1984 GMC C2500 1984 GMC High Sierra 2500 454ci HEI ignitio

KEVIN_TX21

MEMBER
1984 GMC C2500
V8
2WD
AUTOMATIC
100,000 MILES

I am a little stumped here. I was driving to work one morning at about 60mph and the truck just dies. I pulled a couple plug wires and turned it over. I had what I thought was a decent enough spark and fuel shooting into the carburator while I was cranking. Still nothing; no backfire. Nothing. A month prior I was having a slight misfire issue and I installed new plugs, wires, rotor, cap, coil, and ignition module and the misfire issue had been resolved. Now all of a sudden this happens. I pulled it back to my barn and poured some fuel in the carburator and turned it over a few more times. Still nothing. The fuel seemed to be spurting up out of the carb while I was performing this test. Double checked spark. Still good. I have not performed a compression check yet, just because I thought even if I had 1, 2, 3 or even 4 cylinders bad, I should still get some type of fire. No oil leaks or water in oil etc. Good voltage at distributor, pick-up coil about 850 ohms. All I can think is that maybe my spark isn't strong enough (seems like at least one cylinder would try to light off though). Or I somehow managed to jump timing like 180 degrees? I am little lost, any idea where I should start. I still have my old ignition coil, I could slap that one back in and try it?

Monday, August 30th, 2010 AT 8:36 PM

1 Reply

CARADIODOC

MECHANIC
34,330 POSTS

Hi kevin_tx21. Welcome to the forum. The spitting fuel out of the carb is a big clue to a jumped timing chain. I'd check that first. It will cause low compression readings on all cylinders. You can also try running a timing light to see if it is severely retarded. It's unusual for the engine to suddenly stop running due to the timing chain. Usually it will lose power and be hard, (not impossible) to start. Another quick check is to put the vibration damper timing mark at TDC and look at the rotor to see if it is pointing to cylinder number 1 or 6.

It is somewhat common for the wires to break off the pickup coil and cause intermittent no-start or stalling and cutting out when the plate turns for the vacuum advance, but since you already measured it and since you have spark, we can rule him out.

You can also turn the vibration damper back and forth by hand to see how far you have to go before the rotor starts to turn. Anything over a few degrees indicates a stretched chain, but besides that, given the relatively low miles on the engine, I suspect the timing chain and gears are original. The original cam gear likely has nylon teeth on it that have worn or broken off causing the chain to jump even though it might not be loose. You might find evidence of broken teeth if you drain the oil and shine a light in the drain hole or use a coat hanger to poke around in there.

Caradiodoc

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Tuesday, August 31st, 2010 AT 2:13 AM