I am now looking into the O2 sensor and temp sensor being a cause of idle surging. I know where the O2 sensor is, but not sure where the temp sensor is located.
Found old repair receipts and noticed that the idle surge was fixed a while back by replacing the wiring harness that had shorted at the EGR switch over valve (whatever and wherever that is). Also temp sensor was replaced a few months ago.
The idle surge problem started after the techs replaced a faulty water pump. Problem is, it was such an intermittent problem that they couldn't recreate the problem when I complained.
Are any of these components close to the water pump?
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car