Question about 1998 Pontiac Sunfire

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1998 Pontiac Sunfire won't accelerate

The car idles great, but will not increase rpm. It will not go over 1300 rpm either with pedal or manually. We have changed the air filter, plugs, wires, fuel filter. Had a compression test done, as well as exhaust flow, passed both. Has a new Throttle position sensor and coil as well. Any ideas.

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  • 3 more comments 
  • conebear_con Jan 14, 2009

    Already had it to the garage they checked the fuel pressure and it's fine. they also checked exhaust flow and say it's fine also. I forget to mention that earlier, I also replaced the MAP sensor. Any other ideas?

  • conebear_con Jan 14, 2009

    No, there is no check engine light. I was wondering about the camshaft position sensor.

  • Anonymous Mar 28, 2014

    idle or rpm are reading low on gage

  • James Bigham
    James Bigham Jun 04, 2014

    The camshaft sensor is a definite go or no go! Being a 98 it doesnt have a distributor correct what engine does it have the 3.2 3.6 or 3800 you could have bad plugs or wires or the spark distribution system may have bad ground due to old dielectric grease under it you can remove the six screws holding it down and clean with electrical contact cleaner and apply new dielectric grease! If that doesnt do it check the plugs and wires then you might want to look into replacing the coil packs if no improvement with the latter? I had a friend with a pontiac skylark with the 3800 v6 with poor excelleration and that's what I did was start with the distribution system by cleaning mating surface then found coil packs to be faulty two out of three where wearing down causing a weak spark on four cyls!

  • James Bigham
    James Bigham Jun 04, 2014

    The camshaft sensor is a definite go or no go! Being a 98 it doesnt have a distributor correct what engine does it have the 3.2 3.6 or 3800 you could have bad plugs or wires or the spark distribution system may have bad ground due to old dielectric grease under it you can remove the six screws holding it down and clean with electrical contact cleaner and apply new dielectric grease! If that doesnt do it check the plugs and wires then you might want to look into replacing the coil packs if no improvement with the latter? I had a friend with a pontiac skylark with the 3800 v6 with poor excelleration and that's what I did was start with the distribution system by cleaning mating surface then found coil packs to be faulty two out of three where wearing down causing a weak spark on four cyls!

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  • 61 Answers

I assume that you do not have a "check engine" light?

The next thing I would do is check the pressure from the fuel pump. You can have this done at your preferred garage, or by purchasing a fuel pressure gauge from any parts store.

Vent the fuel tank vapors by loosening the fuel cap. Remove the electric fuel pump fuse and start the engine. Allow the engine to run until it runs out of fuel and stalls. After spinning the engine over for a period of 3 to 5 seconds, turn off the ignition switch and install the electric fuel pump fuse. Next, install a fuel pressure gauge in the fuel rail supply line, and secure the fuel tank cap. With the ignition key in the on position, verify that no leaks are present and start the engine. Visually verify the PSI reading on the gauge. The pressure for your vehicle should be 41-47 PSI (Pounds per Square Inch). Hope this helps and good luck!

Posted on Jan 13, 2009

  • Chris Hall
    Chris Hall Jan 14, 2009

    Hmmm, my next guess would have been the MAP.



    I'm guessing that the issue is going to be one of the many sensors that could affect the vehicle's fuel/air mixture. Of course, it's much easier to diagnose the vehicle when it's in front of me because most of the sensors have the same symptoms when they fail (which, in part, are the symptoms you are experiencing).



    Again, I'm assuming you do not have a "check engine" light? The absence of this will eliminate some of the possible sensor failures (if that's what it is).



  • Chris Hall
    Chris Hall Jan 14, 2009

    What method are you using to check for engine codes? If you're using the turn the key on and off three times method or a cheap version of a scanner, you may not be getting an accurate "history code" reading. It is possible that your vehicle is storing useful codes in its computer that aren't easily read and aren't being displayed through a check engine light.



    The symptoms you have explained could definitely be caused by a faulty camshaft position sensor. But, I've never seen a faulty camshaft position sensor not cause a check engine light (in OBDII generation computers). So, if the vehicle's code history (current and cache') are truly clear, I doubt its the problem.



    Faulty O2 sensor(s) or EGR valve could also cause what you're experiencing. Again, they typically throw a code.



    A faulty crankshaft position sensor will cause the vehicle to not run at all. And, typically it throws a code when it goes bad as well. So, in my mind, it too can be eliminiated.



    A faulty Air Charge Sensor (A.K.A. Cold Air Sensor or Air Temp Sensor) can cause the symptoms you are explaining. And, I've seen them fail and not throw a code. Basically, the sensor relays the temp of the cold air being brought in and sends it to the computer. Then the computer adjusts the fuel/air ratio based on the temp of the outside air. It is fairly inexpensive to replace (around $15.00) and might be the issue.



    The last thought I'd have (at least for now) would point back to the fuel supply. You said that you had the fuel pressure tested and the pump was fine. Do you know where the pressure was tested at? A faulty fuel pressure regulator could also cause the issues you're experiencing (although you would normally have at least an occasional puff or black smoke). If the pressure was tested in-line prior to the injector rail, it would not have shown a problem with the regulator. To get a completely thorough analysis of the pump and regulator, a reading should be done at the rail as well as before.



    I know I put a lot of extra info in this comment that may or may not be useful to you. But, I hope something helps.

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