Since I bought my car the idle hasn’t been great, it fluctuates by maybe 100rpm every few seconds but until recently it’s never been enough of an issue for me to care, wasn’t affecting the car in any way

For the past 2 or 3 weeks, the idle has been absolutely all over the place. Some days I’ll wake up, start the car and it’ll stall every time unless I manually add and hold throttle which means when driving, every time I stop I have to restart the car which is annoying. Or alternatively it’ll sit idling at 3000 rpm. Sometimes it’ll stay at 800 rpm as normal but unfortunately that’s less common than these random figures at the moment

I have reset the stepper motor, in fact I do it multiple times an hour when driving because if it does fix itself and go back to 800rpm after a bit of driving it’ll start to go crazy and hit 3k again. I haven’t really had time to look into it further than that so I was hoping someone would have an idea. The radiator fan is always on by the way, I have a feeling that’s related because it only started after the idle went funky. The temps are absolutely fine however so I don’t see why it would need to be on.

Obvious suspects are the idle air control valve but I can’t say I actually know how to test it in these cars since it’s partially electronic. Any help is insanely useful

It’s just so frustrating because it’s just another one of those issues that isn’t going to kill off the car but it severely inconveniences my daily usage of it

It’s a 94 SEi K series, alloy inlet

Where are you based? One of us may have a P-Scan nearby to help with a diagnosis. I’m in Leeds and have joint custody of one.

It may not be the IACV.

There's a TPS reset procedure on the K series ECUs.

First step differs depending on who who's explanation you read.

1. Start the car and let it run, then turn the ignition off (some people say this step isn't necessary).
2. Turn the ignition key to the second position (2 clicks from "off"), i.e. in the "engine run position." Don't do the 3-rd click which is the temporary one that fires the starter motor.
3. Pump the accelerator pedal from its resting position all the way to hard on the floor. Both the "foot down" (to the floor) and "foot off" (back to top) actions need to be done smoothly and gradually so the ECU can read the TPS at lots of different accelerator pedal positions. Don't just stamp the pedal to the floor and take your foot off.
4. Repeat step 3 a further 4 times (so do it 5 in total).
5. Turn the ignition to the "off" position (where you can take out the key).
6. Wait several seconds.
7. Now start the car as normal and see if there's any improvement in the idling behaviour.

Is this the procedure you used when you said you "reset the stepper motor" ?

Erratic idling could also be caused by a faulty temperature or MAP sensor, amongst others.

Johnny 216GSi wrote: ↑Sun Feb 04, 2024 11:02 am It may not be the IACV.

There's a TPS reset procedure on the K series ECUs.

First step differs depending on who who's explanation you read.

1. Start the car and let it run, then turn the ignition off (some people say this step isn't necessary).
2. Turn the ignition key to the second position (2 clicks from "off"), i.e. in the "engine run position." Don't do the 3-rd click which is the temporary one that fires the starter motor.
3. Pump the accelerator pedal from its resting position all the way to hard on the floor. Both the "foot down" (to the floor) and "foot off" (back to top) actions need to be done smoothly and gradually so the ECU can read the TPS at lots of different accelerator pedal positions. Don't just stamp the pedal to the floor and take your foot off.
4. Repeat step 3 a further 4 times (so do it 5 in total).
5. Turn the ignition to the "off" position (where you can take out the key).
6. Wait several seconds.
7. Now start the car as normal and see if there's any improvement in the idling behaviour.

Is this the procedure you used when you said you "reset the stepper motor" ?

Erratic idling could also be caused by a faulty temperature or MAP sensor, amongst others.

That’s right yes, that’s what I’ve been doing. It works temporarily, maybe 10 mins before it starts going crazy again after resetting it

I had this issue on a T-series and it was a vacuum leak. Can't just remember off the top of my head for the K-series, but there may be some vacuum lines which may be split?

im not sure on when the cutover point was - Does your engine have a plastic manifold and throttle body, or the earlier type. Can you take a photo of the engine bay?

rjessett wrote: ↑Sun Feb 04, 2024 11:15 pm im not sure on when the cutover point was - Does your engine have a plastic manifold and throttle body, or the earlier type. Can you take a photo of the engine bay?

It would have left the factory with a plastic one but it actually has an alloy one, unsure when that would have been added but it seems to have worked fine

Paul_1978_yorks wrote: ↑Sun Feb 04, 2024 8:51 pm
I has this issue on a T-series and it was a vacuum leak. Can't just remember off the top of my head for the K-series, but there may be some vacuum lines which may be split?

Will have a look and go through all of them tomorrow, thanks!

I think alloy manifold is 'correct' for your car, in 1994. I have one of those duotone paint SEi's too.
The plastic manifold has different idle air control arrangements too, so would be associated with a different ECU I expect.

Didn't someone on here a few months ago have a problem with the throttle body mounting block, where the rubber detached
from its metal mounting plates? It was T-series, but similar design on K-series. That would admit stray air.

Update on this, removed every single vacuum pipe, washed them out, left them for 2 days to dry and then put them back in. As far as I’m aware, none of them were cracked or leaking. Initially it was fine and actually fixed the issue but then a few hours later it started again. The car also really smells like it’s burning oil, small amounts of blue smoke too. I don’t know if it’s related or if it’s a separate random issue that just started but I actually think the oil burning started at the same time as the revs going crazy

I actually think it could be the ECU more than anything but once again I have run out of ideas

Spray around the inlet manifold/vacuum pipes etc with carb or brake cleaner - if the idle behaviour changes then you’ve a leak somewhere.