Edmunds Answers

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  • avatar zaken1 08/24/11 2:28 pm PST

    Before doing any more work on this vehicle; it is absolutely essential to run a compression test on all the cylinders; in order to confirm that all the cylinders have compression pressures that are above the minimum limits for this particular motor: These figures are published in the factory service manual. All too many mechanics do not bother to look up the factory compression specs on the motor they are working on; because they think all motors are the same (which is absolutely untrue). It is also essential that no two cylinders can have more than 15psi difference between their compression readings. If this motor does not pass either of these specifications; it will not be possible to tune it properly, until the motor is overhauled.

    The throttle position sensor on this vehicle has wide mounting slots which are designed to allow the sensor to be rotated to different positions in order to trim the idle fuel mixture and off idle response. This adjustment is extremely critical; and most shops do not have a clue about how to set it. It sometimes becomes necessary to reset this adjustment after major changes have been made to the way the motor runs. The newer cars do not come with that provision for adjustment; as too many people were messing it up.

    The way to adjust this setting is to warm the engine up by driving the car (not by idling it) until it is at full operating temperature. Then connect an infra-red exhaust emission analyzer to the tailpipe. With the motor idling; loosen the throttle position sensor's mounting bolts, and very slowly and carefully rotate the sensor to different positions while monitoring the hydrocarbon emission levels. When you find the setting which gives the lowest hydrocarbon levels; tighten the sensor mounting bolts on that position, and run the engine speed up to 2500 RPM for about 3 minutes. Then let it come back to idle and see if the hydrocarbon levels have changed. It sometimes may be necessary to readjust the throttle position sensor a few times until the mixture and emissions stabilize.

    You also might find that someone has broken the factory seal on the stop screw on the throttle body which limits how far the throttle plate can close, and changed that setting. This setting interacts with the throttle position sensor adjustment; so it may be necessary to set the throttle stop screw to a wider pisition in order to get the idle speed and mixture right.

    It also could be that one or more of the ignition coils on this motor are defective; or that the wrong type of spark plug has been installed.

Answers

  • MrShift@Edmunds 08/24/11 1:25 pm PST

    Well one suggestion is that they can refund your money for not knowing what they are doing. A shop that has a sign that says "auto repair" can't say "I don't know". If they don't know, they find out, call other shops, do whatever it takes.


    Then you have to find a shop that does know something about cars and they have to embark on a systematic diagnosis, such as reading the car's trouble codes accurately. If no codes show up, they should be looking at components that do not throw codes, like perhaps fuel pump, fuel filter, fuel pressure regulator. Also basic tune up parts should be checked and the fuel itself sampled if necessary for contamination.


  • zaken1 08/24/11 2:28 pm PST

    Before doing any more work on this vehicle; it is absolutely essential to run a compression test on all the cylinders; in order to confirm that all the cylinders have compression pressures that are above the minimum limits for this particular motor: These figures are published in the factory service manual. All too many mechanics do not bother to look up the factory compression specs on the motor they are working on; because they think all motors are the same (which is absolutely untrue). It is also essential that no two cylinders can have more than 15psi difference between their compression readings. If this motor does not pass either of these specifications; it will not be possible to tune it properly, until the motor is overhauled.

    The throttle position sensor on this vehicle has wide mounting slots which are designed to allow the sensor to be rotated to different positions in order to trim the idle fuel mixture and off idle response. This adjustment is extremely critical; and most shops do not have a clue about how to set it. It sometimes becomes necessary to reset this adjustment after major changes have been made to the way the motor runs. The newer cars do not come with that provision for adjustment; as too many people were messing it up.

    The way to adjust this setting is to warm the engine up by driving the car (not by idling it) until it is at full operating temperature. Then connect an infra-red exhaust emission analyzer to the tailpipe. With the motor idling; loosen the throttle position sensor's mounting bolts, and very slowly and carefully rotate the sensor to different positions while monitoring the hydrocarbon emission levels. When you find the setting which gives the lowest hydrocarbon levels; tighten the sensor mounting bolts on that position, and run the engine speed up to 2500 RPM for about 3 minutes. Then let it come back to idle and see if the hydrocarbon levels have changed. It sometimes may be necessary to readjust the throttle position sensor a few times until the mixture and emissions stabilize.

    You also might find that someone has broken the factory seal on the stop screw on the throttle body which limits how far the throttle plate can close, and changed that setting. This setting interacts with the throttle position sensor adjustment; so it may be necessary to set the throttle stop screw to a wider pisition in order to get the idle speed and mixture right.

    It also could be that one or more of the ignition coils on this motor are defective; or that the wrong type of spark plug has been installed.

  • fedup2much 11/20/12 11:48 am PST

    Had the same symptom your describing, after changing two computere, air flow sensor, fuel filter, and some other things, the last thing changed after refusing to replace the catalytic convertor for $800 + dollars they replaced the spark plugs and bingo, it started running great.
    I had to take the vehicle back three times, the last straw was when they said I needed to replace the catalytic convertor, I asked them if they had replaced the plugs and they looked at each other like deers with headlights in their eyes.. It still cost me over $2000.00 for all their easter egging.

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