QuestionQUESTION: I have a 2002 grand prix SE. After 30 minutes of driving, the speedometer starts jumping, which in turn the car starts jerking and the engine starts reving and easing up and losing power. When we pull over to the side of the road, the engine idles nice and smooth and will rev smoothly. Put it back in gear and the car starts jerking and the speedometer starts jumping again.Also the cruise quits working shortly before the whole mess starts happening. We have replaced the VSS on the transmission and had the transmission tested at a trusted transmission shop and it tests good. Any suggestions??
ANSWER: Hi Bruce:
what engine is in this 3100 or 3800? Have any trouble codes set when this happens? In any other modules. If this is a 3800 there is a chance that you have bad spark plug wires emitting EMI into the VSS wires for the transmission. especially if they are aftermarket plug wires and they are too long and draped over the back of the engine near the harness. The cruise is related to the speedo I'm pressure sure if you have an erratic speed signal the cruise will cut out or not work. It could have a bad engine control module as well. the engine control module is the buffer for the speed signal. it comes from the transmission and the engine computer deciphers the signal and the ecm sends the signal to the speedo. as for the erratic idle it may or may not be related. if this has a bad TP sensor it will cuase the idle to surge or be elevated at a stop. More so on the 3100 engine then the 3800. Those are a few things to look at. good luck :-)
---------- FOLLOW-UP ----------
QUESTION: It's a 3100 engine if that makes a difference.nccdj
ANSWER: No not really I would still look at the things I mentioned.Just on a 3800 The spark plug wire is a more common issue with the speedo acting up then on the 3100 engine just because of the way the engine computer harness runs along the engine to the computer.
---------- FOLLOW-UP ----------
QUESTION: bothering you one more time, If it is the engine control mod, wouldn't that throw up an engine light and there for have a code for a reader? We haven't seen an engine dummy light and we checked for a code, nothing.
Answerno, not necessarily, if there is something wrong with the logic inside the ECM it may not set a code. One other thing to take a look at is all the Grounds for the engine control module. there are on the trasmission to engine mounting studs and also make sure the ground from the battery cable to the body ubnder the battery tray is clean and tight... Back to the TP issue if you have a true scan and you catch the car is a failed state take a look at the TP reading and the throttle angle % at idle foot off the gas the TP should be somewhere around 0.50 Volts and the throttle % should read NOTHING but 0% anything more then that you more then likley have a bad TP sensor. That is for the erratic idle not the speedo issue you have...
