RPMs Drop?

[PaulR]

Thanks for your help, Chris. I am not really throwing parts at it at all, I am able to take it right back to the store I got it from and they are fine with that. I have checked for codes multiple times and nothing unfortunately. What else would I be able to test/diagnose?

[chriscarter]

Drive the car with OBD reader connected.

[PaulR]

Drive the car with OBD reader connected.

Hi Chris,

Thanks again for your help. I don't have a reader that has the real time sensor data - is this okay? Do I need to constantly scan the code reader or just watch it for a code to pop up?

[chriscarter]

Should be able to leave it connected and watch for codes to pop up, but I can't be sure without seeing your reader and trying it real-time. Give it a try and see what happens. Again........try to think this through. What input systems are active at 40 mph and how do they influence driveability? Is there a direct correlation between the auto trans shifting and the problem experienced? What else is "on" at the time....A/C, cruise, etc. Do any other warning lights come on? Have you done anything differently in terms of maintenance or repair since this started?

For diagnostic purposes, try leaving the trans in D2 and seeing if the problem still occurs. Change the inputs to see if it affects the symptoms....maybe accelerate hard through the speed range where it happens just to see if it still happens....maybe accelerate very slowly for the same reason. You could also leave the trans in low and normally accelerate to the engine RPM range (not the road speed range) where the problem happens. Do a differential diagnosis in an effort to eliminate certain things and to concentrate on others. It's not just about error codes and CEL indications.

[PaulR]

Since it seems to do it at 45+, and not at all less than that, I can assume the major difference is that it is in overdrive. The fact that the car's RPMs get low when I let off the pedal (say 1100RPM), and then go back to normal (say 1900RPM) lead me to believe the computer might be correcting for something it doesn't see when I let off the pedal. If that's the case, wouldn't the IAC be the most likely culprit? I will check my manual for diagnostic procedures on the IAC. The throttle position sensor doesn't seem to meet most of the symptoms you will experience if it is bad - should I do further testing on it?

Thanks again.