2 dead cylinders

[ditchtiger]

How can a cam with one worn out lobe cause 2 cylinders to not work? 1977 straight 6 Chevy 250. #5 exhaust lobe out. 125psi on all Chambers, to 150 on second stroke. 5 and 6 are dead.

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[v0lcom13sn0w]

more than one mechanical problem is what i would suspect

 

[v0lcom13sn0w]

being this is a straight 6 and you have consecutive cylinders dead you could have a bad headgasket, cracked cylinder wall between the two is probably the most probable cause. not to say that is what is going on but its more common than a broken connecting rod

what i would do is take 5 and 6 spark plug out, fill 5 with water and spray air into 6 look for bubbles in 5.

 

[ditchtiger]

more than one mechanical problem is what i would suspect

You would have thought. Been a mechanic for 25 years. Engine shop and rebuilding for 7 years. Went down the list. Plugs, wires, cap, rotor replaced then a new distributor. Cross checked and rechecked. Came down to pulling the motor to get to the cam.

 

[ditchtiger]

being this is a straight 6 and you have consecutive cylinders dead you have:

a bad headgasket, cracked cylinder wall between the two is probably the most probable cause. not to say that is what is going on but its more common than a broken connecting rod

what i would do is take 5 and 6 spark plug out, fill 5 with water and spray air into 6 look for bubbles in 5.

Did all that. 5 and 6 are not consecutive, 3 is between them. But have a common intake runner. And no it was not a intake leak.

 

[ditchtiger]

Was not using coolant. No bubbles, had the head off and replaced gaskets.

 

[v0lcom13sn0w]

Did all that. 5 and 6 are not consecutive, 3 is between them. But have a common intake runner. And no it was not a intake leak.

in the cylinder arrangement 5 and 6 are consecutive. in the firing order they arent.

 

[Kruel J]

@v0lcom13sn0w knows his stuff.

 

[ditchtiger]

in the cylinder arrangement 5 and 6 are consecutive. in the firing order they arent.

Ya, what I said.

 

[ditchtiger]

@v0lcom13sn0w knows his stuff.

Mechanic?

 

[Kruel J]

Mechanic?

Master Tech.

 

[Kruel J]

Mechanic?

And my personal friend. Listen to him or don't. But @v0lcom13sn0w is the best tech on this site IMHO.

 

[ditchtiger]

Hoping for a connection tomorrow. This is the strangest issue I have come up against.

 

[Kruel J]

Hoping for a connection tomorrow. This is the strangest issue I have come up against.

@v0lcom13sn0w is the man for the job.

 

[orygun]

I would suspect a blown head gasket, too. If not all plugs are removed during a compression test, especially the ones in the "dead" cylinder, a false reading may occur. If compression is good, it could be a vacuum leak at the shared intake port.
Kinda weird. A flat lobe on the exhaust will still usually show compression in that cylinder and when running, will pop thru the carb under acceleration.

 

[Unka-Boo]

You said you had the head off....was it machined? And if so, how much?

 

[orygun]

Love these guessing games when my hands aren't on it.
Broken valve spring/s?

 

[Stomper]

Love these guessing games when my hands aren't on it.
Broken valve spring/s?

It's squirrels.

 

[ditchtiger]

head gasket was good. all the plugs were out for tests. 125psi for all one on stroke. 150psi after 2 strokes. Intake leak looked for early on in this struggle. Spark plugs were only 3 weeks old, replaced the 2 just to cover my self. wires were good. Put on a new set as I was running out of ideas. cap and rotor replaced as they were 15 years old. Then a new distributer. The I pulled the head to look around. No washed out spot or other signs of blown gasket. The engine was not using coolant, no white smoke, not over heating. no 7-up bubbles in the radiator when running. assembled and set valves with engine not running. then reset 5 and 6 with the engine running. No, I and 2 others did not notice 5 exhaust. Its has only lost maybe a 1/4 of its lift. Earlier when pulling wires to locate dead cylinders, 5 was firing a little. 6 was completely dead. Also rotated crank forward and back to check cam gear wear, a little but not bad.
That's when I had enough and pulled it out.

 

[Hop Sing]

This problem sure is a conundrum. Just to be clear, when you say 5 and 6 do you mean physically or firing order?