RB20 Idle issues
 

What problem does this sound like to you?

My GTS-T idles at about 1500, while frequently fluctuating rapidly between 1200 to 1900. Its revs on its own up to 1900, then falls to 1200 and repeats. This all occurs in out of gear of course.

Any input would be helpful. Thanks.

my car does this ever now and then also

x3 but I think mine revvs even higher. I thought it was the gas leak I had (the O-ring on my sending unit) and there was air getting in with the gas but that wasn't it.

Bad IACV?

check and make sure you have no vaccume leaks, and if those are solid then make sure all your grounds are solid

Things to check

Vaccum like said above
TPS voltage
IAC Voltage

+2... get a volt meter and check.

make sure to unplug the tps, start the car, then adjust the iac screw.

Where can the IAC screw be found? My vacuum lines seem to be doing okay. I fixed the O-ring on the sending unit (I though maybe the fuel was getting oxygenated)

My water pump leaks, though. Could that have anything to do with it? Where can I find a Nismo water pump/seal?

All the above;

1) manifold leak, can sometimes be a leaky brake booster.
2) IACV, plenty of info how to fix that & adjust the idle screw on the web.
3) TPS closed switch mal-adjusted. I found nissan datascan & the consult adapter good for all this stuff. (V1.4 of nissan datascan was free, & still works if U can find it). Fire it up, and rotate the TPS at idle until the throttle open/closed indicator just barely toggles closed.
4) Here's one not many know. Under the throttle body is what passes for an automatic choke. I forget its name. It's held on the the intake manifold by 4 bolts. Connector in one end & 2 hoses either side of a flattish bulge in the middle. You can get to it by removing the throttle body. Be warned, it's a bugger to disconnect & get out.... exclude all other possibilities first.

Look down the barrel of the hose connections, you should see a baffle. Stick a multi-meter on the electrical connector, I think it should be 75ohm, but don't take my word, look it up. If there is about this much resistance, connect a 12V power supply to the unit (I don't think polarity matters, but U can find out by testing the connector that plugs to it, which will also tell you if it's getting power!)
Next, immerse the bit that bolts to the manifold in hot water & boil the water while the 12V supply is connected. Don't melt anythang. Note the current. The baffle should rotate around, and the hole between the hose connections either open or close (I forget).

If this does not happen in say 10 min. then this air control valve is not functioning, and will cause hunting, rich running at idle, fast idle and an ineffectual idle speed screw.

If it's not working....

If you pull it apart (be very careful of the gasket and be aware that it will have a tendency to fly apart, with a tiny spring compressing itself completely and a bakelite-like rotatey thingy jumping off its perch. Oh, & don't lose that pivot pin either!

My experience is that the problem with this device is that the baffle has a habit of jumping off its perch, especially at the track. A temp fix is to put it back on its perch with the spring disconnected (U will just have to figure that one out yourself). Then pull the spring into place. Hold the baffle down by the pivot pin, and carefully re-assemble the unit, making sure it does not fall off its perch again and spin around to the fully compressed spring position. A more permanent fix is a thin brass shim washer between the pivot pin head and the baffle, on the side opposite the little frob that fits into the dome on the baffle. That helps hold the baffle tighter on the frob. (Very technical terms here ;) ).

This failure happens on older motors after they've been revved hard at high boost, but noone here would do that.....

The alternative is to replace it. Good luck finding one.

the prob was the AAC valve, replaced it with one from a L28 motor. Idles like a champ now.