Please help me before I torch it!!!!!

[simon t]

I have a 96 3.9l Discovery which will not run properly. IT ticks over - just but as soon as you touch the throttle it splutters picks up a bit then dies til the revs get back to tick over. The problem seems worse when the engine is hotter.
Ive changed the plugs and leads, the coolant sensor seems OK, the throttle pot appears to be working...

Any ideas anyone??

[GreenV8S]

Does it use an mass air flow sensor? I got identical symptoms on the V8S when the AFM started playing up - it would idle OK but barely responded at all to opening the throttle. When I disconnected the AFM it went into fault mode and ran rather poorly, but did at least respond to the throttle as normal.

[CraigAlsop]

simon t said:

the throttle pot appears to be working... Any ideas anyone??

How do you know the throttle pot is working? When I tested a known bad pot on a multimeter, it seemed fine...

[simon t]

Hi Pete,

It's a hot wire system although the symptoms you describe are the same...

[simon t]

Craig,

I got an ohm reading across the two outer pins, when connected to the car I saw a voltage change on the centre pin when I opened the throttle... could still be faulty though?

[CraigAlsop]

simon t said:

Craig, I got an ohm reading across the two outer pins, when connected to the car I saw a voltage change on the centre pin when I opened the throttle... could still be faulty though?

I'm not saying it is, but I wouldn't rule it out, as all the tests I did on one that I knew to be faulty returned fine. I even destroyed it in the process to see what state the carbon track & contacts were - I couldn't measure any obvious problem - I would imagine the voltage change has to be smooth, without any momentary spikes.

[Pigeon]

If it is that, a good dose of contact cleaner should sort it out (Note: proper contact cleaner, NOT WD40, WD40 will ruin it completely!)

[nighthawk]

The hot wire system gets the same results as a vane type meter, if it fails you will have the results V8S posted.

Don't spray anything onto the wires though, especially with the ignition ON!

[GreenV8S]

simon t said:

Hi Pete, It's a hot wire system although the symptoms you describe are the same...

Just like mine then.

On mine the throttle is only used for transient response; with the throttle pot disconnected, it gets a terrible flat spot when the throttle is opened but it runs happily at constant throttle. When the AFM went sour I got the opposite symptoms, it was really sick on constant throttle but would still respond to the throttle if I pumped it - I think it was basically running on the transient enrichment. This sounds rather similar to the symptoms you're getting.

By the way, I'd advise you against touching the hotwire element and don't try to clean it in any way; apparently it is extremely sensitive to surface contamination and I suspect it would be very easy to do irreversible damage. It's an expensive unit and not something I'd be inclined to fiddle with.

[Pigeon]

Yes - in my post "that" referred to the throttle pot, following Craig's suggestion that that might be dodgy but not discernibly so with a meter.

[simon t]

OK chaps, a new throttle pot it is then. I'll update when I've fitted it - many thanks for the help

Simon

[steve_D]

While you have your credit card out get a water temp sender as well. On the 3.5 it is one of the most common failures and difficult to test.

Steve

[simon t]

Hi Steve,

I disconnected the sender and inserted a 300 Ohm resistor into the connector - no difference so I think the sender is ok?

S

[GreenV8S]

Have you given up on the idea that it is an AFM fault? That would still be my first guess.

[simon t]

Hi,

Ordered a throtpot from Paddock Spares today so I'll find out tomorrow. I guess it could be either the AFM or the Pot.
I worked theough the 22 step diagnostics document I found on the web, all of the volt meter, resistance checks and the fuel pressure were OK so unless it's the dissy (Any ideas how I test that at revs?) I guess it's one of the sensors giving a dirty signal or perhaps..... the ECU

F**king thing, I've traded it in for a Toyota and have to take it to the dealer next week

Simon

[simon t]

So.....

Not the throtpot
Not the MAFMFM meter thingy
Not the temp sender
Not the ECU

Air gap in the distributor trigger too small!!!!!!

I guess it was getting smaller as the shaft started to wear and oscilated in the disy housing and was made smaller as everything expanded with heat

Either which way it seems to be OK now I hve made it wider

Funny really as I has a simialr problem a while ago with the speedo on my Tuscan, the needle would just die intermittently and I thought the sensor was too far awy from the rotor so I moved it closer, the problem got worse, moved it further away and fixed it.
Wish I'd remembered about a week ago though!!