So I had this problem with idling. During warmup, the car would start revving by itself, and the rpms would surge somewhat. After the car was warmed up, things would be mostly normal, usually idling around 800-825 rpm (auto TT) but occasionally when I shifted into neutral at a light it would be at 1000-1100 rpm. Shifting in and out of Drive would sometimes make it go back to normal, sometimes not.

Here is a graph I made from recording the ECU readings during warmup. The surges start about at 125 Degrees, then disappeared when the coolant got to about 140.



You can see that the AAC started skyrocketing to 70%, the extra air gets the RPMs shooting up, then it kind of settles with a smaller surging. The X-scale is minutes (after the hour, not since the car was started) and you can see this behavior stops after about a minute and a half later, but with one big surge just at the end.

I have a new air regulator, and recently had done a boost leak test. The engine is as buttoned down as it can be with all new vacuum hoses, new Z1 intercoolers etc etc. Only question was the AAC, so I had disconnected the bypass hose and sprayed a bunch of WD-40 and Carb cleaner through while the engine was running. Made no difference.

The clincher was when I looked at what the ECU was doing when the engine was surging. Seeing the AAC % jump around like that told me where the problem was, the why (dirty/sticking idle control valves) was not immediately obvious but cleaning was a lot cheaper than replacing.

So I ordered a new IAA gasket, and figured I would remove the unit and clean it properly. Good thing I got a new gasket, half the original one stayed on the intakes. I probably used 1/2 a can of carb cleaner, letting it soak for about 10 minutes, and used a straw cleaning brush to really get into it. When the carb cleaner stopped coming out black/gray, I blew it out with compressed air, and put it all back together.

Tested the AAC and FICD solenoids, both for resistance and for clicking sounds when applying 12 volts.

Getting the unit with the new gasket back on was quite a PITA. Let's just say a little Redline grease makes a decent "glue" to hold it on. But little hands and 3 different sized 10 mm sockets were necessary.

Here is the graph of the warmup after I got everything put back together. Idle is now quite stable.



This is basically the same temperature range as the graph above. The AAC steps down from 48 to 40%, and the RPMS are consistently dropping as the engine warms. Once up to 175 the idling is at 750 rpm and 16% AAC.

I was prepared to buy a new IAA unit ($311) if I thought it would help, but as with anything on our cars you never can be sure exactly what is causing the problem. But the IAA was the last piece of the puzzle left, and luckily cleaning it thoroughly solved the weird idle issues.

Anyway, I am hoping this will help anyone who has run into idling issues that while not severe, can be annoying as all heck.

Madkiwi

(cross-posted to all the 300zx forums)

 

Nothing like solving a problem, with hard data to back it up! Awesome!!

 

Great post man, thanks.