Idle Surge every 20 seconds in '01 V70 XC

My '01 V70 XC has a continuing problem where the idle momentarily surges
every 20 seconds. The surge is a couple hundred RPM. It is disconcerting
when I am coasting to a stop or braking. The surge causes the car to
lurch forward. It feels very unsafe. I have gone to the Volvo dealer
about it and the mechanic says he has the latest computer updates loaded
and has no idea where to start. It has 106,000 miles and has always
received scheduled dealer service. This has the low pressure turbo 5
cylinder engine.

 

I found that as my AC Compressor was starting to seize its clutch
would put a dragging load on the engine and then disengage, causing
the engine to race a bit as it adjusted to the freed load.

I don't know how best to test this except to listen for the compressor
engaging/disengaging while watching the tach.

Cheers,
blurp

 

Thanks for the info. I will see if I can coordinate the surge with the
AC switching. Shutting off the AC does seem to prevent it. Your
explanation lets me know why it is much more pronounced than normal.