rpm surging at idle, new IAC didn't help

2001 3.8L

I'm back. Just changed the idler pulley, belt tensioner unit, and fixed a heater hose leak that I discovered (thanks all who helped with that thread).

I have been chasing a rythmnic engine pulsing that occurs at idle. Sitting at a light, parking lot, etc... the engine like a slow ticking clock revs up, revs down , revs up , revs down... harldy moves the tach needle, but it is annoying, and embarassing.

Bought a new IAC, and installed, but that did nothing. Fan belt is pretty new... right size, etc..

Side note... side effect is my headlights will dim, in and out with the rythmn of the engine... and if I have the door open the overhead lights do the same (flicker and dim with the engine changes)

I bought a new alternator from O'reilys in September, but I think the car was doing this with the old alternator too.

Any ideas? Guy at work said MAF

I would clean the battery post connections, and the big lugs down by the starter to make sure that you have good connections.
How old is the battery?.....a weak battery could be creating a load on the alternator & engine.

Check the PCV valve and inspect the hose from the PCV valve to the FRONT (1999 and newer) valve cover.
It is not unusual for the hose to break down and collapse or crack, letting additional air in or restricting airflow.

Another, less likely, but possible cause would be the Throttle Position Sensor.
If you have a ScanGauge, you could select Gauges, TPS and monitor the Throttle Position Sensor value to see if it is changing while this is happening.
The TPS value should stay constant when your not moving the accellerator pedal.
I had the TPS cause issues similar to what you are having on a '85 Chrysler product, but not on any other vehicle.
On the Embarassing side....it happened when I was sitting at a red light next to a police car, and my RPMs reved up so bad that I had to shut the motor off.
I pictured the officer looking over at the sporty looking car next to him thinking I was wanting him to race me (NOT).

bought iac off ebay, and it was not motorcraft.

Can buy a motorcraft brand for $60. wondering if maybe I just got a cheap made in china POS and that is why it didn't work.

I will check the PCV hose and valve.... I think I changed those about 4 years ago when I did the isolator bolt repair...

may order the motorcraft IAC and try it... Van has 170K on it I think... original IAC

I am curious about the dimming lights. Could you take volt meter and measure, while the engine is going up and down in rpm, the voltage at the battery terminals ... then the voltage on the cables, then the voltage at the alt terminals, etc, etc. Please report back with readings

kinda did that at work.

I work with a guy who is a electric guru. Tested volts/ and amps, his tester had a scope to look for noise (suspected bad alternator, or bad voltage reg) voltage was good

Alt tested good, no issues... built in voltage reg...

my original iac has a spring looking plunger on the lower part... the ebay IAC i got didn't have anything in the lower chamber.

Battery is a year old... alt is about 6 months old.

Dimming lights would suggest varying voltage ... or it would to me, at least. Check with the guru.

had a second guy (former Ford mechanic) suggest voltage issue.

when i unplug IAC everything smooths out and no RPM bouncing....

It sounds like the IAC may be OK. Idle problems are usually associated with a vacuum leak, a leak after the MAF sensor, or the MAF sensor itself. The MAF sometime read incorrectly when it is dirty. It can be cleaned with solvents. Air leaks are fairly common. I bad gasket or a disconnected vacuum line can be the cause.