Mercruiser 5.7 350 Where to Start??

Hey guys, I haven't been able to do the 2nd compression test yet. My back is all messed up 'cause a guy in a Mercedes slammed into me yesterday coming home from work.

It's worth mentioning why I did the compression test in the first place. The engine was really tough to start when it was cold and sitting for awhile. It took me about a half hour to get it running, but when it did start it "sounded" great. I hope I didn't do damage in the process of getting it running.

If it was hydro-locked, then a tear-down is in my future. I do have a Bayliner, and the water line is close to the height of the risers. I've had that thought in my mind ... riser blocks may be a wise choice, thanks nutcase.

BRG25 - I don't like to run my engine while it's on a stand, I'd be afraid that the vibration would loosen the stand bolts and the motor would go flying. I lower it onto wooden blocks, put the flywheel back on, hook up a battery, connect a hand fuel pump/hose, and hookup a starter switch ... you can get a starter switch from most auto stores.

I should be able to do another compression test this weekend, and this time I'll write down the cylinder numbers.

 

[bonzoscott]

Re: Mercruiser 5.7 350 Where to Start??

You don't have to start it for a compression test, just crank it. Take all plugs out.

 

[TilliamWe]

Re: Mercruiser 5.7 350 Where to Start??

If it was hydro-locked, then a tear-down is in my future...

If it was hydrolocked, it wouldn't have turned over.