1998 Chevy C1500 350 TBI runs rough strangely

[Bob Kontak]

Rough strangely because stone cold fires and idles fine. Get it on the road and drive for two minutes and it then it stumbles and does so for five minutes and is fine the rest of the drive. So it's consistent, actually.

It starts stumbling in the middle of cruising as if a switch was flipped.

The EGR valve I'm thinking will open about the time it starts running rough but not sure what could be wrong given how the problem corrects itself.

May be an intake leak but I can spray brake cleaner all over the manifold (when cold) and the fuel trims don't move and no change in RPM. Intake leak would be present at initial start up too or is it so rich then it does not matter in operation?

MAF sensor numbers look ok and are linear upon acceleration and I cleaned some crud off the idle air control valve.

Suggestions? I'm thinking replace the manifold gaskets and clean the EGR system.

[Scott Douglas]

I think I'd be looking at something connected to the temperature of the coolant more than the EGR system, Bob. But I'm no mechanic and didn't sleep in a Holiday Inn last night either.

[908/930]

Any chance it is O2 sensor problem, I changed the sensors on my 2011 2500hd to Bosch and I could not drive it, could barely move it. Put the old ones back in it ran ok, ordered a different brand NTK and ran great

Edit, Also check the fuses for the o2 sensor heaters

[LWJ]

How does the TBI lean out as it warms up? Seems like there is a hiccup in the warmup process?

[RANDY P]

I can tell you for a fact if they replaced the EGR with a piece of garbage from the local FLAPS it's likely the wrong one and WILL cause that exact problem. Routine horrible idle, stalling, etc.

Exactly what my TBI truck did until I swapped out for a Delphi branded unit. The trucks EGR runs in reverse of the TBI cars.

rjp