Unstable idle

My MGF has recently developed a problem with the idle originally the problem was the revs were dropping to till it almost stalled then then picked up again then dropped again and so on, I tried stepper motor reset with no improvement so off with the engine panel and investigate further tried spraying wd40 around to see if it was sucking air in any where but that didn't show anything up so I removed stepper motor and gave it a spray with carb cleaner then refitted, now the revs have stopped dropping down instead it's idling high fluctuating between a 1000 and 1200 with a stepper reset again having no effect. I have checked for faults with Torque before and after I removed and refitted the stepper motor and found nothing.

So before I buy a replacement stepper motor are there any other things I should check
Gary

have a good squeeze around the intake and other vac pipes while it's running; use a home made stethoscope to listen around the engine bay for hissing or uneven noises.
also make sure your oil filler cap is fastened securely.

The rising and falling is definitely the stepper motor IACV trying to adjust the idle to the proper level. The change after cleaning the IACV suggests that it needed cleaning, but now the idle is too high.
The TPS reset merely tells the ecu that the idle throttle position(whatever it is) is 3.9% open.(Don't quote me on the figure)
If your throttle is actually a way off 3.9% open the IACV is confused and varies between settings.
Hence the rising and falling.
This is complicated because the stepper motor/IACV actually does have stepped settings and the ecu will try to get you to one of the steps.

If your base idle is wrong then this is an endless loop of the ecu using the stepper settings to try to get a correct reading. Or if the base idle is too low it will continually stall at idle.

If you vaguely followed that, what you need to do is get the engine fully up to temperature after a good blast, come home let it idle, but disconnect the IACV plug. If it stalls the base idle is obviously too low. Let the idle settle for a few minutes and use the torque app or obd2 reader to see what the base idle is. It should be 850 rpm with the IACV disconnected. 875 rpm with the IACV connected (Assuming no other reason ie vac leak sensor problem.)
If the idle is substantially above or below this then the stepper motor/iacv will constantly hunt.
Reset the base idle to 850 using the set screw, no more than 1/4 turn at a time. After each adjustment wait until the idle settles.

Once you've got reasonably close, reconnect the IACV and see what happens for about 5 minutes. If the idle settles to a stable level near 875, great! If not then you'll need to adjust the idle screw a small amount until you achieve a stable idle somewhere between 870-890rpm. Sometimes you might think you are nearly there and the idle will suddenly change. This is the ecu making the stepper motor move to the next step. Hopefully you will be on the correct step already but if not you'll have to keep adjusting.

Once you are happy switch off and reset the TPS, if you still have the torque app connected you'll see the tps sensor change to 3.9% from whatever it was.

I went through all this a couple of months ago and the idle has been rock solid since.

Had exactly this problem and I didnt want to follow the usual crews advice of changining every sensor, coil packs, plugs, etc.
Checked the obvious as you did and got MG Rover solutions in to put it on T4.
The car was a mare and behaving just as if it had just rolled off the productuon line, making me seem a liar! it would NOT misbehave- it handt been started for a few days.
Anyway, Russell made some adjustments as a few things were out but he suspected the crud/oxidisation on the multi-plug for the upstrean o2 lamda.

You can search him on FB and see his comments on my TF160 from yesterday 9/7/18

Hopefully in the last four years and eight months, the issue has been resolved, and the OP has not been waiting around that long for a response :pop:

that post came to me this afternoon...

Spooky :omg: