Grr! Disco idle (again)

[markyc]

Apologies for repeating myself, but first the phone rang and I didn't know if I'd hit the submit button, then I realised I hadn't added a title! Third time lucky...

Hi fellas, A member for one day and a tech query already!
Although I've only had my Disco a week (96 V8i, 200,000 km) the dealer will (hopefully) solve the prob for me but I'm curious about this one.

Gas: Runs great, starts in less than one sec but there's a slight uneveness to idle; not enough for the tach to register but I can feel it.

Petrol: Runs great but more often than not, at lights, the idle goes from around 1100 to zero, in fact she stalls if I'm in neutral.

I've read a bit about a 'stepper motor' and in fact I can hear a motor sound revving and dying, (following the engine rpm) only when in petrol mode and while I'm having the problem. If you listen to your windscreen washer motor as it's pumping water through, you'll get the idea.

Why no motor sound when switched to gas? Is it a crook fuel pump? And until the prob is fixed, am I doing damage doing freeway miles on petrol and switching to gas when I know I'm going to hit traffic or lights etc.?

Ta!

[feral]

I'm not the expert here but I have been told that the stepper motor is used on the 4.6L in a RR. If you have a 96 Disco, it should have the 3.9L. This uses the vacuum/mechanical diaphram on the distributor. These components have a high failure rate and is about $250 to replace.

As I said before, I am not the guru here so maybe someone else can confirm this :roll:

Cheers,
Lyndon.

[MacMan]

If there is a corresponding electric motor noise with the problem, it could be the fuel pump. However the pump will cut out when the engine dies or stops because the fuel rail pressure is no longer dropping. Depends which is happening first - if the electric motor starts to fluff around BEFORE the engine farts off to nothing and stalls then I'd suspect something in the petrol system. If not, it is a symptom not the cause.

LPG running requires a much stronger spark to run smooth, so any slightly uneven running on gas could be an indicator of an low level ignition problem - I found out the hard way, and was surprised at how dramatic the difference can be.

[incisor]

ignition module been checked?

[PhilipA]

3.9 s have a stepper motor, but its just not as big or effective as the 4.6 one.
The function is to stabilize the idle to a target of 800RPM in neutral and 550 in drive, and it adds air if the aircon is switched on etc. If the base idle is correct a faulty stepper should not cause the engine to stall

This may be the stepper, but probably not.
A surging idle is common when the mixture is lean. It may be a crook fuel pump. You would need to test the fuel pressure and look at the CO level to see if the idle mixture is correct.
A gas backfire can damage the MAF, so this would alter the mixture. This is probably likely..
We have had several gas caused problems on this forum, ranging from seized injectors to blown up MAFs and flappers.
So really you would have to check every component in the injection that can be affected by the gas. If thefuel pump is turned off by the gas, the first thing I would check is the relay and circuit which has been added.
regards Philip A

[DEFENDERZOOK]

<span style="color:blue">i thought the fuel pump ran constantly and fuel pressure in the fuel rail
was controlled by the pressure regulator....


it is more likely the fuel pump......
they are known for dieing......
or a blockage in the fuel system...such as fuel filter....


if it was the ignition system it would play up irrespective of fuel type.....</span>

[tcns]

Test the resistance of the air flow meter - I bet you it is out of range

Tom