High Rev on Start Up....Low Idle

Discussion in 'Maintenance' started by daveypantele, Jan 1, 2017.

  1. daveypantele

    daveypantele New Member

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    Feb 15, 2015
    Surprise, Arizona
    I have a 2002 Tahoe 5.3 FlexFuel with about 130 and change. Sometimes...I'd say about 50/50, when I start the truck, she revs up to about 2,000RPM and won't go back down until I shift into Drive or Reverse, which is terrible on the transmission I'm sure. About a half dozen times while I was driving, I'd slow down and turn in to a parking lot and she would die but start right back up and on occasion, when I stop at a light she will drop to about 2-300 RPM and then go back up to 5-600 where she commonly sits steady. She's never died just sitting at a light, it's only when I'm turning and slowing at the same time but I don't believe the steering has anything to do with this as the steering pump seems to be in good shape as well and I haven't a single fluid leak on anything anywhere in or around my truck.

    She does have a minor exhaust leak where the gasket has failed between the exhaust manifold and Y pipe, but this problem has been an issue long before this. I had a code a few times for the infamous knock sensors/intake gasket, and I'm sure that could have something to do with it because she runs roughest when it's very damp and/or raining out. But I want to just get some second opinions as to what ELSE may be causing this issue because it came about a while after the codes came and went (and never came back). I've done a mild tune up including plugs, wires, cleaned the throttle body (without removing it), cleaned MAF and other related sensors, and it hasn't fixed the rev/dying issue. I've also done the old "spray starting fluid on your shit and listen for the rev" trick with no luck at finding any vacuum leaks...quite a few times at different engine temps and atmosphere temps.

    To break this all down simply...here's a time line...
    1st: Code for Intake/Knock Sensors (came and went) (also noted that truck was bought with bad fuel pump float)
    2nd: High Rev at start up, low idle on rare ocassion
    3rd: Mild tune up and cleaning of parts
    4th: High Rev and now dying (only a few random times).
    5th: Exhaust Leak (very minor, no additional issues after...just the same old problems more often as time passes)

    Sometimes she runs like a raped ape and takes off like a bat out of hell, the next take off will be sluggish even with the pedal slammed down. Ocassional drops in RPMS at cruising speed (but rare). So, a lot to take in to consideration and I could just need to get my lazy ass to the tool box and rip that intake off and get to work. Anything else you recommend tearing in to while I'm at it? TPS? MAF? Injectors? Pump?

  2. nsguy

    nsguy GMFS Member

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    Jun 16, 2014
    Fuel pressure regulator?
  3. John Clark

    John Clark GMFS Member

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    Feb 1, 2011
    A vacuum leak could cause this (your intake manifold gasket, if it's bad enough) or it could be a bad idle air control valve. I'm assuming this is a manual throttle (uses a throttle cable not an electronic throttle?)
    Kids-at-Wawa likes this.
  4. z71gacowboy

    z71gacowboy GMFS Member

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    Apr 4, 2010
    West Georgia
    If electronic throttle, check if dirty and gummed up. Clean take off battery post and touch together to drain ecm capacitor and clear eeprom/kam. Restart and let idle 5 minutes, then drive stop normal 10 minutes. Alternatively could be indicative of throttle body module on drivers side of bulkhead having problems or wiring to etc from module...seen both problems but rarely throttle body the cause.

    Lean enough to cause high rpm at start will cause ses, if bulb burnt out use scantool and check for codes or data for lean condition.

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