Engine Code P0850 Engine Idle Hunt Problem
 

I've just finished installing a new engine in my RX8. However there is an outstanding problem that I'm a little stuck with. When idling (and my foot is off the clutch) the engine appears to do this weird idle hunt, continuously revving between 800-1500rpm. Put your foot on the clutch and it goes right away. The check engine light is on and the code is P0850.

There is plenty linking the two on the internet, suggesting the problem lies with the neutral sensor. For a long time I just thought I'd put it back together wrong. I've put a multi-meter across the sensor; in neutral i get a reading in any gear I don't. That suggests the sensors working.

Where do I go from here?

Thanks

Definitely the neutral switch.

Check the connectors. Make sure they are actually connected correctly. When I did my clutch replacement, I plugged white to black instead of white to white and black to black. Blew a few fuses, and in the dark I "solved" it by connecting the wrong ends of each color together.

The whites from the harness to the whites on the transmission, blacks from the harness to blacks on the transmission. For each color pair, it does not matter between the two (white "a" can connect to either white "c" or "d", if that makes sense)


It's possible for the neutral switch to fail, but it's far more common for a wiring harness issue to cause this.

If those connectors at the transmission are fine, check the wiring at the clutch pedal itself. One of the switches is the starter interlock, the other is connected to the neutral switch circuit. I messed up that too a few years back when I meant to work on the starter interlock.

I was gonna suggest neutral switch...

Have you checked your eccentric shaft sensor for debris/connection issues? Did you reset the profile?

This switch does seem to impact idle by telling the ecu how much it can advance timing if engine speed drops below target.

So, I agree with ^ and bet there is a bad ground somewhere between the ecu and the switch, as the switch simply grounds pin 20 on the ecu.

P0850 code
 

Have you gone over what was mentioned in this thread? If not, do so.

Your question is already answered in this thread.