Engine Surging
 

Engine Revs surge between 650 and 1100 while in neutral or when at a stop. Have replaced a number of sensors and the ECU. Did 3 vacuum tests and checked the EGR valve. There are no codes except for the computor seeking the proper rev info. Anyone have any ideas??

my vote was gonna be for the EGR, may be somethig that uses vacume, like the cruise control servo, an 02 sensor that is doin g something called 'hunting'. if you give the year of your car others will step in faster cause its probably happened to them

Could be electical like a battery going south or failing alternator. Worth checking.

Sounds like a vacuum leak.

My '87 did that. It was a broken vacuum line sorta
under the driver's side headlight. Get somebody with
young ears to have a listen. That's what found my
problem.

Sounds like me, Can't hear it thunder anymore. lol

Sounds like the IAC to me. Same thing happen on my Grand Prix SE, changed the IAC and it was fixed. BTW buy it at pep boys for about 30 bucks, others charge up to 200.

That was my problem, only just the plug to the IAC. Check the plug first, unplug it, clean it, try it again.

Also found out the Temp Sensor can be reading temp correctly, but not signalling the ECM correctly. Fuel is constantly trying to adjust due to faulty temp reading to the ECM.

A scanner told me this, but no code because it wasn't completely malfunctioning.

On my '87, the FSM shows two coolant temperature
sensors. One feeds the gauge on the instrument
panel, the other feeds the ECM, which makes emission
decisions based on the info, and triggers the low
temp fan (with dual fans the high temp fan has a
third on/off sensor).

My '01 Nissan Pathfinder is the same way (except
no two-fan option).