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What controls the idle bump when A/C clutch engages?

bknudtsen

bknudtsen

NAXJA Forum User
Location
Issaquah, WA
Is it a controlled vacuum leak? Or does the ECM use the idle air control? I have been having an abnormally high idle bump when my A/C clutch engages, making it difficult to drive in parking lots as the bump causes the vehicle to surge. It also happens as I am coming to a stop at traffic lights.

I replaced the IAC, but it has not helped. If I turn the heater controls to off, then it does not happen.

Thanks,

Brad
 
The ECU sends a signal to the IAC motor to open the orifice slightly, increasing the air flow around the throttle plate, increasing the idle.

Don't think the timing is a factor... but could be wrong...

is your throttle body clean? is your IAC clean? how about grounds?

this a renix or a chrysler FI?
 
bknudtsen said:
Is it a controlled vacuum leak? Or does the ECM use the idle air control?
Click to expand...

Both, The IAC is actually a controlled vacuum leak. The ECM gets a signal that the compressor has kicked on and then sends the step information to the IAC stepper motor, telling it to open up.

I would remove the IAC and clean the throttle body where the IAC pintle hits the TB.
 
The throttle body is a new 62mm unit. The IAC is new as well. Could it be a machining issue with the new throttle body? Maybe the tolerences are off and too much air is being let in?
 
Just installed new parts? Could take some time (day or two) for the computer to adjust. You dont mention a service engine light?
 
langer1 said:
A little more checking and were both right, if the compressor is not loading though the rpm will be to high.
Click to expand...

What other symptoms will there be? No cold air?

edit** I forgot to mention that the low side coolant pressure is about 35psi. My XJ is a '01 if that makes any diff.
 
Last edited:
gjxj said:
Just installed new parts? Could take some time (day or two) for the computer to adjust. You dont mention a service engine light?
Click to expand...

It's been happening for over two months, so I replaced the IAC two weeks ago. No service light.
 
bknudtsen said:
What other symptoms will there be? No cold air?

edit** I forgot to mention that the low side coolant pressure is about 35psi. My XJ is a '01 if that makes any diff.
Click to expand...

That would hard to tell this time of year.
 
If the idle is way high the service light should be on with an idle high code.
It might be wothwhile to go check the codes anyway.
 
What year and engine? On my 87 2.5L it is a controlled vacuum leak through a solenoid valve. On my 88 4.0L it is controlled by the ECU.
 
Last edited:
I put my stock throttle body back on and the problem is gone. Thanks to everyone for their contributions to this thread and forum.

Brad
 
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