97 Grand Prix 3.1 L My car Idles at 2000 rpm cold and warmed up and drives at 1500rpm by itself. How can I tell if its a leak,TPS, or IAC?

6 Answers

  • paul h
    Lv 7
    6 years ago
    Favorite Answer

    You can test the voltage signals from the TPS to see if it is functioning properly for a given operating parameter...or use a scan tool to test function/signals. You may need a shop manual to find the specs...some online sites have them. The IAC operation is in gradual steps called counts which reading scan tools can pick up to see if they are within parameters. The IAC and TPS work in conjunction with each other and the computer and hand off under certain operating conditions when the throttle is closed.

    Testing TPS ... buy a basic multimeter or DVOM needed for testing ...basic models run around 10-15.00 at home centers, etc..and can save a lot of money in doing simple tests.


    I'd start by cleaning the IAC which should be done as part of normal maintenance anyway but most people fail to do and after many years of operation can get fouled and affect idle speeds or performance...the IAC pintle tip and the bore it sits in can get sooted up with dirt/carbon which alters how the computer controls air flow bypass around the throttle plate at idle. You may need to relearn the computer's idle speed adjustment after cleaning or installing a new IAC ...see details in link below. The TB throttle plate area and MAF sensor wire can also get sooted up over time which alters airflow and/or affects idling or computer inputs/performance. Clean the TB throttle plate area as well as the MAF sensor wire...special TB and MAF spray cleaners for these are sold at parts stores and fairly easy to DIY....all should be cleaned as part of normal maintenance and to achieve a baseline in running further tests.

    Add some Seafoam or Techron to the fuel tank per label directions to clean injectors....sold at parts stores. Add a double dose of say 2 ounces of product per gallon of gas for a tank or two.

    Check for vacuum leaks from hoses or TB to intake mounting areas or intake to head sealing areas ...spray some carb cleaner spray around the TB base and intake manifold to head areas while the engine is idling...if idle speed alters or raises up, there is a vacuum/air leak in that area. Clamp off or remove and plug various vacuum hoses near the TB or intake and see if idle speed changes or engine runs better...indicating a leak of unmetered air. Check hoses or plastic vacuum lines for cracks, splits, melted, broken ends, looseness, etc..Repair hoses/lines as needed...splice in vacuum hose to fix bad plastic lines. Check the EGR valve as well...it may be faulty or has a clogged up passage or is stuck open/closed.

    An alternative method in checking vacuum leaks is to use a propane torch ...attach a 2-3' section of hose tightly over the torch head and tape off any vent holes in the head...open the tank valve and run the end of the hose around various areas of the TB and intake while engine is idling to see if idle speed alters...indicating a leak.Keep the hose end away from any sources of sparks/ignition like alternator, distributor, etc...

    IAC and TB cleaning...


    Pics of various sensor locations and descriptions of operation....


    Relearning idle speed....


    IAC operation....

    "The purpose of the Idle Air Control (IAC) valve is to control engine idle speed, while preventing stalls due to changes in engine load. The IAC valve, mounted in the throttle body, controls bypass air around the throttle plate. By moving a conical valve, known as a pintle, in (to decrease air flow) or out (to increase air flow), a controlled amount of air can move around the throttle plate. If RPM is too low, the PCM will retract the IAC pintle, resulting in more air being bypassed around the throttle plate to increase RPM. If RPM is too high, the PCM will extend the IAC pintle, allowing less air to be bypassed around the throttle plate, decreasing RPM.

    The IAC pintle moves in small steps called counts.

    During idle, the proper position of the IAC pintle is calculated by the PCM based on battery voltage, coolant temperature, engine load, and engine RPM. If the RPM drops below a specified value, and the throttle plate is closed (TP sensor voltage is between 0.20-0.74), the PCM senses a near stall condition. The PCM will then calculate a new IAC pintle position to prevent stalls.

    If the IAC valve is disconnected and reconnected with the engine running, the idle RPM will be wrong. In this case, the IAC has to be reset.

    The IAC resets when the key is cycled ON then OFF.

    When servicing the IAC, it should only be disconnected or connected with the ignition OFF in order to keep from having to reset the IAC.

    The position of the IAC pintle affects engine start up and the idle characteristics of the vehicle. If the IAC pintle is open fully, too much air will be allowed into the manifold. This results in high idle speed, along with possible hard starting and a lean air/fuel ratio. DTC P0507 may set. If the IAC pintle is stuck closed, too little air will be allowed in the manifold. This results in a low idle speed, along with possible hard starting and a rich air/fuel ratio. DTC P0506 may set. If the IAC pintle is stuck part way open, the idle may be high or low and will not respond to changes in engine load."


    More repair info at website above...

    The bottom line is that you want to get any cleaning of items done first to establish a baseline of proper engine operation before doing any repairs or testing....or just replacing parts and hope it helps. Given that the car is 17 years old and likely with high miles and not knowing the repair/maintenance history, I would say you need to start by cleaning needed items and checking for vacuum leaks...then move on to other testing of items/circuits if needed. The conditions you describe could be caused by a number of issues or multiple problems which may not show up as a trouble code or check engine light. Weak fuel pressure could also cause poor performance...how old is the fuel filter/air filter? Replace those if due. You may also need to run a fuel pressure test to see if fuel pressure is OK or not as a baseline test....parts stores sell or rent pressure testers.

  • 6 years ago

    My car only stalls out if I start my car and immediately try to drive it.

    After its been warmed up for like 30 minutes it wont stall.

    What's the best way to check for vacuum leaks?

    My check engine light isn't on so would Autozone be able to receive an error code?

    My car also sputters and shakes when I try to give it gas.

    I put some fuel injector cleaner in my car because of the stuttering.

    Its hard to explain how rough my car drives when I press the gas.

  • 6 years ago

    Might be a vacuum leak. So it may not be the tps, or maf. or iac.

    Usually the Iac motor is the culprit if your car stalls out when you stop. tps and maf sensors will sometimes cause a surging idle, or constant runabilty problem.

    could be a vacuum hose, or leaking plenum/intake gasket

  • 6 years ago

    Check for codes: many auto parts stores as in autozone do free code checks l. If it's a tps it will throw a code on the dash. Any bad sensor will inc. idle speed sensor. Map sensor and maf sensor. A leaky air hose usually won't and have to manually inspect and listen for a leak.

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  • 6 years ago

    Either a vacuum leak or stuck IAC. That isn't how a TPS fails.

  • Evil C
    Lv 7
    6 years ago

    More than likely you have an intake manifold vacuum leak. These engines are famous for it and they don't always set a code for it for some odd reason. You are most likely looking at new intake manifold gaskets.

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