2 dead cylinders

[Ura-Ki]

I had an old 300 ford that would have 2 adjoining cylinders go dead when warmed up and running. Would start up perfectly, idle nice and smooth and seem perfectly normal. Once it got warmed up it would loose power and run really rough. Did a compression check and it showed normal, no oil leaks into the coolant, no coolent in the oil, and no vaccume leaks or any sign of cross over. Pulled the head and had it all checked out, no problems. Checked the cam with a dial indicator, was worn but within specs. No bent rocker arms or bent push rods, and the head gasket was fine.

Pulled the engine and tore it down, looking for any issues, and didnt see any. Took it to my machinest for a full clean and inspection. Turned out cylinders 3 and 4 were cracked and would only leak down when warmed up as they would swap hot gas and not get a complete air/fuel charge!
Tossed the whole thing in the scrap pile and swapped to a 400M series and was mucj happier, still wish i had that old Ford F-250 Ranger, was a super nice truck!

 

[Reacher120]

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.

If your "pulling wires" to locate dead cylinders, that could be electrical. I've had a brand new Dist. cap not make connections inside on a chevy V6, drove me nuts until I used a spark tester at the end of the plug wire. You can also have a broken connection at either end of your wire ,of course.
Just purchased a used Subaru; drove original owner nuts replacing parts. Would run fine for 15 minutes, then start running rough. Had to be towed home. Turned out it was a defective brand new coil purchased on Amazon. Put on a used PicnPull coil, driven 6 months now w no problems.
Cars are like human bodies, can have more than one thing wrong at the same time.
FWIW.
Good luck

 

[ditchtiger]

Rotated and changed wires, different caps and rotor, plugs, problem always stays at the same place.

 

[ORhutner]

This is like a great thriller mystery novel. Hope for your sake we get to hear the grand finale soon.

 

[DirectDrive]

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.

125 psi on all chambers, yet 5 and 6 are dead ?
Wut ?

 

[ditchtiger]

Took the head in. Only thing bad is number exhaust spring is broke.

 

[deadeye]

Saga Finally Ended! Mighty 6 Lives Again.