Unintended Acceleration

Any ideas on this one??? We would be driving the van and it would just start accelerating up to at least 50-60 mph w/o putting your foot on the gas. Cruise control is off...when you are stopped, you've got to push the brake pedal pretty hard to keep it from moving...Finally, when you put it in park, the engine is just racing @ very high rpm's. After turning the car off and restarting it would be fine ..it did it about a year ago, 7-8 times and then CarMax replaced the alternator and reflashed the PCM...it was ok for the last year. Last week, it did it again. We just had the intake manifolds replaced this week...they told me that maybe that was related to the engine surging...does this sound plausible?

I'm tempted to get out my camcorder and tape it the next time it does it...then I can leave it in park until the motor explodes/dies and then take it back to Carmax and see if they believe me then...

Our 2001 venture now has 65k miles...in 1.5 years since we bought it from CarMax we have had the following done:

alternator replaced
pcm reflashed
two windows motors replaced
flickering lights --- the tsb ( alt. screw) did not fix our problem.
wiper motor replaced
wheel cylinders leaked brake fluid and ruined rear pads
upper/lower intake manifold replaced

Total repairs - ~$2000 and counting in 18 months.

My other car is a 99 Grand Prix GTP with 136k miles - supercharged, 29mpg, and has cost a total of $450 in repairs in 6 years....

my other van (was a voyager) did something similar. mine was a faulty throttle positioning sensor (TPS).

I am not sure if the venture has the same set-up or not. Hopefully one of the experts here can shed more light on it

I'd suspect the TPS.

I am still trying to figure out why Carmax thought the alternator would have anything to do with it.

I can see why someone has suggested the TPS as I think their thinking is that the PCM thinks the throttle is open more than it is but I don't understand how it could accelerate the vehicle significantly since the throttle plate itself would need to be open to allow enough air into the system for that to happen.

For some reason I tend to believe that the throttle is open when this occurs. Perhaps it is sticking, or the cable is sticking or the Crusie itself is engaging (even though it is off).

It's simple to eliminate the cruise control from the picture, just disconnect the cable. I don't think that is the problem though.

The accelerator pedal and/or cable also could be sticking, but I don't think that's it either. You said that when you shut off the engine it returned to normal. A sticky pedal/cable would rev again when restarted.

What you say is true about the TPS, unless the throttle plate doesn't seal properly or air has another method to get around the plate. You should be able to take of the air filter cover/hose and look in there. Is there a lot of buildup? If you open the throttle by hand and release, does it snap back quickly and smothly, or does it catch?

Thanks for the responses everyone - i'm going to check the throttle and see if I notice anything out of the ordinary...

It's simple to eliminate the cruise control from the picture, just disconnect the cable. I don't think that is the problem though.

The accelerator pedal and/or cable also could be sticking, but I don't think that's it either. You said that when you shut off the engine it returned to normal. A sticky pedal/cable would rev again when restarted.

What you say is true about the TPS, unless the throttle plate doesn't seal properly or air has another method to get around the plate. You should be able to take of the air filter cover/hose and look in there. Is there a lot of buildup? If you open the throttle by hand and release, does it snap back quickly and smothly, or does it catch?

Actually if I can add one more requirement:

Not only is there a route for additional air when this phenominum occurs but the PCM "knows" about the air becasue it is adding fuel as well. This would mean that the air meter or TPS or both need to be in the equation too.

I'm having the SAME issue in this thread? Is it the TPS?...and is that something I would want to take somewhere to have repaired or is this something that is simple to replace?

My mechanic actually looked at it this morning and told me that he saw the identical issue 4 months ago in a Venture...in that instance, there were no codes set for the TPS or anything else but after it was replaced, the customer hasn't had an issue since...Our van also had no codes set - but I told him to go ahead and replace it and we'll see what happens...I'll post again in a month or so with the results.