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Intermittant rough running

This is a discussion on Intermittant rough running within the General Help forums, part of the LSx Technical Help Section category; I have a 99 Sierra giving me problems and all I can get on other sites is "fuel pump." So ...

  1. #1
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    1999 GMC Sierra

    Unhappy Intermittant rough running

    I have a 99 Sierra giving me problems and all I can get on other sites is "fuel pump." So I thought I would give you guys a try.

    Its a 99 Sierra with the 5.3.

    The problem - 1. occasional hard starting (getting more often), I can't find any common factor for this, does it hot/cold, wet/dry.

    2. Truck dies or comes very close to it in traffic - the biggest trend with this one is that it does it after constant speed cruising for a bit and then hitting a traffic light, after i come to a stop it will wait a second and then the idle will stumble real low and sometimes die.

    Everyone on GM-trucks says fuel pump. Why do I think its not? - its been doing this, and getting slowly worse over about 6 months now, I think the fuel pump would have died; also the fuel pump has only got 15-20k on it.

    I was hoping to get some other ideas here. Sorry for the long post and thanks for the time

    Oh, and gives no codes.

    Thanks

  2. #2
    the chief oldguy's Avatar
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    Was the filter replaced with the pump?
    Is the fuel rail pressure within spec?

    If all of this is good, it could be a TPS fixing to crap out. I've had them go bad intermittently and not give a SES code.

  3. #3
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    I replaced the fuel filter a while after I got the truck, (the pump was replaced right when I got the truck). I would think the fuel rail pressure is fine, as it runs perfect otherwise(took a 700 mile trip a week ago and it never died or even tried to, just some hard starts), but I don't have the tools to check it.

    I did read on another thread that mentioned the TPS also so I may have to see if there is a way to check that also.

    Thanks

  4. #4
    the chief oldguy's Avatar
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    The Throttle Position (TP) sensor is a potentiometer. The TP sensor is connected to the throttle shaft on the throttle body. The PCM calculates throttle position by monitoring the voltage on the signal line. The TP sensor signal changes as the throttle valve angle is changed (accelerator pedal moved).


    The TP sensor signal voltage is low at a closed throttle position. The TP sensor signal voltage increases as the throttle valve opens so that at Wide Open Throttle (WOT), the output voltage should be above 4.0 volts.
    The PCM calculates fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor may cause intermittent bursts of fuel from an injector. This may cause an unstable idle because the PCM detects the throttle is moving.

    You could try testing the output voltage at various throttle positions but these are cheap and easy to replace so I would just gamble and replace it.

  5. #5
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    Thanks for the info, I will give it a try.

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