Engine Idle Speed - Hunting

[MPoxon]

My Griff recently has begun to have an issue with the idle speed. It will often idle at around 1400 for 20 seconds, then drop down to around 1100 for another 20 seconds and then settle down to the correct 950RPM. The annoying thing is that it does this when driving along so driving it slowly is challenging. If i dip the clutch while driving at 30 then often the revs will increase to 1400.

Funnily enough sometimes when you drive the car it is fine and drops straight to 950RPM.

Any ideas?

[blitzracing]

It could just be a dirty stepper motor, so first off try cleaning it. If that does not fix it then it could be a throttle pot out of range (ie the ECU does not get the correct throttle voltage level for idle) or the car moving signal is still being fed into to the ECU, as its programed to raise the idle when the car is moving to reduce engine breaking, and simply sticks at this level when the car stops.

Ive covered in in some detail here:

http://www.g33.co.uk/fuel_injection.htm

[MPoxon]

Thanks for the advice and the link Mark, much appreciated. I shall give that a go.

[Richieboy3008]

Throw it away?

[blitzracing]

Wrong forum mate, not the Nissan one.

[MPoxon]

[redacted]

[blitzracing]

You don't need to live with an over high idle, its definitely a fault. One of the following:

Air leak, sticky stepper, throttle position sensor voltage too high, speed signal still present, base idle set too high. Its also been rumored that certain fueling fault conditions make the ECU hold the idle high to protect the catalysts, but Ive never managed to replicate this fault but this would throw a fault code if it happened however. The speed signal is the only one that might cause a mechanic a problem diagnosing, as on some TVR's there is a box of electronics under the dash that generates a fixed speed signal at anything over 3 mph, and apart from some poor soldering, the way the electronics is designed can lead to a situation where the speed signal is sometimes present even when the car is stopped. Ive not tried measuring it with a test meter, but this is the signal, that drops to near 0 volts when the car stops. Its found on a yellow wire pin 6 on the ECU plug.




Plug layout here:

http://www.britishv8.org/Articles/Rover-14CUX-EFI....

[geofft46]

i have a 4ltr that does exactly the same thing,its like having cruise contol at about 30mph but no way of turning it off.if i dip the clutch it sometimes stays about 1500 sometimes it just drops to 900, i have cleaned stepper motor and chamber then replaced stepper motor and replaced throtle position switch none of these have fixed it,but with new stepper motor it doesnt happen as often

[blitzracing]

What it should do is hold the idle at around 1500 rpm if you dip the cluch and let the car keep rolling and this is to reduce the engine breaking when changing down. It should then drop fairly rapidly as you come to a stop when the speed signal dissapears below 3 mph. Interesting about it keeping the car moving at 1500 rpm as I suspect a lot more air is getting in to give 1500 rpm under load, compared to 1500 rpm between gears, but off the top of my head I dont know why. Its certainly worth checking the throttle pot shut voltage first of.