Discovery V8i high idle - part 1 - possible stepper motor problem?

I bought this '94 Disco V8 as a non-runner. The previous owner cooked the engine (3.9 serpentine / cats) and had put in a replacement (3.9 pre-serpentine / non-cat), but hadnt finished installing it. It has the original engine wiring loom and the cats are still in place.

The problem I have is that I cannot get the idle down below c.1100rpm. It failed the MOT emissions test miserably giving 4.25% CO (limit is 3.5% for non-cat, which the analyser noted the car as being). HC was 88, so well within limits.

I have had the plenum chamber off, given it a good clean and sealed back down properly to eliminate air leaks. All vacuum pipes are also fine as far as I can tell. I have tested swapping out the stepper motor, throttle potentiometer and coolant sensor from the original engine, but the only difference is that the replacement engines stepper motor makes it idle just a tad lower than shown in the video at around 1000rpm. I read that idle should ideally be 800-900. Engine has new sparkplugs and new leads.

There is no erratic revving, but when I do rev, it hesitates before rising (timing?) and then takes a while to settle back down once Ive throttle-off as can be seen in the video, the revs drop in stages.

Near the end of the video, I squeezed the bypass pipe (throttle body-rear of plenum) and the revs did seem to drop. I have also tried the trick to get the stepper motor conical core out to clean it, but it would not move. This is making me suspect the stepper, but the doubt I have is whether both steppers could be playing-up?

As I cannot get the idle down, I cannot set the dynamic timing correctly (see my other videos for distributor/oil pump saga).

The video is in 5 parts:
1.Cold start.
2.Warm engine
3.Tour of engine bay
4.Test of squeezing bypass pipe
5.Final look at tacho

Im a bit stuck at the moment. Apart from needing to sort some ABS sensor air gaps, the engine idle and then timing is all I need to do before I can get it down for an MOT again. Any ideas/thoughts greatly appreciated. Thanks.

You can find part 2 of this at    • Discovery V8i high idle - part 2  ... there is also now part 3!