zero compression one cylinder 5.7 Black Scorpion

[nailem]

I just bought a boat knowing that one of the cylinders has zero compression. it is a 5.7 Black Scorpion with only 130 hours on it.
all other cylinders have 150-160psi
i pulled the rocker cover off and valves are moving up/down

tried to bump the motor to were i though both valves were closed blew in compressed air and could only hear a little blow by mostly into the intake.

took the rockers off both intake and exhaust and and blew in compressed air. i could easily pump it up to 40-50psi before it started to leak past my air gun. could not really hear air in intake or exhaust.

took compression test with rockers off and could get 60psi remember 0psi before.

so what do you think? bad/cracked valve seats? bent valves? or could one of the hydraulic lifters not be collapsing all the way? i guess this would not be because i still only had 60psi with both rockers off?????
thank you in advance

 

[Don S]

Re: zero compression one cylinder 5.7 Black Scorpion

Adjust the valve right per specs and see what the compression is.

 

[nailem]

Re: zero compression one cylinder 5.7 Black Scorpion

will do tomorrow Don but shouldnt i get more than 60psi with the rockers off?

 

[Don S]

Re: zero compression one cylinder 5.7 Black Scorpion

Probably not. If the valves don't open, how do you get air in to compress?

 

[Bt Doctur]

Re: zero compression one cylinder 5.7 Black Scorpion

no, because the intake valve is not opening

 

[nailem]

Re: zero compression one cylinder 5.7 Black Scorpion

got it. thank you. will do proper valve set-up tomorrow and post results.

 

[TilliamWe]

Re: zero compression one cylinder 5.7 Black Scorpion

Good luck. (My guess/bet is tulipped valve in that cylinder)

 

[nailem]

Re: zero compression one cylinder 5.7 Black Scorpion

i did valve adjustment as per manual and still have zero in #7. i did pour some marvel mystery oil in intake and it held for well over 30min.
i also noticed when i poured the oil in #2 i could see it travel over into the intake of #4 across the gasket. the compression on that side is all 150s but is that normal?

the boat was used mostly for wakesurfing. could this have caused the potential problem. also when i went to pick the boat up it had a lot of water in the gas. he said he started it and ran it in the driveway. it did start fine and idle out of the marina but once we gave it throttle it showed the sings of water.

 

[Bondo]

Re: zero compression one cylinder 5.7 Black Scorpion

it had a lot of water in the gas. he said he started it and ran it in the driveway. it did start fine and idle out of the marina but once we gave it throttle it showed the sings of water.

Ayuh,... Water in the gas causes a Lean condition...
My guess is, it detonated, 'n burnt the valves...

 

[Don S]

Re: zero compression one cylinder 5.7 Black Scorpion

If you do a leakdown test on that cylinder, you will be able to tell where the problem is. You don't need gauges, just 100 psi of air.
Get the cylinder up on TDC of the compression stroke, apply air to cylinder and listen where the air goes.
If loudest in exhaust, exhaust valve problem
If loudest in carb, intake valve problem
If loudest in valve cover, piston has a hole in it.
For 0 compression, you might as well pull the head, it's going to be real obvious what caused the problem. Things like blown head gaskets will usually hold 20 to 30 psi. on a compression test.

 

[nailem]

Re: zero compression one cylinder 5.7 Black Scorpion

i did introduce compressed air just using my tire filling hose and was able to get 30-40 psi easy before it started to leak around the end of my hose and the spark plug hole. but i did this with the rockers off. i will find TDC and do over. looks like the head will need to come off though either way. What all gaskets should i get ready for re-assembly?

 

[Don S]

Re: zero compression one cylinder 5.7 Black Scorpion

Pull the head and find your problem before you order any gaskets. You could end of needing to replace a piston if detonation was the cause. It may not cause your 0 compression, but it could have the edge burned off and the rings visible.

 

[nailem]

Re: zero compression one cylinder 5.7 Black Scorpion

ok im to the point were im not sure if i want to pull the heads my self or have it done. will not have any time until this weekend and than may be out of town next week. good thing there is still ice on the lake. where is a good place to find heads, valves, gaskets, ext. just so i can get an idea of cost im looking at. what would i be looking at for a cost to have the work done assuming it has lets say burnt valves.

 

[Bondo]

Re: zero compression one cylinder 5.7 Black Scorpion

where is a good place to find heads, valves, gaskets, ext. just so i can get an idea of cost im looking at. what would i be looking at for a cost to have the work done assuming it has lets say burnt valves.

Ayuh,.... Call, 'n ask your local Machine Shop, they'll supply most all of the motor parts....
For gaskets, either yer local Parts House, or Mercruiser, depending on which gaskets...