Idle Air Controll Solonoid/Valve hanging open, but I think you would have hanging idle issues as well if that were the case.
When an auto car is engauged into D, the computer sends a signal to the IAC to increase throttle for the intended load, as well as it does this when you kick the A/C on to keep RPM sound.
Here is a wild guess, but have you checked the charging and battery system because a bad diode in an alternator or a internally shorted battery can cause the computer to go nuts. When I do a battery or alternator at work, esp with new cars, sometimes I have to clear codes because the faulty alternator/battery caused the sensors to read poorly and the computer raised the amber flag on the dash.
When an auto car is engauged into D, the computer sends a signal to the IAC to increase throttle for the intended load, as well as it does this when you kick the A/C on to keep RPM sound.
Here is a wild guess, but have you checked the charging and battery system because a bad diode in an alternator or a internally shorted battery can cause the computer to go nuts. When I do a battery or alternator at work, esp with new cars, sometimes I have to clear codes because the faulty alternator/battery caused the sensors to read poorly and the computer raised the amber flag on the dash.
The little screw setting the throttle blade angle will make it do that if it's opening the blade to far. Ask me how I know...
Back it all the way off the plate that is connected to the throttle blade. Put a .010 feeler guage between the two and tighten until you feel it pulling the feeler guage a little. Then remove the feeler guage and turn the screw 1.5 times and leave it alone after that.
or
Throttle position senor voltage is above 1.1 with the engine running. That has happened to me too. I set the TPS at .98 volts. To do this put a straight pin through the green wire and another through the black wire coming from the TPS. Then measure the voltage across the two with a volt meter. The voltage will be higher with the car running. It's not supposed to be more than 1.05volts running. Most are right at the limit running. I have also set the TPS with the engine running to 1.00 because .98 with just the ignition on wasn't working. It turned out that I had to really crank the TPS around to get the 1.00voltage with the engine running. I could set .98 with just the ignition on......the TPS was replaced and all was good.
Idle Air Control valve has caused the same thing for me. I just cleaned it out with brake cleaner. One of my cars was sucking oil and the oil was eventually getting into the IAC and the Throttle body causing that problem.
I recently read that a fuel injector wire can cause it, although I have never had that happen to me that I know of.
Clean the mass air probe with electrical cleaner. I had trash on that once that caused the engine to hold RPM.
Oh yeah, I had a BIG K&N filter attached straight to the Mass Air Unit. I don't know if you have done this though. That caused all kinds of strange RPM problems. I had to use a smaller one to get it running right. Ahh....one more thing that actually worked. Panty hose over the Mass Air then put the filter on top of the panty hose. I saw that trick at the drag strip. Use strong panty hose...
Hope one of these will help.
Jim
Back it all the way off the plate that is connected to the throttle blade. Put a .010 feeler guage between the two and tighten until you feel it pulling the feeler guage a little. Then remove the feeler guage and turn the screw 1.5 times and leave it alone after that.
or
Throttle position senor voltage is above 1.1 with the engine running. That has happened to me too. I set the TPS at .98 volts. To do this put a straight pin through the green wire and another through the black wire coming from the TPS. Then measure the voltage across the two with a volt meter. The voltage will be higher with the car running. It's not supposed to be more than 1.05volts running. Most are right at the limit running. I have also set the TPS with the engine running to 1.00 because .98 with just the ignition on wasn't working. It turned out that I had to really crank the TPS around to get the 1.00voltage with the engine running. I could set .98 with just the ignition on......the TPS was replaced and all was good.
Idle Air Control valve has caused the same thing for me. I just cleaned it out with brake cleaner. One of my cars was sucking oil and the oil was eventually getting into the IAC and the Throttle body causing that problem.
I recently read that a fuel injector wire can cause it, although I have never had that happen to me that I know of.
Clean the mass air probe with electrical cleaner. I had trash on that once that caused the engine to hold RPM.
Oh yeah, I had a BIG K&N filter attached straight to the Mass Air Unit. I don't know if you have done this though. That caused all kinds of strange RPM problems. I had to use a smaller one to get it running right. Ahh....one more thing that actually worked. Panty hose over the Mass Air then put the filter on top of the panty hose. I saw that trick at the drag strip. Use strong panty hose...
Hope one of these will help.
Jim