Intermittent miss

[hatchetjack]

Have a '76 GMC 350, 252k+ miles on it. It will start fine and run sometimes up to 25 miles or so, then begins to miss randomly. Sometimes you can shut it off and it will run perfectly again for awhile. I've replaced the module, coil, plugs and wires, cap. Also the pcv valve and air filter unit. Throttle position doesn't seem to make any difference, nor does ambient temperature far as i know. Any thoughts on this one? Thanks

 

[personaltrainer]

wish I could help ya.


I guess all I can do is bump your post..
I'd like to know ideas for this as well.

 

[ryoken]

i know it seems ignition related like a module but could it be a fuel issue? what carb? might have some crud in the bowl..

 

[hatchetjack]

Its the Q-jet, not the original carb, but a rebuilt one from Autozone a few years back.
I'm at a real loss on this one. I've been doing mechanics for a long time, and never run into a miss like this. It acts just like the module, but its been replaced with one made by Wells (same manufacturer as the one that was in it, and had lasted for several years).
I guess its possible that there's some crud somewhere in the fuel system that just takes a long time to show, maybe the next thing to do is take the line loose at the fuel pump and blow some air back into the tank.
I was hoping that someone here had run into the same sort of thing, i really don't want to just keep throwing money at it. When i find what it is, tho, i'll post here.

 

[Thunder]

Could be the pick up coil in the dist. If you have a ohm meeter test between the green and white(yellow?) wires that come off it. Resistance should be 500-1500 ohms. replace if out of specs. Pick up coils will act like a bad module when bad.
run fine when cold and miss after warmed up and resistance builds.

 

[dyeager535]

Sounds like your symptoms are different, but here's my story.

Q-jet, rebuilt, everything good as can be inside and out.

Driving along the freeway at 60MPH, try to accelerate up a hill, vehicle would start to sputter and miss. This was pretty much totally random as to how often it occurred, but always seemed to occur when I needed more gas.

Last time it happened, was climbing a very steep hill. Started sputtering, let off gas, which usually "fixed" the problem. Not this time. Ended up in low with the 465, trying to climb this two lane road up a pass, and it finally stalled.

Got it backed off the road, popped the air cleaner, hit the throttle, and no pump shot from the accelerator pump. Carb was empty. Got it pointed downhill, coasted about 2 miles in gear, when it finally started.

I *suspect* that the float was sticking for some reason, (maybe rubbing on a distorted float bowl wall, or "loose" float tolerances) although on disassembly, there was nothing visibly wrong.

So fuel problems CAN manifest themselves oddly, but problems like mine are definitely not the norm.

Definitely I'd lean more towards the ignition system, new doesn't=good anymore.

 

[hatchetjack]

Replaced the pickup coil and that took care of it, thanks much, Thunder!!

 

[diesel4me]

The big pink wire...

One truck I had did run similar to what you described--I put in a new pick up coil,it ran better for a few weeks--then,just when I thought I had it licked,it started missing intermittently again!

An older GM mechanic told me to run a new 10ga wire to the HEI,he said the big pink hot wire to the HEI would sometimes get corroded or have high resistance somewhere along its long run between the distrubutor and the ignition switch(he also said the ignition switch might do it too)--after I put in a new wire,I never felt it skip again the whole time I owned it--guess that wire was crappy somewhere in the harness or something...

 

[hatchetjack]

Thanks!! I'll certainly keep that in mind if it starts again. May just be a safe thing to do anyway, given the age and miles on it.