Thread: 3.9 idling too fast

1991 Range Rover, idling too high as the title suggests. I have the luxury of having a dead 1997 3.9 Disco (slipped liner) plus a living 1994 3.9 Rangie with which I can interchange bits for diagnostic purposes. Here's what I've tried so far-
Removed the plenum chamber from the Disco and cleaned out all the idle passages and other air passages. Removed the throttle linkages and butterfly, everything is spotless; swapped the plenum chamber onto the Rangie.

Swapped the air-flow meter from the disco onto the rangie; also cleaned this out with carby cleaner.

Removed the extra-air valve and cleaned it out with carby cleaner.

Set the ignition timing to 9 deg BTDC.

Blocked off the idle bypass hose and set base idle to around 600rpm.

The motor starts well when cold, runs fine, except that the idle always settles to a too-high speed. the tacho is not accurate, reads around 400rpm but I'd estimate the idle to be around 1000 in drive, and the car has to be held back hard with brakes.

Seems to me that the ECU is dictating the fast idle speed via the extra-air valve. When i got home from work the other night, I screwed the idle speed screw in until I got a nice slow idle, but next time I drove it it settled again to the same old high idle. On initial startup, hot or cold, it revs to around 1500 for a few seconds then settles to the fast idle described above.

I've swapped the extra-air valve with the 94 rangie, it idles perfectly with either valve installed. The 91 RR settles down to the same too-fast idle with either valve installed.

I'm wondering where to look next. Could the under-reading tacho be part of the same problem? I think the ECU gets it's engine-speed signal from the coil, and the tacho from the alternator, so wouldn't have thought they would be related.
The problem has arisen since removing and reinstalling the auto and transfer case. Thought I might have squashed something between the motor and firewall when the motor was lowered, but all seems intact.

I'd appreciate any suggestions, won't have much chance to fiddle with it for around a week now but I'm thinking maybe swap the ECU from the Disco next.

Got any vacuum leaks?