87 pcm no comp on #3

[11.50fox]

I was running my boat last summer and ran it hot{270} I know wow that's hot.took it to the lake to kinda test it out last week, and I have a small miss under a load. also it was hard to start.i changed the plugs and the miss is gone
I have done a comp. check and number 3 has 9.5bls, all others have 109-126lbs. took off the heads expecting to find a blown head gasket{on the exhaust side} or a burnt valve I found neither of the 2. I have them at machine shop now. the "head" guy said the look good no sticking valves no bent or burnt valves. there checking to see if they are warped. if not there going to do a valve job on it, just cause its off the motor.... if it comes back not warped what could it be?..oh yeah while the heads were off i got the water out of them{from when i pulled the heads off} then sprayed with pm blaster to ensure no rust. the walls of the cyc. felt good no groves and the piston looked good no holes or chips. the pb blaster is still sitting in the cyc. today so it cant be a stuck ring or stuck piston.... what else could it be?

[AirTool]

I'm going to complement you on the most descriptive subject line ever. Maybe you are more technical and AC, AR, and OCD than me.

Before I read your post....I thought...this guy spun a cam lobe down flat.

After the compression check, I would have pulled the valve covers and watched the rockers before any other work.

Did you check that?

[11.50fox]

it runs perfect when you get it to start in the yard and under a load. so I wouldn't think it to be a cam lobe worn flat. other than it is hard to start it runs perfect. after you get it to crank you can switch it off and then crank it {1 min after }with a 1/2 turn of the motor it fires up.

[kend]

It can run pretty good on 7 cylinders, I'm with Air on checking the rockers. Did you turn that cylinder all the way down where you could make sure there were no cracks; they can look really small when the motor is cool. Otherwise, I guess you put it back together and cross your fingers

[11.50fox]

when I took off the head #3 was all the way down at the bottom of the stroke. I did notice that it loked like it could have had a blown head gasket. but it was on the exhaust side and looked to be from the number3 cyc to the very back cyc.{lol don't know the number}
but if it were a blow head gasket between both cyc. the both would have read low on comp. right?

[Nautiquehunter]

When my 89 351 was new it had a problem with a sticking valve. Sometimes I would loose a cyl it ended up being a burred or distorted valve guide.

[DanielC]

It is too late now, but after finding low compression, you should have done a leakdown test.

[11.50fox]

the head guy at the my machine shop checked the #3 valve and all. no sticking valve or anything. he said that it sounded like a stuck valve. but nothing was stuck.

[AirTool]

You need to check the cam lobes next. Anything else is a waste of time.

In 85, I worked with a guy that drove an 80 or 81 z28. It was an automatic and would still scratch second gear. But the idle had a very slight imperfection only a trained ear could hear...mainly because it was faint and perfect. One of the cam lobes was as round as the bearing.

[11.50fox]

checking cam lobes today. a cam lobe wore off wouldn't make it low comp. would it? asking cause I don't know....

[AirTool]

checking cam lobes today. a cam lobe wore off wouldn't make it low comp. would it? asking cause I don't know....

If a cam lobe wears down, the valve doesn't open.

There are various scenarios depending on if it is intake or exhaust and how much your rings leak... but basically your cylinder is dead.

Edit: What air/fuel mix is it going to compressed if the intake valve didn't open?

[11.50fox]

gotcha.... going to check today

[TRBenj]

Lots of tail chasing being encouraged in this thread... lost compression in a cylinder so inspect for cam wear- what? Thats nonsensical.

DanC was right- should have done a leakdown test before taking the head off- that would have told you WHERE the compression was being lost (intake valve, exh valve, rings, etc). If the heads check out, then cross your fingers and hope for the best... hopefully you just smoked the gasket and didnt crack the block.

An overheat that bad may have done other damage- no water showing up in the oil? Exhaust manifolds seem particularly susceptible to cracking after an overheat that bad.

[Zach@n3]

Go get a machinists straight edge and check the block deck for flatness while you have it apart for good measure.

[Westlotorn]

I ran my 78 Ski Nautique with a bad head for a full summer, barefooting and hard skiing it pulled strong as long as my plug did not foul out. I had to clean the plug every weekend but it ran very strong with a bad valve.
Compression that low tells you either the valves are open when they should not be or closed on the intake when it should be open.

A flat cam lobe would not be from an overheat so it is either fine or was bad before your overheat. Your heads are off so you can inspect the cam in the block before making your next decision.

Even if you had a cracked head you would have more than 9 lbs compression. When you pulled the heads were any head bolts loose?
Loose head bolts after over heat show the head gasket was crushed when the over heat occurred. ( Very Common ) You can measure your gasket for crush. Usually you find this near the exhaust valve on the fire ring of the dead cylinder. If you find areas .005 smaller than other areas on the fire rings you found your leak.
The fire ring of a head gasket is the Steel ring surrounding each cylinder. Most stock gaskets will measure around .039 thick.
On a boat with fresh water cooling and a blown, crushed, fire ring the compression can blow into the water jackets and go un noticed. In a closed cooling engine this same issue would pressurize the radiator and cause fast boil over. It would over pressurize the system.