An accidental cure !!!

[Merlin]

My 1990 RRc has been very erratic on tick over for some time now.  I put a thread on earlier but none of the iteas put forward cured it. It ticks over OK after start up (when cold) but when I pull up at a shop or anywhere when it's warmad up the engine revs up to 1800/2000 & then slowly drops down to normal 550 ish. Also when I start up when it's warmed up it does the same.  I have done the following  -1) cleaned & oiled the Stepper Switch & fitted a new washer--2) checked the timing-3) replaced all plug leads, dizzy cap & plugs--4) new coil.  + a general clean up of all pipes/wires/ flame trap etc relavent to that area.
None of these worked BUT I then adjusted the "Base idle speed setting", following the steps in the"Grey Bible" --FUEL SYSTEM, 19 Fault Diagnosis (page 4) I barely adjusted the setting & the car was ticking over great.  I reved it up a few times & it returned to proper tick over straight away, I took it for a run, still with the hose blanked off at both ends & performance was normal in all ways, stopping, driving, starting.
I have left the Air by-pass hose blanked off at both ends for a week now & the car is running great IN ALL WAYS.
Will this do any damage if I leave it like this permanantly ? & what effects is it likely to have, if any.
Also can anyone tell me how this has happened !!!
Cheers--Merlin

[Range Rover Blues]

Yes, I'd say you havn't adjusted the base idle speed setting properly as it should only run at about 400rpm which is not enough to charge the battery.

The idle stepper motor increases the idle to 800 plus or minus a bit and keeps it there irrespective of the load on the engine.  The ECU knows when the heated front screen and/or Air conn is switched on to give it a fighting chance of preventing the engine stumbling, plus it knows when the car is in Neutrla or park on an auto.

If your engine is reving up when the steeper motor is back inplay I'd suggest the ECU might thing somehting is being switched on or that the rev pickup from the coil is faulty giving the ECU a false reading of the engine's speed.  It's also possible that the stepper is sticking.  The ACU only knows where the stpper is by how far it's move IIRC so if it sticks it may have to 'experiment' with it's position, hence the over-reving.

[Merlin]

Thanks for that RRC, most helpful.
What's next I wonder!!!!