Changed the air filter today as the old one was seven shades of grey (kind of like the sky in Scotland this summer) and now it idles about 2000 revs. Hand over the intake trumpet and the revs drop. Methinks the whole thing has previously been set up with the old, fouled bits and now needs reset. Only prob now is I can't find an idle adjuster screw. The throttle linkage stops against a little nub that pokes up through the edge of the plenum chamber. If that is threaded it is certainly an awkward place to try and fit a screwdriver. Any help greatly appreciated.
O.K. so we're talking about a 3.9 efi, so probably hot wire system. Sounds like you need to reset your base idle, it might also be worthwhile checking your CO trim.
Hope you don't mind a bit of a long read -
Setting the base idle is done by first driving the vehicle for a couple of miles and getting it up to normal temperature.
Turn everything electrical off, including A/C if you have it.
Remove the air bypass hose and blank off/seal where it is normally connected. I just sealed one end of the pipe and the "open" connection.
Start the engine and adjust the idle speed to 525rpm + or - 25rpm, the adjustment is made turning the idle screw on the plenum housing, near where the flexi house from AFM connects, there may be a tamper proof plug you need to remove but if not just put the right size allen key in there and give the screw a twiddle, one way will increase revs the other decrease.
Once you have set base idle, switch off engine, reconnect air by pass hose, fire her up and hopefully the ecu will tell the air bypass valve (stepper motor) to do its' thing and put the idle speed up to roughly 700rpm. Simple eh
Now you might want to check your CO trim, that's the screw on the Air Flow Meter, again possibly under a tamper proof plug (if it's still there). What you'll need to do is get a Digital Volt Meter (DVM). Carefully pull back the rubber cover on the plug that is connected to the AFM, you are looking to get a reading of between 1.0v and 1.5v measured across the sensor ground and CO trim.
The wires going into the AFM are as follows:
Red with Black - Sensor Ground
Blue with Green - Air Flow Signal
Brown with Orange - Ign live +12v
Blue with Red - CO Trim
If you're interested the Air Flow Signal and CO trim inputs to the 14 CUX ECU correspond to pins 35 and 22 respectively.
I think I set my CO trim to about 1.25v, I seem to remember there is more data about this on the RPI Engineering website.
Don't forget to check/re-check your ignition timing and make sure there are no vacuum leaks - these could affect setting the base idle speed as well as making sure the engine is up to temperature before you set anything.
The other thing you might want to consider is disconnecting the ECU for a couple of minutes then plug it back in, turn the ignition to "on" listen for the relays switching, then fire the engine, let it idle for a minute or two and see if the idle comes down, if not, drop it in gear and see if that settles it down.
The usual disclaimers apply here, as ou'll appreciate without seeing the engine and wiring I can only make suggestions based on what would be standard, but I hope that helps.