Idle Surge from start

Hi all, thanks for having me. Looking for some guidance.
My XJ40 3.2 on start up revs high then drops to bounce between 1100 and 1500 rpm. I have cleaned allair intake system, including idle valve and have replaced the stepper motor. I have checked all vacuum connections and pipes, around gaskets etc without finding anything wrong. I have checked voltages at stepper motor, throttle body, MAF and checked resistances of temperature sensors. I have checked continuity between stepper and ECM for harness issues and so far found nothing. I have adjusted my old stepper motor to fully closed and refitted but not connected. Car would fire but cut out until I adjusted the manual bypass. Now starts and stedy idle at 800rpm until warm when it rises to about 900rpm. I did try the battery disconnect procedure for resetting the ECM and the procedure for stepper set up. Unfortunately I dare not drive with this surging so doing the short run part of the reset procedure is not done. Vacuum at idle is about 17inHg and steady. Blocking purge line to inlet manifold makes no difference. The car drives fine with the stepper motor disconnected but I want to get it functioning correctly. I must be missing something fundamental, so hope someone may point me in the right directions.
Thank you

Sorry but I’m a bit confused, are you saying that the idle is now a bit high at 900rpm when hot or are you still having surging?

Hi Keith.

Is it working now because the stepper motor is disconnected?

Surging that cuts out above 1500 is usually (not engine specific) the cut-out speed for the injectors, so the idle would be even higher, but the fuel is cut above 1500 to enable engine braking and the engine then recovers (1200), until it reaches the cut-out speed, etc.

My guess is the stepper motor sets too high an idle… where the command is coming from I don’t know. As a result the engine is fighting itself. I‘m thinking some Signal failure, but I think you looked into it. Something around the throttle? You did about anything I would have come up with. Is the stepper about fully closed in normal hot operation? Is there a proper adjustment procedure (with values)?
Until then you should be fine without it, as long as the weather isn’t too cold. It goes up when it’s warm but not dramatically… but I do understand you want it working right.

Good luck!

Hi Robin, thanks for reading, sorry if I’ve confused you, the surging does not occur whilst I have the old stepper motor refitted as it is in the fully closed position and not connected to ECM. The change to 900 rpm from my setting of 800 I put down to the engine reaching working temperature. I have not yet tried again refitting the new unit, connecting to ECM and going through the set-up procedure again. I am hoping my execution of the set-up procedure is flawed because a signal input malfunction or an ECM malfunction is an expensive fault to repair. Never give up though, someone somewhere will provide that eureka advice?

Hi David, thanks for reply, one area which you suggested may help…position of stepper motor after engine reaches working temperature. When last removed the valve was almost closed, but being a cone air could still blow through easily. I will again go through set-up procedures as command signal issues may prove expensive. Not giving up though. :thinking:

I assume you can hold it in your hand and see how far it opens when the engine is hot and running, but if the idle is way high with the stepper connected and live we can both guess what’s going on. Best investigate the cause for the open valve - does it read the wrong idle rpm? Does it have the wrong values for the valve position? Does some fault cause the valve to go fully open as soon as it is powered?

Keep going…