This 2002 Buick Century came in with the complaint that the idle speed was higher than it was supposed to be and sometimes it was very high. I looked the car over and saw that a new IAC valve and and new TPS had already been installed on the engine. I did a quick check to make sure there were no obvious vacuum leaks and none were found. I turned my attention to codes and data. No codes were stored. I did find several lines of data that were out of the expected range. The TPS voltage and the TPS Percentage on the scan data were higher than they should have been with a closed throttle plate.
The longer the ignition was on the higher the readings went.
Notice the MAF g/s were high for an idle condition but if you look at the engine rpms, the engine speed is zero. The engine was not running during this part of the test.
Keep in mind that all of this was happening in the shop with the throttle closed and engine off.
Notice that the MAF reading has fallen back down even though the TPS reading was high. Although I was aware of the MAF fluctuations, I wanted to keep on track with just the one TPS fault. Experience tells me that finding and repairing one fault will normally rectify multiple problems. Especially when it was looking like the PCM was going to be the culprit.
I checked the voltage at the TPS itself and found a reading of about .6 volts. I went to the same wire at the PCM connector and found the same general reading. I case you are wondering I sharpen the tip of a pick to a very fine point and just barely pierce the insulation to get my readings. I continued on with testing of all the power and ground circuits. They were all okay.
I replaced and programmed the PCM, including doing a theft relearn and a crankshaft variation relearn. With the engine running and warmed up all data is as it should be and the idle is normal.
I did not put a lot of details into this article, as far as wire colors and pin locations, because you have to have a scan tool to do the diagnostics on this particular problem. My assumption would be that if you have the scan tool you should also have the data sources.