98 350 vortec reving itself

black98z71 · October 5, 2005

My '98 k1500 IS Reving up on its own and not reving down, thus making my truck hard to stop. When it acts up, the truck accelerates to 25 w/o even pressing the gas. I suspected TPS, and it did not have any play in it as some have suggested to diagnose a bad one. I am guessing a vacuum leak, but has anyone ever heard of this?

JimsZR2 · October 5, 2005

My '98 k1500 IS Reving up on its own and not reving down, thus making my truck hard to stop. When it acts up, the truck accelerates to 25 w/o even pressing the gas. I suspected TPS, and it did not have any play in it as some have suggested to diagnose a bad one. I am guessing a vacuum leak, but has anyone ever heard of this?

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Be sure to check the EGR valve too. Sometimes those get dirty and cause a high idle.

My 99 S-10 ZR2 does it occassionally and I brought it too the dealer once and they could not figure it out.

Larry · October 6, 2005

I would go after the Idle Air Control valve as it controls idle RPM’s. You very well could also have an intake manifold vacuum leak. Spray some carburetor cleaner around the intake during idle and look for any changes in the idle speed. Be careful not to burn the truck to the ground, as carb cleaner is very flammable. Typically a stuck EGR valve will cause a rough erratic idle not a high smooth idle.

My 1998 Z71 with a 5-speed manual always had an idle flare when depressing the clutch. Very annoying and much worse when the A/C was on. Unfortunately, the idle flare was programmed into the PCM calibrations to help reduce hydrocarbons during shifting. As much as we all hated to see the old 350’s go away, GM could not program enough scabs into the PCM to keep it emissions friendly.

JimsZR2 · October 6, 2005

I would go after the Idle Air Control valve as it controls idle RPM’s. You very well could also have an intake manifold vacuum leak. Spray some carburetor cleaner around the intake during idle and look for any changes in the idle speed. Be careful not to burn the truck to the ground, as carb cleaner is very flammable. Typically a stuck EGR valve will cause a rough erratic idle not a high smooth idle.

My 1998 Z71 with a 5-speed manual always had an idle flare when depressing the clutch. Very annoying and much worse when the A/C was on. Unfortunately, the idle flare was programmed into the PCM calibrations to help reduce hydrocarbons during shifting. As much as we all hated to see the old 350’s go away, GM could not program enough scabs into the PCM to keep it emissions friendly.

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Mine does that same thing! It's annoying!

Where is the Idle Air Control Valve located on a 99 4.3L W code engine?

Larry · October 6, 2005

On the throttle body towards the front……opposite side of the throttle cable. The weather pack connector has 3 wires going to it.

Jeremy WI · October 16, 2005

My '98 k1500 IS Reving up on its own and not reving down, thus making my truck hard to stop. When it acts up, the truck accelerates to 25 w/o even pressing the gas. I suspected TPS, and it did not have any play in it as some have suggested to diagnose a bad one. I am guessing a vacuum leak, but has anyone ever heard of this?

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I saw this once on a truck with a dirty MAF, guy overoiled his K&N. Once the MAF was clean, it quit revving itself