Thread: 3.9 surging on idle

From my limited experience it sounds like a tuning issue...if it hasn't been done for a while I'd replace things like plugs and leads first to brand new ones (can't hurt anyway if they're due) and that alone sometimes works wonders, it did for me when I had that same exact problem on my previous Disco which was a 3.9 V8. Also check the coil to make sure it's giving off a good healthy spark. Check timing with a timing gun or a get a garage to do a tune up if it's due and if you have LPG on it make sure you get good spark plugs which are recommended for LPG (a couple of users on here will advise which ones) and if you want to get really thorough adjust the spark plug gap to suit LPG (only if you have LPG).

The plugs and leads alone have worked for me in the past a few times when this happened.

Thanks Young Angus.I should have said that the engine has just had everything done to it tuning wise.I just fitted a new engine with 70,000 kms on it and have replaced what was needed and adjusted everything that could be.The engine is running beautifully but occasionally the revs go up and down a bit when in drive and just standing still,like at traffic lights.I have also cleaned the idle control valve.The only thing still to do is replace the ignition amp with a bosch one I have already bought but not had the time to fit.
Regards
Shane.

Ahhh okay no worries, well in that case I'm not sure what else it could be but things like the ignition amp and other electric ignition controls could be at fault but like I said I'm not sure...I'm sure others will have more helpful suggestions

check the stepper motor near on plenum try cleaning it out ...(check for how to amongst the threads)

Originally Posted by sostok

check the stepper motor near on plenum try cleaning it out ...(check for how to amongst the threads)

Thanks.I have checked and adjusted the stepper.The engine was idling really well today on a short drive so what ever is going on is a bit irregular in dead.

Clean the sensor wire in the air flow meter with throttle-body cleaner; check for air leaks on the inlet manifold and hoses and have a read of this site Rover 14CUX Hot Wire Mass Flow EFI: Service and Troubleshooting

and this one Standard ECU

Mine did the same thing intermittently but doing one of the above fixed it. The road speed sensor can also affect idle, according to the workshop manual,

Cheers Charlie

Mine's doing the same mate. next step for me is checking for vacuum leaks. let me know if you have a win!

Hi
I have chased the same issue down on and off over the last 12 months and eventually found that it was the stepper motor .Same symptoms as described below where it would idle faultlessly in park or neutral and when cold but as soon as it got hot the idle would start to surge up and down after about 30 seconds at idle when in drive or reverse.
I have not as yet replaced the stepper but what I did do was drive the valve to full extension then GENTLY unscrewed the thread till I removed the plunger from the body of the motor.I applied a thin film of high temperature grease, refitted it and it ran faultlessly for 2 months then gradually started the surging again.I have repeated the process twice and on both occassions the problem went away completely.I would imagine the issue is wear in the plastic tread of the stepper motor causing it to stick.
I am not suggesting that this is the fixall solution but it did isolate the problem and eliminate all the other countless possibilities.I am currently keeping an eye out for a new stepper at the right price but in the meantime I can drive the car without it stalling and playing up in traffic.