98 4.3l high idle

Hi, I have a 98 c1500 4.3l 5speed manual trans. 117,000 mi. that is getting a high idle. It doesn't do it when sitting still, but while driving around town the idle goes up to about 1200-1500 RPM. Then when I come to a stop after a few seconds it will drop back down to about 750 RPM. I know there is a TSB for this exact same problem. The ECM has already been reflashed per TSB. I bought the truck used about three years ago with 100,000 mi. The guy I bought it from bought it new and had it serviced at the dealer and gave me a printout of all repairs made. Flashing the ECM was one of the first things done. It just started doing this about 6 months ago. I 've changed the IAC solenoid, dist. cap, rotor, plug wires. Plugs are Delco platinums replaced at 100,000 mi. and look fine. Anyone have any ideas?

I also have this same problem with on 96 C1500 4.3L w/MT. I bought this truck new and its done it since day one, although its usually not very noticible unless running the AC or hauling a load. I had taken it in to the dealer twice early on, apparently before the TSB was issued and finally gave up on it until now. I'm going to look into the TSB, because I know that mine has never been flashed.

Mine just says 98, so I punched in 96, and it's not listed. Seems strange though. I'm thinking of going to a carb. and manifold, since there is no emmissions testing in my state. Also will be easy to convert to E85 if that gets going. GM and Edelbrock both make carb. manifolds for the 4.3l Vortech. I was already to do the injector upgrade till I heard that the newer style require a higher fuel pressure. Hopefully we'll get some feedback from the group. I'll let you know if I come up with something.

I have a 95 that I had this problem with. Turns out, it was a vaccuum leak. You can purchase a can of Chem Tool and spray around your intake, and TBI, or injector housing. If your engine bogs down, you have a vacuum leak. Mine leaked at the left rear of the intake, and later my TBI gasket leaked. Leak was very punishing to torque.

I have a 95 that I had this problem with. Turns out, it was a vaccuum leak. You can purchase a can of Chem Tool and spray around your intake, and TBI, or injector housing. If your engine bogs down, you have a vacuum leak. Mine leaked at the left rear of the intake, and later my TBI gasket leaked. Leak was very punishing to torque.

Thanks for the info. I checked the hoses they all seemed tight. Diddn't try the spray and diddn't check around intake. It doesn't seem to do it till it gets good and warmed up.

I would check the gasket on the intake and TBI. Mine also idled up the warmer it got.

I would check the gasket on the intake and TBI. Mine also idled up the warmer it got.

I'll try tightening them up. I visually tried to look and tried the carb. cleaner. Mine definately gets worse the warmer it gets. Thanks.

If it gets worse the warmer it gets, then it could be the temp sensor reporting a colder temp than the engine is running at. The colder the engine, the more fuel it needs.

You can try this, unplug the engine coolant temp sensor (I think it is in the intake manifold) and jumper the harness with a wire. This will tell the PCM that the engine is very warm. If the idle is normal, the temp sensor may be bad.

//2000CAYukon

If it gets worse the warmer it gets, then it could be the temp sensor reporting a colder temp than the engine is running at. The colder the engine, the more fuel it needs.

You can try this, unplug the engine coolant temp sensor (I think it is in the intake manifold) and jumper the harness with a wire. This will tell the PCM that the engine is very warm. If the idle is normal, the temp sensor may be bad.

//2000CAYukon

Thanks! I'll give that a try. I tightened my TB and upper intake bolts. It seemed a "little" better but was still doing it.

I think I may have got this licked. I changed the coolant temp sensor (about half covered in sludge) and drove it around and it diddn't do it once. I also cleaned the MAF with brake clean, and I'm going to check the "lower" intake bolts also. Thanks!

Well I thought I had this licked. It seems every time I try something it works for about a day until the computer relearns. It does seem to be a little better, it was getting pretty bad. I hope it's not time to start throwing money at it. I could take it to the dealer but they want $90 right off the bat. I have OBDII web browser for my laptop. And it's not poping any codes.

If you tightened up the intake/TBi bolts and it seemed to help then there is you problem. You can get those bolts as tight as you would like but without a properly functioning gasket you will still have vacuum leaks. My TBI gasket actually blew out on me twice before I did my rebuild.

I have a 95 that I had this problem with. Turns out, it was a vaccuum leak. You can purchase a can of Chem Tool and spray around your intake, and TBI, or injector housing. If your engine bogs down, you have a vacuum leak. Mine leaked at the left rear of the intake, and later my TBI gasket leaked. Leak was very punishing to torque.

OK, I'll try that. Your intake leak was that just the upper, or was it the lower?

OK, I tore into the the throttle body and upper intake. No gasket leaks. It looked like the fuel press. regulator was leaking. Wet sludge buildup in that area only. It seems to run a little better. But still doing it. It idles fine when not moving, or befor it warms up when driving. The minute it goes in to closed loop when driving it starts idling high until I push in the clutch and let it idle for about 10-15 seconds, then drops back down.