INTERMITTENT erratic idle speed on my 1990 3.9l RR, Lucas Hotwire EFI.
190kms.This problem has been around for 3 yrs and I have never got to the bottom of it. Even a speed garage has failed.
Once the engine is warm, the idle speed will vary from [say] 800rpm down to
200-300rpm and up again over a couple of second. Left for [say] 10 seconds the engine may stop. Happens about 80% of the time when coming to a intersection to stop. The only thing that temporally cures this erratic idle is 2-3 quick burst on the throttle up to 5000rpm and bit of exit dirty smoke, then it seems reasonably happy until the next intersection. It always appears to compensate when putting into 'Drive, AirCon' etc under these conditions, but the overall erratic idle will continue.Always idles beautifully when cold, say 900-1000rpm's.
Drives beautifully once under way, no missing or engine dying here.
Yes, new Bypass air valve fitted and the 4 wires checked back to Computer.
Water & Fuel temperature sensor ohms / temperature as per book. [measured twice and have monitored corresponding voltage back to Computer plug under these conditions.nice and stable so no intermittent wiring.
Also monitored the Airflow sensor 'live' wire, no problem here.
Distributor. Fitted a HDI Distributor 2 yrs ago for 4 wheeling, but this didn't cure the above.
Not aware of any minor air-leaks around intakes /plenum chamber etc.
Idle adjusted, re Austin Shackles reply
Have had the % mixture checked and adjusted twice over 3 yrs to mfd's specs.
Could almost say that fitting new sparkplugs was good for a very short while?
I believe RR's need a new camshaft about this mileage and I do have a noisy lifter when cold.
Possibility of a faulty fuel injector? burnt valves?
Any thoughts greatly received, John Milburn