Patterned drop in RPMs when idling Options

[rjames]

My car idles steady when cold, but after warming up a bit the idle will drop every other second from 900 to ~600. *Sometimes* when the engine is warmed up this doesn't happen. Additionly the volt meeter also drops in unison sometimes (or at least it did today). I haven't measured voltage at the battery while this is happening yet since I only noticed the voltage drop this morning on the way in to work.

It's been doing this since I've owned it for the last 6+ years. If it was a vacuum leak I would expect the RPMs to go up, not down, right? I've done a lot of work to this car, including:

replacing all hoses (vacuum and fuel)
replaced fuel pump,
set fuel pressure,
new battery,
replaced the relay board
new voltage regulator.
new plugs, wires, set timing and replaced the points.
new injector
new injector o-rings.
plus other things I'm not remembering at the moment.

All of the above work was not done to fix the idle issue, but to solve other issues or as routine maintenance. None of them had any effect on the idle. Car runs good and strong, otherwise.

Any ideas of what I should check to remedy this?

[Jeffs9146]

Have you done anything to the CHTS yet?

[rjames]

Have you done anything to the CHTS yet?

Wouldn't that only come in to play when the engine is cold, or if it were increasing in RPMs vs dropping? Drop doesn't happen when engine is cold.

-nevermind, just re-read the writeup on the pbanders site. I'll check it and post results.

Function: Senses engine temperature and sends signal to the ECU to provide mixture compensation. Proper part for your application and proper functioning is extremely important!

Failure Modes

Shorted: The ECU interprets a shorted sensor as a signal to lean out the mixture (about 30% leaner). The car may run and start in this condition, but will have poor idle and drivability. Check by disconnecting the sensor from the wiring harness and checking the resistance to ground. Note that shorts are often intermittent, caused by nicks in the sensor wire and by exposed contacts to the wiring harness touching ground. Check by inspection.