Question about 1993 Pontiac Grand Prix
The engine runs fairly smooth at idle but when you put it into drive it runs rough. We have replaced the fuel filter, fuel pump , fuel regulator, fuel injectors, plugs and wires, and the throttle position sensor as well as the map sensor. The only codes thrown were the tps and the map sensors. The compression checks out great and the engine does not smoke. At an idle if you full throttle it the engine cuts out and surges. We cannot find any vacuum leaks either. Funny, it passed emissions as well earlier in May of this year as well. Now what?????
The 3.1 had some EGR problems, EGR valve stuck open, blown diaphrams, Plugged passages Im not saying that is all your problems and I believe yours has whats called a linear EGR about $350.00 from the dealer (and that is the only place you ever consider buying such a critical part for the emissions system regardless of the cost)
That car is so antiquated as far as computers go that relying on the codes is nearly pointless! Map code can set because of low engine vacuum which you will have with a poor running engine, Check for 5 volt reference voltage at the TPS and MAP with the key on and engine off if no 5 volt reference I suspect a Computer. Computer Ground is on the Thermostat housing and needs to be clean and secure.
There is a legit test in the GM factory manual called the TAP TEST take the ECM (computer) and slap it with an open hand, if the symptoms change the ECM is bad it's very common in the early-mid 90s for the computers to go bad in one way or another the circuit boards come loose and suffer also from cold solder joints.
Make sure the wires are on the correct plugs and hopefuly you used factory plugs and wires and No split fire or bosch plugs only AC delco
Make sure you have good spark on all 6 cylinders.
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Posted on Sep 19, 2008
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