1990 Chev 3500 350 tbi idle/off idle missing

[log_cabin_sam]

Hey, got an issue with my truck: It started idling way high not that long ago, and lately it is not liking more than half throttle without missing, backfiring, etc. Also, just tonight, I stopped at the store for five minutes, and came out, and it didnt' want to start, it was hard cranking, and kept catching at tdc (it seemed) on and off. Lightly backfired a couple times too. It did start though after a few tries. But that is the first time it hasn't started right up for me.

Background: Has been flashing an egr fault off and on for a year or so, although it was not on in these cases. Not long ago, I put new plugs, and plug wires on it. I think that's it.

It wasn't doing this before the plugs and wires, although none of this started right afterwards either, so maybe unrelated. I read about egr issues, and it seems to be acting opposite of a stuck egr valve......any ideas?

[CrazyHorst]

The high idle, and then failure to idle is classically a vacuum leak.

The TBI-to-intake gasket is one place to start as are any vacuum lines which lead away from the TBI. Also check the larger diameter hose which goes to the brake booster.

There also may be a vacuum line which goes to the auto transmission.

Napa sells silicone vacuum lines which will outlast the truck and are flexible enough to compensate for different port sizes.

[log_cabin_sam]

Update: I had a message that wondered on the status. Well, my high idle seems to be vacuum related, as suggested. I played with the vacuum lines, and it got better for awhile. But came back, although not as bad. I think the solution will be to replace all of them, again, as suggested. The other half of my problem, was a bad/old/ruined cap and rotor unit. omg, it was thrashed inside. The contactor in the center was burned and worn down, and the arm was half burnt off too. the cap contactors were worn burned, and corroded badly. Needless to say, it starts great again, after replacing those units. Haven't been driving it much lately due to gas prices, so not in a rush to do the vacuum lines.

[log_cabin_sam]

I did eventually find out about the high idle problem. It was my coolant temp sensor. I ended up replacing it, and the truck then started and idled perfectly normal. So simple. Live and learn.