What happened?

catherinetDecember 29, 2011

This is the first time this has ever happened and it was a little scary. I've heard about cars taking off on their own and running over people, so I want to catch a problem before anything bad happens.

We live out in the country, no garage. It hasn't been that cold. I have a 2001 Honda Odyssey van. I got in it the other day, after not driving it for maybe 3-4 days. I started it up and it raced horribly. I think I even felt a tiny attempt to lunge, but it was in Park. I didn't even think to check out how fast the rpms were going, and just shut it off.

It hasn't happened again, although I've only driven the car twice since.

I don't even know if the gas pedal was stuck. All I knew is I had to turn the car off fast.

Do you think I should be concerned about this? It only happened that once and it was in Park.

Thanks for your input.

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Check to see if you a "drive by wire" model or one with an odinary throttle cable hooked up direct to the accelerator pedal. There is no physical connection between the gas pedal and the throttle plate in the "drive by wire" models; The main throttle plate is controlled by an electric actuator.

If you have a drive-by-wire model, yes, you do have a concern. See the dealer immediately.

If you have the old fashioned type directly connected to the accelerator pedal, ordianry mechanical sticking may have been the problem.

There is another source of high enigne speed and that is the idle air control speed (IACS). The idle speed on most late model cars is controlled by the engine computer. There are various schemes used to decide how to set this device, but it can be in the wide open position if the contoller 'forgets' where it was when shut down. Also, it is opened some for a cold start. If this device gets sticky or one direction fails, all kinds of mischief can occur.

I had a GM car from the 1980s and this is the way they set the IACS. If you installed a new IACS, the mechanic would install the unit with the pintle partly open. This could result in an idle speed that was too fast or too slow. After the coolant passed a given temperature and when the car was traveling over 45 mph, the engine controller would pulse the unit 75 times toward the closed poaition and then back out 35 pulses. The assumption was that the 75 pulses was more than enough to insure the unit was seated closed and that is where my touble lay. I had unit with a plastic screw thread that had worn down. It was slipping and not driving the pintle home, but wouldd back it out most of the way. The idle speed was out of control. Under normal circunstances, the next time the engine was put at idle, its speed would be checked and a small correction made to trim the speed. But with a stripped thread on the positioner screw, the algorithm did not have a chance. Small adjustments are made many times during engine operation to maintain idle speed under varying loads.

In my 1998 Jeep, the idle air flow is boosted when the A/C compressor is actuated. Idle air flow is also boosted when the shift lever is moved from park or neutral to a gear (My vehicle has an automatic transmission).

There are a variety of schemes employed by different manufacturers. When the battery is disconnected, this may trigger a new cycle of auto-setting of the IACS.

A wide open IACS can raise the idle speed to the neighborhood of 2000 to 2500 rpm.

    Bookmark   December 29, 2011 at 11:40PM
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Thanks jemdandy.

    Bookmark   December 30, 2011 at 7:35PM
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