Mercruiser 5.0 low compression #1 cyl

S

shorn

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1994 Mercruiser 5.0 runs fine at speed but idles a little rough & sometimes dies. Hard to restart when warm; must open throtle fully. #1 cyl only 30 psi and leakage probably past intake valve. Other cyls 135-150 psi. Will a valve adjustment help the issue or must the heads be pulled? How long a job is it to pull the heads? Will continuing to run the boat (carefully) cause further damage?
 
Don S

Don S

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Re: Mercruiser 5.0 low compression #1 cyl

If it used to run ok, then went bad, no a valve adjustment won't help.

Will continuing to run the boat (carefully) cause further damage?
Click to expand...

What ever caused that cylinder to go bad is probably working on the others. Expect it to get worse.
 
grendelsniper

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Re: Mercruiser 5.0 low compression #1 cyl

if there was a way you can pressurize the one cylinder with exhaust and intake valve closed and see if you get any air throught the carb or exhaust will tell you may have a burnt valve, then all you need to get would be a valve job that would be the best way, or just pull the heads and have them pressure tested if no visible damage is noticed. also could be a head gasket
 
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shorn

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Re: Mercruiser 5.0 low compression #1 cyl

How long will it take and how difficult is it to pull the heads? I assume you do both heads even though the other cyl compression are ok?
 
grendelsniper

grendelsniper

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Re: Mercruiser 5.0 low compression #1 cyl

It is a in depth ordeal the whole top of the motor comes off all exhaust intake distrubutor make sure you mark when you pull the distributor or put it at #1 cylinder it could take someone with mechanical skills probably half a day to have them off, when installing you need to adjust the valves and set timing, there are torq specs and sequence on bolting down the heads. hope this helps and not scares you from doing it. but it can be done.
 
Bondo

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Re: Mercruiser 5.0 low compression #1 cyl

shorn said:
How long will it take and how difficult is it to pull the heads? I assume you do both heads even though the other cyl compression are ok?
Click to expand...

Ayuh,... Welcome Aboard,....

Ya, the entire top of the motor is removed, 'n the heads set to a Machine Shop for a rebuild....
On a scale of 1 to 10,.... it's about a 14....

Hopefully, it's Only the heads...
 
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shorn

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Re: Mercruiser 5.0 low compression #1 cyl

Is there a real advantage to just replacing the engine? Obviously, I don't know if there are other problems that might be known once I pull the heads (piston, rings, etc.). Other cylinder compressions: 2 135, 3 125, 4 135, 5 145, 6 150, 7 145, 8 145. I can get a remanufactured marine 305 engine for $1500. Reconditioning the heads would be around $600. I imagine the labor for changing the engine and pulling and replacing the heads to be similar. Engine has about 500 hours on it. Thoughts please.
 
FreeBeeTony

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Re: Mercruiser 5.0 low compression #1 cyl

Bond-o said:
Ayuh,... Welcome Aboard,....

Ya, the entire top of the motor is removed, 'n the heads set to a Machine Shop for a rebuild....
On a scale of 1 to 10,.... it's about a 14....

Hopefully, it's Only the heads...
Click to expand...

Really think it's a 14??
 
grendelsniper

grendelsniper

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Re: Mercruiser 5.0 low compression #1 cyl

if you decide to just replace it, than I would go with a 350 more power and torq, best bang for the buck
 
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