1991 mercruiser 260hp blown head gasket?

[ghettoyacht]

Two part question here.

I have a 1991 mercruiser 350ci 260hp engine in my 23' rinker cuddy cabin.

#1


Over the last couple years I have noticed a miss at almost all rpm ranges. In the past two years i have replaced: plugs, wires, cap, rotor. Also recently I took the carb apart,(after I replaced the fuel filter/water separator. I found alot of black grainy stuff in the bowls so I cleaned this and the jets out meticulously. This made it run a little better but still the miss.
When first trying to start the engine after a week or more downtime, the engine would stop turning over at what seemed like the same place every revolution getting easier to get by this spot every time it went around. When it would start it seemed to me like it took a few seconds to "clear out" kind of like a 2 cycle.

So I decided to do a compression test. I pulled all the plugs, noticing 5 and 7 looked like they had a little more carbon build up than the rest. I pulled the coil wire out and set the throttle wide open. I put the compression gauge in # 7 first. Turning the engine over I noticed water coming out of 5. I disconnected the hose from 7 and there was water dripping from the hose.
I checked the oil next thinking it may be milky. It was not. It looked fairly clean and midway between add and full. I continued with the compression test. 1-235, 3-230, 5-225, 7-225, 2-235, 4-240, 6-240, 8-230. This made me decide not to pull the engine but to just pull the heads. Does anyone see another step to take before pulling the heads?

#2
Can I do anything with the heads/ cam/intake while i'm in here to make some extra torque? What gives the 350 magnum the extra hp?

Any input will be appreciated. Phil

 

[Howard Sterndrive]

Re: 1991 mercruiser 260hp blown head gasket?

1-235, 3-230, 5-225, 7-225, 2-235, 4-240, 6-240, 8-230.

if that's in PSI, that can't be right.... an engine with compression that high would struggle to run on pump gas... you're in diesel territory

Buy a new compression tester. the water must've damaged it

 

[Don S]

Re: 1991 mercruiser 260hp blown head gasket?

DO NOT pull the heads until you know what the problem is.
Yes you need a compression tester, but assuming that it's at least consistant and just showing 75 to 100 psi high, the numbers would be good.
Block off the cooling hoses going to the exhaust manifolds & risers, drain the cooling system and pressure check the engines cooling system to see if it leaks into the cylinder.

 

[ghettoyacht]

Re: 1991 mercruiser 260hp blown head gasket?

if that's in PSI, that can't be right.... an engine with compression that high would struggle to run on pump gas... you're in diesel territory

Buy a new compression tester. the water must've damaged it

i thought that seemed high, some of my seadoos 2 cycle engines run 155 psi. i just realized i've never comp tested a 4-stroke engine before. (except the v twin in the exmark)

 

[6meter]

Re: 1991 mercruiser 260hp blown head gasket?

Maybe those numbers are metric? The good thing is that they are consistant. Normal sbc numbers would be around 120-140. Do Dons recomended procedure.

 

[Aloysius]

Re: 1991 mercruiser 260hp blown head gasket?

Water doesn't necessarily leak INTO a cylinder..if the head gasket is leaking between a cylinder ands a water passage, the 2000+ psi pressure in the cylinder leaks INTO the water jacket when it's running.

 

[ghettoyacht]

Re: 1991 mercruiser 260hp blown head gasket?

Water doesn't necessarily leak INTO a cylinder..if the head gasket is leaking between a cylinder ands a water passage, the 2000+ psi pressure in the cylinder leaks INTO the water jacket when it's running.

doesn't that usually cause overheating also? only 1 time has this even tried to run hot since i've owned it. a couple years ago when i noticed the temp heading towards 200 i got off plane and it cooled off. the rest of the trip back to the trailer i ran on plane until it went up to 180 then backed off until it cooled back down. at no time did the temp go over 200 and it ran fine during this and after the repair. it's been more recently (this season) that the missing started. the fix for the overheating was to seal the gasket for the impeller housing in the outdrive.

 

[Don S]

Re: 1991 mercruiser 260hp blown head gasket?

doesn't that usually cause overheating also?

That would be a problem you would see on an engine with a closed cooling system, the pressure would blow the antifreeze out of the heat exchanger. But, with a raw water cooled engine, it wouldn't be noticed unless it was a sever leaking head gasket, but that would show up in the compression numbers.

 

[ghettoyacht]

Re: 1991 mercruiser 260hp blown head gasket?

i just got around to replacing to gauge on my compression tester with a 10 dollar item from lowes. its one of the compressor type gauges that maxes out at 160. the engine is pegging the gauge. should i go on with the cooling system pressure test or worry more about my high compression?

 

[Alpheus]

Re: 1991 mercruiser 260hp blown head gasket?

No you should find out who built that engine.

Those numbers are way to high.

Did some idiot put a hot rod car engine in that boat???

 

[ghettoyacht]

Re: 1991 mercruiser 260hp blown head gasket?

i dont think so. it ain't screaming fast or have that lopey idle. i'm looking for another comp tester to back this up. also i have ran 87 octane for 3 years and no detonation.

 

[ghettoyacht]

Re: 1991 mercruiser 260hp blown head gasket?

just for reference i checked my suburban for compression with the same gauge. its a 6.0 with 150,000 miles. it checks out at 160 psi.

 

[Alpheus]

Re: 1991 mercruiser 260hp blown head gasket?

Get yourself a new gauge...

 

[ghettoyacht]

Re: 1991 mercruiser 260hp blown head gasket?

Get yourself a new gauge...

again?

 

[6meter]

Re: 1991 mercruiser 260hp blown head gasket?

Simple fact is stock sbc motors just won't create that much compression. Something's wrong. Your tester, your testing procedures, or somebody has seriously fooled with that motor. A stock starter would have trouble turning it over.

 

[wire2]

Re: 1991 mercruiser 260hp blown head gasket?

Maybe those numbers are metric? ....

No, metric PSI equivalent is kiloPascals (kPa),
and 150 PSI converts to 1034 kPa.

 

[ghettoyacht]

Re: 1991 mercruiser 260hp blown head gasket?

So I checked a few cylinders when the engine was warmed up. Now the psi is around 140. Still seems high, but I'm not too worried. I also changed the plugs again. must have had a bad plug cause it is running great again- no miss. You know if that was a pwc, the first thing i would have done is change the plugs.

it still doesn't explain the water in 5 and 7 but at least i think bad head gasket is ruled out. Probabably this winter i will pull the exhaust risers and manifolds and check them out. Thanks for all the input on this subject.