87 pcm no comp on #3

[Westlotorn]

Thinking about your issue some more, in your overheat if you were running hard the cylinder may have been exposed to detonation and fractured the top compression ring. Look closely at your spark plug for signs of detonation or the outer edges of the piston for signs of detonation. Even with broken rings you should get 50 lbs or more in your dead cylinder so this may not be your issue.

[core-rider]

11.50fox
I'm having a hard time following some of the info here, and excuse me for potentially making you sound clueless about engines, but I assume that of all those I don't know until I learn otherwise.

You keep referring to #3 cylinder has low compression, but then reference the cylinder beside it but didn't know what number it is. You also mentioned removing both cylinder heads... This has me worried that either the machine shop is checking the wrong cylinder head or wrong cylinder on each head. Did they check ALL the valves on both heads or just the valves on "cylinder #3"?

I agree a leak down test should have been done before removing the heads, but too late for that now.

I don't understand how wiping a camlobe could cause loss of compression. If neither valves opens (because of a wiped camlobe) compression cannot be lost; unless it is lost by other means like blown HG, stuck open valve, hole in piston, etc.

What engine is this exactly? You say 1987 PCM... Chevrolet... Ford... small or big block... I need a few more details. Have you tried rotating the assembly by hand to see if all pistons move? I know 270°F is hot, but this should be all cast iron parts. If the heads/block were aluminum I could possibly see a warped head or block, but not cast iron. Did you inspect the sparkplugs closely on each cylinder for signs of detonation?

I just can't understand a cylinder with virtually no compression and there not being obvious signs of blown HG, stuck valves, detonation damage, etc. if this engine ran fine the season before. Something is missing and I hope more info will bring it out from hiding.

[AirTool]

There can be many causes to the ops problems... but I still think he should check that intake lobe. I fail to understand what could be compressed if the valve never opened.

TDC
Stroke 1 - Valve didn't open
BDC - pulled a vacuum,...maybe some ring leakage in
Stroke 2 - compression
TDC -nothing happened,...nothing to compress. Same volume/mass as start.
Stroke 3 - power stroke
BDC - vacuum again....or some ring leakage
Stroke 4 - Exhaust stroke....exhaust valve opens (so what)
TDC - start again with an empty cylinder.

[TRBenj]

There can be many causes to the ops problems... but I still think he should check that intake lobe. I fail to understand what could be compressed if the valve never opened.

TDC
Stroke 1 - Valve didn't open
BDC - pulled a vacuum,...maybe some ring leakage in
Stroke 2 - compression
TDC -nothing happened,...nothing to compress. Same volume/mass as start.
Stroke 3 - power stroke
BDC - vacuum again....or some ring leakage
Stroke 4 - Exhaust stroke....exhaust valve opens (so what)
TDC - start again with an empty cylinder.

There is nothing wrong with inspecting anything and everything while the motor is torn down, but that said, I'll reiterate:

A wiped cam cannot cause the low compression and has nothing to do with the issue at hand. You might as well encourage him to double check his safety lanyard and make sure his transmission isn't low on fluid, so long as we're sending him on a wild goose chase.

[core-rider]

There is nothing wrong with inspecting anything and everything while the motor is torn down, but that said, I'll reiterate:

A wiped cam cannot cause the low compression and has nothing to do with the issue at hand. You might as well encourage him to double check his safety lanyard and make sure his transmission isn't low on fluid, so long as we're sending him on a wild goose chase.

While I'm not certain this is a camlobe issue... The more I've thought about it the more I can see a worn camlobe causing a low compression issue. I would still like more details on the engine in question. 11.50 fox, I hope you get this figured out and back on the water without too much cost involved.

[Westlotorn]

When camshafts lose a lobe it generally rounds off the lobe reducing the amount of lift. I have never seen one reduce till you had zero lift but I have seen hundreds of these that had one or more lobes rounded off reducing the amount of lift. Worst case the valve would still open 25% of the designed lift. This would still give you partial compression.
The common issue with these flat cams is Valve Train Noise. They get torn down for repair because of lifter noise. You can try and adjust the lifter as many times as you wish, it will stay quiet only till it wears enough off to make noise again. Usually less than 10 minutes of run time.
Unless you had very noisy lifter issues for a long period a flat cam did not cause your loss of compression.
If I remember correctly the OP reported 9 lbs compression. Maybe he better check to make sure this connecting rod is not broken if the valves and piston are still looking good.

[AirTool]

The cam lobe I witnessed wiped was as round as the bearing portion.

It's not a "wild goose chase" at this point he already has the bleeping head off.

It may not be the problem....but its not hard to check.

good grief

[core-rider]

Maybe he better check to make sure this connecting rod is not broken if the valves and piston are still looking good.

The cam lobe I witnessed wiped was as round as the bearing portion.

It's not a "wild goose chase" at this point he already has the bleeping head off.

It may not be the problem....but its not hard to check.

good grief

Both of these are reasons why I asked if he has tried to turn the engine over by hand. If both heads are off the he can see the lifters and see if they move or not while spinning over the engine as well as if the pistons move.

While on the subject... It could be a stuck lifter causing the valve to not close. This would act just like a stuck valve, but would go un-noticed to someone looking for stuck valves because that isn't the problem. All of these issues can be checked by simply rotating the engine over by hand.

[Westlotorn]

Agreed