Irregular stumble or miss at idle

This is more of an annoyance than a threat to driveability (so far) but it has me and the mechanics I deal with stumped. 94 GMC 350 with about 175 thousand miles. Prior to my emissions test I cleaned and gapped the conventional plugs, inspected the cap and rotor and gently cleaned off a small amount of accumulated ‘snow’ on the contact points and put a couple of cans of Seafoam in the tank. I passed the test without issue but for some unknown reason the truck has developed an irregular stumble at idle especially when hot. I replaced the plugs and the stumble became less noticeable but still there. Mechanic has verified no problem with cap and rotor, ignition wires, coil, PCV valve, coolant temp sensor but did discover a very small leak in the intake manifold gasket which has been remedied but with no effect on the stumble. I had the truck out for a two hour highway drive today at sustained speed of 70mph and it ran beautifully but soon as I came back into town and hit the first stop light I noticed the stumble plus the tail pipe is too sooty after that much hwy driving. There are no stored codes and the SEL has never come on. Thoughts please?

My wag you need to do a better cleaning of the fuel injectors than sea fome. I would try this.

Problems with idle while hot are often due to lean conditions due to undiagnosed vacuum leaks, but since your mechanic has already checked the entire vacuum system for that presumably … hmmm … Since replacing the spark plugs helped, I’m thinking you still have some ignition electrical gremlins. Your shop must have no access to a sophisticated engine ignition diagnosis instrument, so suggest to replace cap/rotor/condenser/points/plug-wires and see if that helps. Another thing to try is look for signs of carbon tracking on the underside of the cap, and look at the engine running when the truck is in a very dark environment. Do you see any visible sparks buzzing around in the engine compartment?

There’s a number of things that could cause a problem like this. My preference is to always run a compression test to make sure the engine is mechanically sound. The spark plugs are out so kill that bird right then and there.

If even one cylinder is dropping in relation to the others an erratic idle or stumble may occur.
In most cases with something like this it may smooth out once the RPMs rise.

The next step, or even prior one before the comp. test, would be a vacuum test with a vacuum gauge.

“put a couple of cans of Seafoam in the tank”

Was that the recommended amount for your tank?
Maybe a high concentration of Seafoam is causing it.
Run the tank down to 1/4 or so and refill. See if the stumble goes away.
Otherwise, +1 for a compression test.

Just wanted to let circuitsmith know I think he is on to something. After re-filling the tank and immediately emptying it in an extended highway run the stumble has disappeared and the truck is operating beautifully. One can was probably all I needed.