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94 Posts
Discussion Starter · #1 ·
'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

94 Posts
Discussion Starter · #3 ·
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.

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.

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