Engine surging with A/C on

[200Proof]

I have a 2012 QX that I bought used in November and with it heating up now I am using the A/C more and have noticed that when i am sitting at a light the engine surges. The rpm's are usually around 750 while stopped but then will suddenly drop down to just above 500 and when this happens the whole vehicle shutters, it feels like you let off the brake a little and then step on it again, you can feel the shutter in the gas pedal and in the seat. Then the rpm's will go back up to 750 and the process repeats itself. I'm thinking it's the fan on the radiator kicking on and off but I've never had a vehicle where you can feel it when it happens. If i put it in neutral while stopped the rpm's stay steady and this doesn't happen.

Is this normal or do i need to take it to the dealer?

 

[Silverback]

Not normal.

 

[roughyear]

definitely NOT normal... that use to happen to me in my Audi.. was a loose vacuum hose (engine compartment)

 

[Dread_Parrot]

Ias

You are describing what sounds like a Idle Adjust Solenoid. It opens the throttle just a bit to bump up the engine speed when the AC compressor kicks on. That is what it was called on my old Montero. It would also allow the a little fast idle if it was cold, making it easier to start. The ECU would turn it on to allow the engine run at about 1000 rpm and then as engine warmed it would dial back the idle speed to around 700 - 750 rpm.
Turning on the AC always turns on the IAS to increase engine idle speed for AC. It wasn't an expensive part and not that hard to replace. Mounted on the side of the throttle body, it had a electrical connector on the bottom of it. It went bad after 11 years, and $22 for the part.
I would say yours or whatever the QX uses is a more modern equivalent, is covered under warranty.

Interesting part is some cars and trucks, maybe better designed as it is a smooth operation and transition that you don't even feel the increase of the engine speed, or kick of the compressor. QX is pretty smooth, all that torque the engine has, surely helps smooth it out.
Hope that helps, good luck and let us know what the dealer says.
Parrott

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[theta]

I noticed the same thing on mine recently. A slight surge when at the stop light. At first I thought the person behind me bumped into me. I'll have to check out the rpm and see if that is associated with it.