1978 GMC C3500 Engine misses at operating temperature

NEVADAOUTLAW

MEMBER
1978 GMC C3500
V8
2WD
AUTOMATIC
68,000 MILES

I bought 1978 RV on a GMC 3500 chassis. It starts good and runs great all the way up to speed, UNTIL it reaches operating temperature. Then it has a huge miss. The more gas the worse it gets. If you bury the 4 bbl. It would die if it wern't moving. It is a 454 cid, with a large quadrajet. I have changed all of the following. Fuel pump, fuel filter, air filter, dist. Cap and rotor, coil, the module in the dist, spark plugs. Watching the choke, it seems to be working fine. It looks like there is an EGR valve on the intake. If it is bad, could it cause this problem? I'M not up to speed on smog equipment. Suggestions needed. Thanks Edward

4 Replies

2CARPRO JACK

MECHANIC
11,533 POSTS

How are the plug wires? The egr shouldnt do that, but you can pull the acuum line off it to eliminate it as a possibility. When hot does it idle ok? Or only miss when on the as?

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Monday, August 11th, 2008 AT 2:19 PM

MERLIN2021

MECHANIC
17,250 POSTS

Yes a bad EGR can cause problems, but check on the back side of the crb for the large vacuum port, pull the hose off and clean it out, use a piece of wire coathanger with it running and jam it in the port a few times, see if the suction improves. If it's spark related with engine running, use a pair of insulated pliers and pull the plug wires off one ata time, when you get to the misfire the idle won't change. Put the wires back on as you go, disconnect only one at a time. Any backfires going on? On a 78' you can pull off the vacuum hose to the EGR and test drive it.