Base idle/stepper weirdness - is it haunted?

'93 LSE 4.2. Recent top end rebuild, runs OK apart from erratic idle/bog down/stall.

Been here before, but this time the previous solutions didn't work.

Everything is sparkly clean, heads, plenum, rams etc, were acid dipped.

Retimed static ign, checked airgap; vac advance and weights both work. Stepper works (I have 2 and test them by leaving one screwed in and connect the other, switch on & start and the plunger moves in and out no probs). AFM hotwire is cleaned, and all connections seem clean tight and dry. No vac leaks that I can find. Thermistor resistance is within the various temp values as shown here:- Rover 14CUX Hot Wire Mass Flow EFI: Service and Troubleshooting.

Recent diagnostic test showed trouble codes:-
DTC - 0021 / Status - Present / description - ECM tune selection resistor.
DTC - 0048 / Status - Present / description - Stepper motor/air idle valve.

(@1100 rpm):-
Coolant temp ---- 72.42C
Calculated position of air idle control valve , (nominal 15 - 25) .. 171 steps.
Bank A fuelling feedback status ...... +17.37%
Bank B fuelling feedback status ...... -1.01%
Av injector opening banks A & B ....... 3.14ms

(@1947 rpm):-
Coolant temp ---- 74.58C
Calculated position of air idle control valve , (nominal 15 - 25) .. 154 steps.
Bank A fuelling feedback status ...... +19.6%
Bank B fuelling feedback status ...... +11.55%
Av injector opening banks A & B ....... 3.38ms

The discrepancy between the Bank A & B fuelling status seems quite big and I assume that the correct values should be around zero - ie neither rich or lean. Is that correct?

Decided to try resetting base idle as recommended after major work. BUt clamping the air idle tube has no effect on idling at all (engine hot) other than a strange ghostly continuous 'whooo' noise (sorry, it's the only way I can describe it). As far as I can tell it's coming from the AFM/trunking/plenum area, removing the air filter has no effect. Unclamping the pipe stops it immediatelyI can't track it down any nearer because of the nature of the sound, tried listening through a bit of hose, but it's too indistinct.
Now I may have got the exact sequence for setting the base idle wrong, there are several versions around, does anyone have the definitive procedure?

Turning the idle screw a couple of turns either way seemed to make no difference. The throttle butterfly valve wasn't disturbed and there appears to be no play in the shaft.

I need to see if I can find the tune resistor mentioned in the fault code 021, I believe it's in the loom about 6" from the ECU, but no one's sure when they stopped fitting them, it all seems a bit of a mystery . . .

Any ideas anyone? All suggestions will be appreciated no matter how outlandish . . . (matches/petrol and similar solutions are not yet relevant)

Thanks - Alastair

I may stand to be corrected but I would suggest you could have an inlet leak. The reason I think this is likely is because an air leak after the AFM would upset fuel mixture and the uneven idle speed would cause the ECU to try and compensate by winding in and out until eventually the system can't keep up with the fluctuations and the mixture goes too far out and the engine stalls.

The tune select resistor is quite easy to find, trace the loom back from the multi plug on the ECU, it should be in a clear plastic sheath on the outside of the loom. I assume you have Lambda sensors (also called Oxygen sensors). With lambda control the system will control tickover by adjusting overall fuelling via long term fuel trim value, then in the mid range a short term trim value is used (closed loop)which gives small rapid corrections to fuelling and then at WOT (full power) there is no lambda control (open loop). If the lambda sensors are failing (lazy) fuelling may become unstable at idle causing the problem you describe. The other factor in variable fuelling will be the throttle potentiometer.

So, I would think check for inlet leaks - base of the ram housing, plenum chamber and air inlet pipe (AFM to Plenum) then do a compression test. It might then be worth checking lambdas and throttle pot.

The only other thing I can think of could be (possibly) a cam or valve issue but I would have thought you would have some other symptoms (like odd noises).

Hope that gives you something to go on.

Thanks Kev for your suggestions - yes it's got lambdas, hence the fuelling values in the diagnostics. I agree that it feels like an inlet air leak, but I'm b******d if I can find it, there is a definite hiss around the AFM to plenum tube (through the hose stethoscope), and I've sprayed it with WD40 with no effect on the running, taken it off and blown through it underwater - no bubbles. Might try wrapping it with clingfilm next.

The intake manifold/plenum and ram housing were assembled with blue hylomar, so I'm pretty sure there's no leaks there. There's no odd mechanical noises.

I'll look for the tune resistor tomorrow, I know the value of the resistor, then try the base idle again - if I can find the right procedure.

A

I know you say AFM is clean but have you checked the CO Trim value and air flow signal although you should note with lambdas CO trim is effectively ignored. For catalyst engines, CO trim should be 1.8v, at idle, the air flow signal should be something like 1.7v (on a 3.9 so maybe a bit higher on 4.2) and then up to something like 4.5v at full load.

Ignoring your lambdas as I dont have them on mine , I had the same problem and the was the Throttle potentiometre being to worn. in addtion it didnt help, and made the syptoms much worse that the timing was 1 tooth out. the symptoms were exactlyt he same as yours , even the whistle!