Question about 1993 Pontiac Grand Prix

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1993 pontiac grand prix 3.1 liter

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?????

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  • dwiley41 Sep 22, 2008

    Checked EGR, it was fine. Had a 5 volt reference at both the tps and the map. However I did find that the coil was bad on the #1 and #4 cyllinders. Runs much better now but in park when you full throttle it it still cuts out. In drive though it is not noticable. It runs well enough that I think we will leave alone for now. Thanks for the help. The note to check the plug wires helped me to find the bad coil. Oh, something strange though. When I energized the wiring to check the egr, all of the wires operated the solenoids correctly. However when I energized the fourth wire a strange thing happened. The a/c fan inside the car came on. Any idea why that would happen????

  • dwiley41 Sep 27, 2008

    Now we have a different problem with the car. Transmission (125 c) The owner was able to drive her car home running great for the first time in 3 years. After driving the car 1 day running well , she parked her car in the parking lot of a Walgreens, went in and came back out and now has no reverse or park. The car will still start, but it starts in drive. There is no park at all and no reverse. It also creeps forward in neutral. We pulled the pan and found a few small pieces of what looks like clutch material but not much. Drove it out, briefly, and the car has all the forward gears. I think I am going to get a Molatov cocktail......Any ideas? Oh, by the way, The reason I had not checked the coil in the first place is that the owner's boyfriend of the car had told me that he had replaced the wires and checked spark and compression already. He is a young mechanic that just graduated so I assumed he knew what he was doing......

  • dwiley41 Oct 04, 2008

    Just in case anyone out there had this problem, it turned out that the transmission needed rebuilt due to worn clutch plates. Evidently after we got the engine running correctly the extra power finished off the already worn out clutch packs and the reverse band. We rebuilt it for about $150 when transmission shops were going to charge us about $1800 - $3800.

  • Anonymous Mar 21, 2014

    it starts up idles low 500rpm give it throttle n only gets 2500-3000 rpms I think I hear a slight nock.i have done the following new plugs,map sensor,camshaft position sensor,camshaft acturator solenoid,cleaned furl line and enjecter rail,fuel filter.and all this made no difference.The check engine light is on code is po300 which is random or multy misfire.can any1 help? please



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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.
Contact me at with the results and any feedback or questions

Posted on Sep 19, 2008

  • Randy Harkins
    Randy Harkins Sep 22, 2008

    you should never do a full throttle test the cutting out is the govenor to save the engine from overspeed, I am glad you found it and dis not assume you had a bad coil because you mentioned having changed wires and plugs and I thought you would have checked that but anyway thanks for the feedback and good luck and please if you like visit my website I created it while I was in business in my diagnostic shop for 14 years.

  • Randy Harkins
    Randy Harkins Sep 27, 2008

    Ok I'll look into this and get bck to you tomorrow ,sorry busy all day today sorry.



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