IAC Troubleshooting: 1999 Miata
 

I just helped a friend uninstall the JR M45 supercharger kit from his '99 Miata. We also removed all the JR electronics that were installed and returned all engine components and ECU wiring to factory. The car runs great and everything operates fine, except when the engine is trying to return to idle from driving. After pushing the clutch in, the engine catches at about 1500rpm, stays there until the car is fully stopped, and then drops to normal idle after 4 or 5 seconds. It also has a pending p0507 (Idle Air Control System RPM Higher Than Expected) code, but no CEL yet.

The IAC valve got a new gasket when it was moved from the dummy TB back to the OEM TB. The option of the IAC valve being at fault has crossed my mind, but I don't have another one to test that theory with at the moment.

There are no visible or audible vacuum leaks in any of the lines, haven't done a spray test yet, but I did at least try to thoroughly inspect every line. The boost/vac gauge is also still installed and the way it acts doesn't point to a vacuum leak. The fuel injectors were also swapped back to OEM, don't smell any fuel around that area, but could an air leak around one of those cause this issue?

Some research has said this condition can be caused by a faulty PCV valve, but it seems to be ok after pressure testing it.

I've also noticed something that may or may not be related, the speedometer hesitates when accelerating from a stop, it takes a couple seconds to start registering speed. And when coming to a stop, it takes a couple of seconds to drop to '0'. Sometimes the engine returning to normal idle and the speedo returning to '0' happens at the exact same time, but not every time. I'm going to swap in a spare electrical sender I have and see if it changes.

Any other ideas on any of this?

I had the same issue when I swapped turbos and it turned out to be a vac leak. I started reasoning that the IAC was not able to handle the extra pull from the bigger turbo and it was creating the issue but then I remembered that FM sells their turbo kits with the option to leave in the stock ecu so that wasnt the issue. Check all vac lines and while your at it, have your friend get a PCV valve from a 323 GTx as its loads better.

Edit: I just realized you took the blower off lol. Still a vac leak

I did check all the lines as well as I could, but I am going to do a spray test tonight to be sure.
No reason for an upgraded PCV on a stock engine, the OEM piece didn't leak while being pressurized and opens normally too.

*edit* I should also point out that I've already attempted to adjust the throttle cable as well to let a little more slack into the line. No effect. The butterfly does close all the way, or at least it does on the outside.

I had the same issue when pulling out my Adaptronic ecu and reverted back to the stock ecu (everything else was relatively stock.) Drove around for a few kms and the issue went away. No idea what it was.

Got this one fixed. After suggestions on the local forum regarding the idle adjustment screw, I read the JR install instructions and confirmed the idle adjustment screw gets adjusted after installation of the supercharger. Tried adjusting it back with little luck tonight. Checked everything else and had no improvements. Decided to jumper the 10 and ground one more time and adjust the idle lower. It worked.

Jared, you was wrong! :p