I'm new to the post EA81's and just bought my son a 2000 RS. Thing runs and idles great, but I keeping getting an idle circuit malfunction code. Seems to kick on at all speeds. Most recent light popped on doing about 40mph in 4th gear. TPS? Dirty IAC valve? O2 sensor acting up? As mentioned, no idle fluctuation and the thing runs great. This is also a transmission conversion car.....5-speed from automatic.

Edited September 10, 201213 yr by ettev

My guess would be they didn't connect the Neutral Position Switch wiring, OR they didn't ground the MT identifier pin on the ECU.

All of the manual transmissions have a switch on the side that reports to the ECU when the trans is in gear or neutral. It's actuated by the shift rod inside the trans as it moves forward or back. Very simple, two wires, its either in gear or it's not, and it has NOTHING in common with the inhibitor switch found on the automagics. (don't get 'em confused) Also has NOTHING to do with the Neutral Safety Switch under the clutch pedal.

The ECU uses this switch to determine fuel ratio and set idle speed. Basically, the ECU is trying to bring the engine back to idle RPM after you take your foot off the throttle, but because the trans is in gear while you're coasting it can't. The RPM is high and the ECU sees this as a fault with the idle control.

 

Find a wiring diagram and ECU I/O pinout for that model, check first if the P/O even connected the wires, and then check that they're in the correct places.

The NPS is on the drivers side of the transmission at the back near the tail housing where the shift rod enters the case.

Fairtax4me, I do believe all grounding and transmission wiring is done as you indicate.

Double check it if you haven't already. Beyond that, it's entirely possible the IAC is bad or is in need of cleaning. There are a few different kinds.

I know the cone valve type that screws to the side of the throttle body is somewhat prone to failure, but tends to cause noticeable rough idle issues.

Another type bolts to the top of the throttle body and has a 3 sectioned O ring that can swell and degrade and cause problems. A new O ring is only a few $ from a dealer though.

OK. The odd thing is this car runs and idles perfectly. Still thinking something in the tranny swap bypass wiring. We'll figure it out.