84 260 Zero compression cyl #6

[TribalFlounder]

Hey all, long time reader first time poster. Here's the numbers:
1984 SeaRay SRV 245 Sundancer
1984 Mercruiser 260 Engine SN: 6728341 - 637 Hours original
Outdrive SN: 6700080

Symptoms: Misfire on number 6, after running for several hours will start to do a hard misfire while underway, creating the throttle blip style miss. Decreasing throttle will cause blip to slow down, increasing will extremely increase it. Misfire is extremely bad when engine is cold, harder to recognize when fast idle - 1200-1500 RPM.

Pulled compression on #6 and it is reading 0. Gauge is good, not too sure about accuracy tho. Finger over hole does not produce any pressure while cranking, and nothing is coming out of the exhaust port for cyl 6. Pulled leaky valve cover off and see that push rods are functioning normally for #6.

Top of head is relatively clean, oil is clean, not burning any oil. Using a boroscope to look at the exhaust valve I can see that it is closed, but looks overheated.

Question is what could be causing this and what are the approximate probabilities? My theories are:

Stuck intake valve
Destroyed valve seats
Warped head
Piston rings
Lower connecting rod separated and piston lodged.

Has not overheated in my ownership, and has not done this the whole time (one year) I have owned the boat. Did not ever hear any loud clanks or bangs that would indicate that the piston had separated from the crank.

Any suggestions? A boat this size limits my capabilities of pulling the engine, as no engine picker I know of will reach that high.

Thanks all, Jeremy

 

[Bt Doctur]

Re: 84 260 Zero compression cyl #6

being a center rise V8 motor I suspect some water got into that cylinder and "puckered" a valve. Injecting air will tell if intake, exaust, or piston/ring issue.

 

[TribalFlounder]

Re: 84 260 Zero compression cyl #6

So inject air and if it comes out the exhaust = exhaust valve, carburetor = intake valve, or no pressure buildup/ air out of dipstick = blown rings?

 

[Bt Doctur]

Re: 84 260 Zero compression cyl #6

yep, thats the plan

 

[TribalFlounder]

Re: 84 260 Zero compression cyl #6

Thank you very much, I'll try it tomorrow and report back.

 

[Walt T]

Re: 84 260 Zero compression cyl #6

If you had a borescope you should be able to see what it was. No matter what you're gonna pull the head so why bother with pressurizing it?

 

[JustJason]

Re: 84 260 Zero compression cyl #6

If you had a borescope you should be able to see what it was.

Borescopes don't do uturns when you run them down a plug hole. And you can not see valves with them.

 

[TribalFlounder]

Re: 84 260 Zero compression cyl #6

I was able to see the exhaust valve from the exhaust port with the boroscope but it was too big to go into the combustion chamber through the spark plug.

 

[TribalFlounder]

Re: 84 260 Zero compression cyl #6

Put air to it while also cranking the engine and through out a full rotation I had air coming out of the exhaust consistently. Looks like I am either in for a valve job at a machine shop or am buying a new set of heads depending on what the local (Fort Hood, Texas) machine shops will charge. Thank you BT Doctur for enabling me to pinpoint it!

Jeremy

 

[alldodge]

Re: 84 260 Zero compression cyl #6

Borescopes don't do uturns when you run them down a plug hole. And you can not see valves with them.

Pretty sure of your self are you. Maybe you meant it takes a little more doing to get the camera to make a U-turn. Or maybe you can see the valves without doing a U-turn.

 

[Bt Doctur]

Re: 84 260 Zero compression cyl #6

most likely puckerd it from a bit of water inteusion.

 

[TribalFlounder]

Re: 84 260 Zero compression cyl #6

Update for any future forum searchers. Pulled the heads and the exhaust valve on #6 had the seat completely dislodge itself from the head. It then became stuck around the valve stem keeping the valve from closing. Damage was isolated at the head.

 

[Walt T]

Re: 84 260 Zero compression cyl #6

Cool! I assume you'll have the head completely done? Why not both? Check your exhaust manifolds and risers while you're at it.

Oh and my borescope can see valves easy and its a very small tube with a light and camera that allows me to turn and twist and see all manner of stuff in there. I used it on diesels mostly until I got out of the truck business.

 

[TribalFlounder]

Re: 84 260 Zero compression cyl #6

Taking both heads to a machine shop to get a quote, if too high I'll go with replacement heads. Manifolds and risers were just pulled out of the boxes last week. The oldies had hit the end of their life.