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Anyone ever had an experience with their vehicle like i had? unexplained acceleration when waiting

…in traffic? i was at a traffic light with my foot on the brake waiting for the green light, when suddenly the truck revved up and tried to lunge forward! i kept my foot mashed down on the brake and it seemed to calm down to normal. what the heck happened? i couldn’t figure out what on earth was going on!

anyone ever hear of such a crazy thing and what could have caused this? now i’m wondering when it might happen again! your thoughts? and no my foot didn’t switch from the brake to the accelerator peddle at the time.

the vehicle is a 98 tahoe.

The high rev’s might have been caused by a faulty Idle Air Control valve moving into the wrong position.

This happened to a co-worker as they were pulling into a gas station. The engine RPM’s rose so fast that they didn’t have time to apply the brakes and ended up slamming into two gas pumps.


is there a way of testing the part and do you know what the repair cost would be? tia

It’s not something that occurs often. I checked, and there are no complaints for this defect. As Tester said, maybe the IAC valve just wore out.

Try tapping on the IAC valve with the handle of a screwdriver while the engine is idling. If the idle speed changes replace the IAC valve.


As mentioned above, could easily be caused by a faulty IAC. Another possibility is that the ECM is having difficulty adjusting the fuel/air mixture. The result can be engine surging. This can be caused by faulty O2 sensors, MAF sensor, temperature sensors, or an improper adjustment of the idle speed screw. If someone has recently tried to correct an engine problem by adjusting the idle speed screw, the last one is the place to start.

Is or was the check engine light on?

Did anyone check for stored fault codes?

Did this happen only once?

My gf’s 1998 Honda Passport (which is gracefully falling to bits) will occasionally do a variation of this–suddenly, generally while idling in park, the RPMs will jump to about 2,000 and stay there for an indeterminate amount of time. It has been doing this for years and with over 250K on this vehicle and many other issues, it hasn’t been worth the time or effort to troubleshoot. The check engine light does not come on during these episodes, which happen about 1-2x every 3-6 months, usually in warm weather. She is used to this problem and it seems to mostly occur in park so it hasn’t tried to take off on its own. Other than that, it runs beautifully, especially considering the somewhat indifferent maintenance she’s given it over the years. I have been sitting in the passenger seat waiting for her to come back from an errand, with the A/C on, and have experienced this ghostly behavior a couple of times. Once, I shut off the engine and restarted it, and after a few seconds, the idle returned to normal.

So to sum up, I have experienced this with a vehicle, and I don’t think you had your foot on the gas when it occurred. I would also suspect the IAC, but you also may have some sort of intermittent vacuum leak that could be causing the problem. Some of the emission controls use engine vacuum to operate, and they only kick in at certain RPMs and operating conditions. So your engine’s computer decides that it needs to purge the charcoal canister or some other task and activates a solenoid that diverts engine vacuum “pressure” to accomplish this task. There’s a crumbling vacuum line with a crack in it at the other end of the solenoid valve, and when the solenoid valve opens, suddenly more air is being sucked into the engine, increasing the idle before the computer can compensate. Not saying this is necessarily your problem, but this is a possible scenario.

I had a '98 GMC Sierra that did this same thing. My wife first noticed it but it started happening to me as well. It was a bad IAC. It was easy to swap out and cost about $60.

Is there a belt on the a/c compressor ?
I’ve had customers in the past, when it was easier to do that, remove the a/c belt for whatever reason.
Whet the compressor was qued for ON ( such as in defrost mode ) the idle would kick up yet there was no compressor load so the result was high idle.