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 âž± 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.
