The car seems to have a weird spot in it's RPM. It idles just fine.

With the car stopped, if you rev the engine to 1500 and hold it, it will rev on it's own up to 2500, then drop back to 1500, then again to 2500, and back to 1500 all on it's own. It does this about 15 times per minute.

This also happens when the car is breing driven at low speeds.

I suspect the Throttle Position Sensor - is there a way to test it?

I also suspect the ACV / IACV - is there a way to test that too?

Here's how to adjust the TPS. Instead of using a wire on the connector, I use a sewing needle and multimeter leads with alligator clips, it makes it a lot easier. http://www.twinturbo.net/net/v...60871

I doubt that is your TPS or IACV causing the issue, but both worth testing. If you remove the connector on the IACV and touch 12 volts - & + to it, you should hear it click indicating the solenoid is working. It is worth setting the idle the proper way after you confirm the IACV unit is working. Warm the engine, unplug the IACV and let it idle. Adjust the idle screw on the side of the unit to 650, and then plug it back in while it's running. It should increase and then settle to about 725~750. It should idle dead smooth with no fluctuation or miss.

The very first thing to do is run ECU diagnostics including the o2 sensors test. A bad o2 can cause hunting idle and poor power etc., but so can vacuum leaks and some other things like bad Coolant Temp Sensor or corroded connectors.

http://www.ttzd.com/tech/diagnostictech.html

Really, each component of the EFI system should be checked and tested to prove and eliminate any source of problem.

In the manual exist the specs to test each component starting around page 178 of Fuel and Emissions Control section. P42 is Trouble Diagnosis.

http://www.300zx-twinturbo.com/cgi-bin/manual.cgi

I had a 77 280z that did the same thing. (Those were electronic feul injected, and I worked on it ALL the time to keep it running right.) It would surge and then idle down and it especially seemed to do it when you were coming down through the gears like if you came to a stop light it would slowly come from 2k rpm down to idle speed and then start to creep back up. My whole problem was old brittle vacuum lines. I had replaced a couple of sections of line, but I finally just spent an entire weekend replacing every single bit of rubber hose I could find on that car. Not only did it run better after that, the surging stopped and my heater worked better because the vac operated flaps in the duct work were all opening and closing correctly. Not saying that your problem is a vacuum leak, but it could be worth checking into.

That brings up a very good point. I started a new thread seeking a good Vacuum Line Diagram. The one in the Online Service Manual is hard to read.
Modified by leerandallstewart at 3:27 PM 1/17/2010