Question about 1999 GMC Sierra

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Truck runs very lean "no power" until the motor warms up. Usually a quarter mile. I have had it to two shops no results.

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  • Chad Foster
    Chad Foster May 06, 2014

    It will gain some power after flooring it. It idles fine. and after it gets warmed up it runs perfectly.

  • Robert Coffey May 07, 2014

    I have to agree with Zach. It sounds like a lazy O2 sensor. The shop should have checked the codes. If they don't have the capability for that, you might have to bite the bullet and take it to a dealer.



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Sounds like it might be a problem with the O2 sensors not warming up fast enough possibly. Is the check engine light on? Try pulling codes from the PCM and see if there is one pertaining to the O2 sensors and even if there isn't, have a shop check the O2 sensors on an oscilloscope to see if maybe you have a "lazy" one. Also, check the fuel pressure while it's warming up and see if it's within specifications before, during, and after it is warmed up.

Posted on May 06, 2014

  • Bryan
    Bryan May 18, 2014

    GMC=Get My Coveralls. Yer intake manifold gasket is blown. Running cold air gets in till heats up and seals. GM factory intake gaskets are paper thin to cause us money. Knock sensors are in intake I took mine out and grounded them and ties to firewall to avoid more BS later to take off intake gaskets just for this.


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It could be many things.... I will start to look at O2 sensor heater.... if you have a scanner to read data. Don't start engine the voltage should be around 450mv key on wait 3 to 5 minutes....the O2 sensor voltage should raise toward 900mv. if it doesn't then bad heater detected.

what else gaz, Map sensor ,Maf sensor,tps sensor,temp sensor,
hope it helps

Posted on May 06, 2014


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