3.9EFI starting idle & mixture problem - help!

Hi all.
I have a problem with the initial idle on my 93 RR classic. It revs to around 2000 rpm initially after firing and stays like that for about 10/15 seconds or so before falling (in steps) back to its proper level. It does this whether it's hot or cold, although in the very depths of the last snowy winter it didn't seem keen to go down for quite a bit longer.
It idles ok once the usual idle speed is reached and drives normally. My only comment is that the idle seems a bit "breathless" occasionally - almost like it's running a bit too lean.
I changed the stepper motor but this hasn't made any difference. There don't appear to be any cracks or splits in any of the hoses but I've only done a brief check.
I noticed that one of the lamda sensors was loose in the cat and tightened this but again this hasn't made any difference.
I'm sure it's connected to an issue it had at last MoT - the first emissions test it read over 4% co. It was then revved to 2500 for 30 seconds or so and the test was repeated - this time the result was virtually zero co!
All and any help or advice very gratefully received.
Cheers, Mike

Sounds like a sensor issue of some kind.

It's not as complex as the later Thor Motronic system, but still a lot to go wrong.

The Thor system raises tickover for 20 seconds when cold then drops back, so yours sounds as though ti it trying to do the right sort of thing.

Check the MAF sensor (on the inlet pipe from the air filter) and 'possibly' the temp sensor as well.

I don't know if you have a camshaft 'and' crankshaft pickup, but if so, the camshaft one would be worth a check. The crankshaft sensor normally causes the engine to stop if it fails, while the camshaft sensor failure has a fall-back position in most systems.

HTH

Peter

Thanks Peter
Can't find any mention in the workshop manual (the genuine LR one!) of a camshaft or crankshaft sensor. I think I mkght try to get hold of a couple of s/h repalcement sensors as you suggest.
I also wondered if it might be the throttle potentiometer?
Cheers
Mike

What injection system has it got?

There should be a speed pickup 'somewhere', but it would also depend on the type of engine management system fitted.

Lucas Sagem and Bosch Motronic both had them, but the EFi may have been a simpler system.

Found this on Rimmer's site:

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

There are two types of Efi system fitted to the Range Rover:
The first system - Lucas L-Jetronic - uses an airflow meter, inside which is a hinged flap that measures, by deflection, the flow of air into the engine, thus varying the voltage signal to the ECU.
The second system - introduced to coincide with the 'new' 3.9 litre engine in 1988 but also fitted to some 3.5 litre engines - incorporates a 'hotwire' air metering system, which contains no moving parts. Instead, it employs two sensor wires - one of which is heated - to measure air mass as it passes over the wires and into the engine. A voltage signal is then sent to the ECU proportionate to the voltage required to maintain the temperature of the heated wire as it is cooled by the air flow.
The flap type, fitted to the first 3.5 Efi (non cat'-equipped) models from 1986, is similar (though not identical) to the system fitted to the Rover SD1 Vitesse from the same time. The hotwire type, fitted to later 3.5, 3.9 & 4.2 vehicles (with or without cat's), despite appearing quite similar, shares few parts with the flap type system.

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Peter

It's running a stock hotwire system
Cheers, Mike

Thanks, Mike, that helps us narrow the options down a bit.

Peter

Mines doing the opposite and Idling too low some times , particuarly when warm started.

Want to give me some rpms lol

fett said:

Mines doing the opposite and Idling too low some times , particuarly when warm started.

Want to give me some rpms lol

Click to expand...

If the engine is breathing ok and the ignition componants are also ok and the by-pass air valve is not sticking then check and reset the base idle.

If none of the suggestions work, try a substitute ecu.