Home 5 Engine 5 4. RPM Surging with A/C On | 2004 GMC Envoy

Surging | GMC Envoy


🚗 2004 GMC Envoy 4.2

  • When A/C is turned ON, the engine starts surging (rpms go up and down).
  • With A/C turned off, seemed  to idle normally.
  • Customer provided no car history.


🩺 Diagnosis

Could it be that the A/C compressor builds up so much pressure that it causes the engine to have a noticeable hard time coping with that unexpected load?

  • I hooked up A/C low and high pressure gauges ➱ they have showed 30psi and 150psi respectively.
    • If something was clogged, it would quickly go above 400psi on a high side. So, no problem there.
    • Just to make sure, I disconnected the A/C relay; the surge remained.

    What else engages when you turn A/C on?

    • Fans. This kind of fan is driven by the engine with a clutch on a fan that regulates the fan’s speed.
    • I disconnected the fan, surge remained.

    PCM Update?

    • Possible. There is even a TSB for it (#03-06-04-057). Will save that for last.

    What else then? Here I had to dig a little and found that PCM expects certain airflow based on a throttle plate opening.

    • When the A/C switch is turned on, PCM expects more load on the engine and raises rpm slightly to compensate for that load.
    • PCM controls the airflow using the throttle body, adjusting the opening angle of the butterfly valve.
    • Dirty throttle body might confuse the PCM’s airflow expectations for a specific throttle plate opening angle… which ultimately results in surging.

    So, I removed the hose from the throttle plate and it looked somewhat dirty.

    • Cleaned it with a carb cleaner. Put everything back, started the car, and turned A/C on.
    • RPMs were higher this time but some surging was still there.

    Then, I performed “Idle Air Relearn” procedure by disconnecting the battery for a minute.

    • Tested again, and what do you know, it worked!
    • No more surging.


    Dirty Throttle Body. 2004 GMC Envoy

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