M50 auto tranny: scary surges

[Bruce Kennett]

all:

1992 525iT, auto tranny, M50 motor, 159k miles.

twice now i've been sitting at a light, in "Drive" with my foot on the brake, and the car has suddenly surged up to around 3-4000 revs.

last week, the first time it happened, it was VERY scary: a mother with two little kids and a stroller was crossing the street right in front of me. i managed to push down hard on the brake pedal and prevented the car from lunging ahead. the surge went back down and never repeated itself.

today it happened a second time. as i was heading out of a parking lot, i got to the edge of the street to make a right-hand turn, and put my foot on the brake to come to a stop. waited for 2 or 3 cars to pass. then the car suddenly surged as if i had hit the accelerator. this time i put it in neutral and motor speed got up closer to the red line, then subsided. (in hindsight i suppose i should have turned off the ignition instead but it was faster to get it into neutral. and it didn't hit the red line anyway.)

any guesses as to what it might be? car has a brand new fuel filter. in fact, in between the times of these two incidents, i put in a brand new fuel tank and fuel lines. it feels much more like some electronic prob with speed governance than it does fuel flow in from the tank.

bruce

[BillionPa]

well, its kind of hard to go that high without air into the engine, so some sort of massive intermittent vacuum leak or the ICV would be the only 2 things that would cause that.

[SnakeyesTx]

Initially I was going to say the throttle position sensor, but since air was mentioned, I'd check that, and the Mass air flow meter. I've had vehicles where the TPS controlled idle and shift timing, and MAF's probably work in conjunction with that kind of info too, so its worth a look.

[Bruce Kennett]

turned out to be a bad idle valve. so with luck this should fix it.