My R is exactly the same.....
Winding the idle screw all the way in drops the idle from 1500 to 1300.
I presume therefore that the idle screw is working (i.e. it adjusts the idle!) - so I'm on the hunt for other causes.
I have been trying to fix this for ages.
What does your boost gauge read at idle? Does it show much Vacuum or not?
After prodding and checking all my boost pipes for leaks (didn't find any) and changing my BOV from a single piston Bailey to a dual-piston Forge my Vac at idle went from just under the zero on the gauge to about -0.6 on the standard gauge.
So I presume I solved a leak somewhere - but the idle stayed the same
I've checked the MAF connections are clean - but don't have a sensitive enuff multimeter to read in mV to check the output from it.
I'm gonna take my bleed valve out next to make sure that's not leaking.
Does your idle drop at all from a cold start to being warm.... when cold mine idle's at about 1800rpm then drops to 1500 or 1300 (depending on whether I have the idle screw all the way in!). This makes me presume that the auto-choke is switching off - which makes me presume that the engine temp sender to the ECU is functioning.
As always - check for leaks / split hoses and that the famous rubber bungs are in place. Also check the throttle cable is not over adjusted and has some free play when on the idle stop.
My next steps are gonna be:
1. Remove bleed valve and reconnect Boost Solenoid
2. Check throttle adjustment bars front and rear.
3. Check timing
4. Check Engine Temp Sensor resistance
5. Check MAF voltages
6. Dump it in the canal and claim the insurance!!!
I think it's just a process of elimination but it can be long-winded and can end up being a dodgy ECU.
Just change one thing at a time otherwise you'll never know what caused it!
Anyone got any other suggestions?
Cheers
GeeTee
8)