I've been having a problem with my 94 z28's idle. After I drive the car for around 30 minutes and come to a stop, sometimes the engine revs/surges up past 1000rpm. It's enough that I need to hold the brake very hard to keep from lurching forward. I made a Freescan log today of the car idling after a fairly long drive. I left the car in gear for most of the time and you can see around the 29900 mark that it revs to 800rpm (550 desired). I then shifted it to park and it revved up to 825rpm (650 desired) around the 30580 mark. I've done a lot of reading and I'm thinking it might be the IAC, but then again I'm a noobie when it comes to all these readings. Any help would be greatly appreciated. Thanks.

Here's my log

Also just wondering, shouldn't my battery voltage be higher than 12.3?

 

So, what are your IAC readings at idle?

Battery voltage should be more like 14V when the engine is running.

 

Thanks for the response. When the car is in gear and stopped, the IAC is around mid 30's, then jumps to the high 60's when the engine unexpectedly revs on its own (0 TPS). When in park, the IAC is around mid 40's, then jumps to the high 60's again on its own (0 TPS). About the battery voltage, should I be seeing near 14V even with the A/C on?

 

You've got a couple apparent issues, neither of which is likely to be causing the idle surge.

Left BLM in Cell 16 (idle) is 144, vs 129 for the right BLM. Something is causing the PCM to add 14% extra fuel to the left side of the engine..... vacuum leak???

Inlet air temp sensor appears to be suffering from heat soak.... 114-135egF.

As the idle drops, the system voltage drops, as low as 11.8V when the idle dips to 500rpm. Weak charging system or battery.

There are a couple places where there is sporadic data that makes no sense at all. In record 29839 the idle RPM suddenly jumps to 2655rpm for 2 frames. In record 30051 the spark advance suddenly jumps to 34 degrees for a couple frames.

Your MAP seems to be a bit on the high side (38-49kPa). High MAP, erratic MAF readings (6.69-9.91 GPS) may all be the result of the erratic idle, although the MAP might be part of the problem.

You seem to get one brief period of surge from record 28440 to 28500. The only things that appear to change are the IAC and the right O2 voltage. The right O2 voltage seems to hang up at an average value of less than 100mV for a period of a couple of seconds. Hard to tell if that's the response to the IAC opening up suddenly, because the MAF sensor is picking up the increased air flow, and the PCM should be adding the extra fuel, but apparently it isn't.

The question comes down to why does the PCM suddenly decide to open the IAC in that area between 28440-28500. The PCM doesn't just suddenly decide to open the valve without a cause, but the cause is usually a low idle problem, which isn't the case.

Was your AC or defroster turned on when you were letting it idle? Is it possible the AC clutch kicked on and the PCM increased the IAC in response to the AC clutch coming on?

 

Wow, I don't know how I missed those 2 frames of 2655rpm! Thanks so much for taking the time to help me out. Yes, the AC was on low the entire time. By vacuum leak, do you mean I should check my intake elbow, filter, etc? The only thing I can think of related to fuel is that I had 24# Ford Motorsports injectors installed all around after one of the stock ones got clogged. I'm running the stock tune with lowered fan temps for the 160 thermostat via Madwolf. As for the IAC counts and erratic readings, I'm gonna make another log on my way home today, this time with the AC off at idle. Hopefully things will be a little more conclusive.

 

The thing that always bothered me with the FMS or SVO injectors was the way the O-ring seemed to fit very loosely in the intake manifold. Is it possible you have a vacuum leak (leak meaning any air that gets into the combustion chamber without going directly through the MAF and the throttle body) at the injectors on the driver's side of the engine?

 

I'm not really sure about the fitment of the injectors, because I had my regular mechanic install them. Unfortunately, I'm really clueless when it comes to the fuel system. I looked at another log of a short drive, and the car starts off with BLM L/R both at 128. Then after 1 second, the left jumps to 144 and holds, while the right stays at 128. Once on the road and cruising, they even out at around 128-130. About 3-4 seconds after the TPS is at 0 (stoplight), the left BLM will jump up into the 140's again. The right BLM stays at 128 the entire drive. Would it help if I pulled the PCM fuse to reset the BLMs?

 

You realize that that are 18 different BLM "Cells", and that as you move the throttle and the engine load (MAP) and RPM changes, the PCM moves from one cell to another based on MAP and RPM? That's why they change constantly as you drive. Look at your log, and there is a Cell reference for each record. In the one you posted, the PCM was in Cell 16 the entire time, because your engine was idling all the time. As you drive, its going to move over a range of Cells from 01-15, and to 18 during certain open loop or PE mode condtions, and to 17 on decel.

That's one of the purposes of running a log over a wide variety of driving condtions. It allows the engine to operate in more of the Cells, and often you can get some sort of idea of the problem by how the BLM's vary from cell to cell. Whatever you do, DON'T reset the PCM before you log, because that will put all the cells back to 128 and as you drive each Cell will be relearning and changing too much to be meaningful.