Motor problems, dead cylinder

[GsxrMike]

Ok guys, I'll try to list as much info as I can. Chevy 350, #8 cylinder is dead. It has spark, it has air & fuel, but doesnt seem to fire at all. That tube on my header never seems to get very hot. I can run my motor for a few minutes and keep my hand on that tube of the header the whole time. Spark plug looks sooty (installed new and rechecked after 5 minutes of run time). I did a compression test last night and it came back 160. I did a poor mans leak down test and pressurized the cylinder with 90 psi with rocker arms removed. The only place its hissing is on the idle air port on the throttle body. Intake valve spring seems fine, not broken, looks just as tall as the others. When I look into the combustion chamber with a flash light it looks very sooty as well. Where should I go from here? I was leaning toward a head gasket issue but I can't make sense out if the results I have. The motor sat for a while so a rusty intake valve and/or seat is the other thing that I was thinking but I'm unsure about that since I have good compression. Header tube isn't clogged because it blows out just fine when I push valve down during leak down test. Seems like it should still fire but isn't. Thanks in advance!

 

[diesel4me]

Fuel injected?...if it has an injector for each cylinder and one isn't delivering fuel for whatever reason,it can make that cylinder seem "dead"..

A bad cam lobe will allow enough air in to show good compression,but maybe not enough fuel / air mixture is reaching the cylinder if an intake valve does not open up far enough--a rocker arm stud pulling out of the head can have the same effect (on older engines)..--if an exhaust valve loses its lift,the burnt gases will back up in the cylinder and tend to escape thru the intake valve and intake next time the intake valve opens...usually you'll get a backfire thru the intake if thats the case,under acceleration..

If it has HEI ,its possible the pick up coil or reluctor may be damaged on the "tooth" that picks up the signal to fire the #8 cylinder..improbable,but not impossible..also its easy to mix up the plug wires,make sure they didn't get swapped between #6 and #8...

 

[GsxrMike]

Throttle body injection, valves are both moving as they should. I haven't measured the lift but they seem fine via visual inspection. Ignition is not my strong point but it does have spark and it's wired correctly (plug wires). I'll pull the cap today and check everything out. The distributor is the same one I was running carbed, and it's not hooked up to the computer. Not really sure how that happened because someone else made my harness.

 

[diesel4me]

If your running Bosch Platinum plugs,toss them away...seen many of them brand new out of the box,refuse to fire in some cylinders on GM V8's,cant explain why they act weird like that,but I've seen it happen many times..just another thought..checking the cap & rotor is a good idea too,the cap especially could be detouring #8 spark to ground via a carbon track or track,and its not always visible to the naked eye,trying another one is about the only way to test for that..

 

[454Sub]

I have cracked the porcelain on a plug and couldnt figure it out till I fired it up in the dark. Same problem you describe. Maybe try a compression check, could be a stuck ring or hung up valve.

 

[rampage]

Have you tried swapping the plug wire of the dead cylinder with one that's working? Even though you are seeing spark it could be weak.

 

[GsxrMike]

Well, the sob runs great now. Pretty sure the intake valve got some rust on it or something when it was sitting. Must have clear it out messing with it. I'm just going to play with the timing a bit and then go play in the snow.

 

[magik235]

Excellent. Have fun in the snow.