Hello

I have a discovery 1 v8 3.9 auto. I have a problem where when driving the revs drop to around 500 rpm at trafgic lights. Intermitantly it will cut out and sruggle to restart. Once running it runs fine again. Any ideas ? I have put a aftermarket idle air control valve on it are these much good or could that still be the fault ? Any ideas or thoughts greatly appreciated.

Thanks chris

If your engine revs drop and and it eventually stalls, it could be a restricted or blocked crankcase breather, so as a simple fix is spin off the flame trap after removing the hose has and washing it in petrol and inside of the hose if necessary. Cleaning the breather is not on the service schedule until there a stalling issue, which happened in my 3.5 RR days and the dealer service dept told me what to do over the phone and it worked… simples.

Next, its a symptom of a pending issue with the ‘spider’ immobiliser which ends in the engine failing to fire up, so if u have trouble starting any time then starting ok for the rest of the day for example, which I put up with for a few months until the engine died, a piece of wire from the battery got it running, that was made permanent with the supply from the ignition switch.

Next, check the ‘base idle’ which should checked is ok, only readjusted if necessary after changing the idle control valve, its something that is set in the factory and should never require touching again, info on that simple procedure for the disco 1 is in a workshop manual.

Finally, don’t have any experience on electrical aftermarket parts, so can’t help, or on idle control valves but I expect they would be OEM Quality now days.

Many thanks for your reply. The idle control valve is in a lucas box. The car is in spain but the breater sounds like a good shout. It idles at about 500 rpm intermitantly before it stalls mainly at or pulling up to traffic lights.

Thanks chris

Chrisvr46 said:

Many thanks for your reply. The idle control valve is in a lucas box. The car is in spain but the breater sounds like a good shout. It idles at about 500 rpm intermitantly before it stalls mainly at or pulling up to traffic lights.

Thanks chris

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Similar in my time also slowing at junctions, very inconvenient with an auto.

Have you had the same issue on yours ?

Thanks chris

Chrisvr46 said:

Have you had the same issue on yours ?

Thanks chris

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Nope not on the disco, but once I thought it was going to
I now have a spare flame trap so every engine oil change I replace the existing with the clean replacement just 5 min job while the old oil is dripping from the sump, then the existing one which looks ok but very oily soaks in a small jar of petrol which soon drys, then put on the shelf for next time. Thats my bit of preventative maintenance… when a auto stops its an obstruction it cannot be cranked out of the way like manual.

Mine is a fairly late 96/97.

Clean out all the idle bypass path around the screw and the whole way through. I used a foaming intake cleaner and some long cable ties afterwards to poke through. It's gets gummed up.
Make sure you have no vacuum leaks. set the timing (I use 10BTDC).
Clean the breathers, as above, including the restrictor in the line to the plenum. And the seal on the dipstick. Make sure the breathers and hoses are connected right too - fuel pressure, little breather from rocker to air filter, bigbreather from "flame trap" to plenum and intake etc.
Check the vacuum diaphragm on the dizzy is intact and clean the plenum port for it.
Check the throttle plate is closed correctly (from the intake to the plate top and plate bottom should be within 0.5mm of vertical (proper closed!). The screw UNDER the plenum adjusts it! - pain in the arse - I made a tool from a small allen key.
THEN adjust the base idle - it should be about 500 RPM with the hoses blocked. You can also use rovergauge to force the idle valve shut, then unplug it quickly, before setting the base idle.
If you set the base idle too HIGH, it actually idles worse on mine.
I have a restrictor in my idle VALVE hose and at 600 RPM idle in Drive, the valve is still only open about 12%? 700rpm idle in Park, is about 8%? all with AC off.

Do you have a CAT or NON-CAT tune? If on a cat tune, with o2 sensors you need to make sure you have no vacuum leaks too, otherwise it screws up the fuel trims.

+1 on Vac advance on the distributor.
Check the base voltage on the throttle pot. This needs to be right so the ECU switches on idle stabilisation. If the voltage is too high the ecu won't correct the idle speed.

HTH