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1992 Range Rover Classic - Unstable Idle


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Good morning gentlemen,

I am new to the forum (long time voyeur), so apologies for diving right in with my own problems. Selfish I know. I am posting this in a couple of LR forums as I'm not sure where I feel most comfortable yet.

Anyway, I have bought myself a 1992 Range Rover 3.9EFI 2 Door. Some pictures if you're interested (and to break up the text):



I bought the car and drove 3 hours home with no issues, well, apart from a wet floor, but that's for a different thread.

The engine was running ok, idled lovely, just seemed a little hesitant under load and pulling in 3rd there's quite a judder. I thought I should give her a 'stage 0' anyway to get a known baseline.

Last weekend I replaced the fuel and air filter (genuine), plugs (NGK), leads, cap and rotor (genuine).

I also took the dizzy out to give it a once over. It was very clean and actually the cap and rotor looked very clean, but I replaced them anyway as I had them.

The grease for the weights was a bit old and sticky and the mech advance didn't seem all to healthy. The vac advance was working and holding a vacuum fine. After a clean and re-grease the mech advance seemed a lot nicer.

Anyway, I got it back in the car this weekend (rotor to HT1, less the pitch of the gear, TDC). Everything seemed to hit the marks I made before removal.

Anyway, now for the issues (thanks for getting this far).

She started fine but came down to about 250rpm and nearly stalled, but saved herself by revving up to 2k+. This kept happening in a continuous cycle. Near stall, high rev, near stall, high rev etc etc.

Incidentally when revving up high she sounded sweet and felt smooth.

I tried to hook up my strobe to see if I'd managed to get the timing way out. Clearly with the lack of a stable idle I wasn't going to see much. What I did notice was the strobe flashing, it was very inconsistent on HT1, but when revving up at 2k+ it went back to consistent. Then back down to near stall and hardly any flashes from the strobe.

Things I've checked:

HT leads and their connections. Ok and separated.

Ignition amp wires to the coil. Old but connected.

Base timing. Cap off and rotor at HT1 at TDC.

So the question is, what have I messed up?

Any advice would be gratefully received.


- Richard

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  • 2 weeks later...

What brand was the distributor cap? In not genuine Lucas, then it could be the cause. Try refitting the old one. Likewise, leads can be a big problem if not genuine, and even genuine can have a bad one, so removing one at a time might reveal a dodgy one. The mass airflow sensor can cause problems - make sure it can move freely and that the electrical connections are clean and secure.

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  • 4 weeks later...

Just jumping in here, the '92 has a MASS air flow, no moving parts! the earlier Volume Air Flow would not have been fitted on a car this "new".

I'd assume by 92 it would have lambdas, in which case once running the EFi should be running closed loop and ignoring the MAF, but you can check this by unplugging it before starting up. With a broken MAF it will start and run better with it unplugged.

Now the ECU controls engine speed with a stepper motor, it's a black screw-in jobby in a housing on the back of the plenum, there's 4 wires in it. you can try unplugging this and see if the idle speed stabilises, it may cut out though.

When you turn on the ignition you may hear this resetting, it's supposed to so don't worry. It has no feedback so it screws all the way in then counts the pulses that drive it out. If the valve gets sticky it can get confused.

But I would try putting all the old stuff back on and try again. Also try cracking the dizzy onwards just a tiny bit. When working on an ignition system like this one it's best to change one thing at a time and let it settle into it's new home, a bit like buying a puppy.

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