Engine loping

[Loekee75]

My '92 started loping in PARK, at the most about 4-500 rpms above idle and back down again, and typically after the engine was at its normal operating temperature. It doesn't do this all the time. I had swapped out the '92s TPS with the sensor from our '93 Lesabre (this was to diagnose the cause of a code 21 in the Lesabre), and I noticed the loping started shortly after that. I scanned and got no TPS codes, nor anything else other than the O2 circuit (13) I've had for a while now. Could cold weather be a factor? Thanks in advance for the input!
Oh, and oddly enough, another code I had for as long as I owned the car, PNP switch/ circuit, 31) is now gone, and for the first time ever my car can be started in neutral! The PNP does control ECM operation as the ECM changes engine operation depending on what gear the tranny is in...so there's another mystery!

[HotZ28]

Did you check the voltage of the original TPS at idle? Did you check & set the replacement TPS to the same voltage? If not, the ECM is seeing a different voltage from what it used to set “driving parameters”. (This is the learning mode the ECM goes through when it starts developing driving parameters). The ECM might eventually adjust to what it is now seeing, if not, check the TPS for correct settings and then pull the battery negative cable for 30-sec. I always turn the ignition switch on while the battery cable is loose to drain all residual power, then reconnect the cable. The ECM will start developing new “driving parameters” the next time you drive it. Be patient, it could take 50-250 miles for the learning mode to fully develop!

Originally Posted by Loekee75

Oh, and oddly enough, another code I had for as long as I owned the car, PNP switch/ circuit, 31) is now gone, and for the first time ever my car can be started in neutral! The PNP does control ECM operation as the ECM changes engine operation depending on what gear the tranny is in...so there's another mystery!

I am not sure that I understand what you said here, so let me see if I can describe how the PNP switch should relate to the ECM. The PNP/switch (should) send an input to the ECM indicating what gear you have selected and then the ECM responds accordingly. The gear selected has very little, if any effect on engine running parameters, however it does have a large influence on the transmission operating parameters! Does that make better sense?

[Loekee75]

I didn't check the TPS's voltage or reset the other one, because I figured the ECM would adapt to the new one after a few miles as it does other sensors. How does one go about setting the desired voltage in a TPS, that's something I've never done.
Everything you said in regards to the PNP switch made sense, Bo!
I could have sworn I read in the service manual that the PNP also affects engine parameters, but I could have easily confused it with any of the other gazillion switches and sensors the car has! Irregardless, the car never had any issues with the tranny when the code 31 was set. The only problem I had was the inability to start the car in neutral, and I figured that was the only issue I had when I had the code. Funny thing is, as a general rule, I disconnected the negative battery cable when I replaced the switch, still got the code, and I had the code for almost 3 years since I replaced the switch. Now all of a sudden the code is gone, and I'd be intrigued to find out how and/ or why. Well..it is a Buick, right?

[HotZ28]

Here is an in-depth and recent discussion from the LeSabre forum (same TPS as on PA) and how to check the voltage. Remember, they are not adjustable per say, however, you do need to verify that you have the correct voltage on both the reference signal and the variable signal. In most cases, the TPS can be wiggled enough to change voltage a few tenths of a volt.

[Loekee75]

Alright, thanks a lot. I'll check it out later today!