V8 Running Problems - Getting Low On Ideas!

[V8 Disco]

Hi again - still have problems with my 3.9 Efi... The engine starts and runs sweetly, except for an intermittent and sudden drop of power. The revs drop toward idle, then power comes back just as suddenly and either the cycle starts again (resulting in kangarooing) or, just as often the problem dissappears for a while (can be 5 times a day or not at all for five days). The engine has never stalled - I cannot help but think this is significant, as any sustained loss of fuel/spark would require a re-start, especially in an auto which IIRC cannot restart by virtue of continued forward motion (bump start).

Work done so far:

All main ignition and Efi connectors checked and seem ok
Immobiliser spider replaced with LR replacement
Fuel pump relay replaced
Plugs replaced
Leads replaced
Dizzy cap replaced
Ignition coil replaced
Ignition amp replaced
Coolant temperature sender replaced (the one that feeds the ECU)

But still it fails...

Please can anyone suggest what to try next? What happens when Throttle Pot fails? Same for MAF? Intermittent fuel pump failure maybe?

It's been three months now, the dealer that sold the car to me is trying to get it sorted, but we're not getting anywhere.

[steve_d]

It's one of those time when all you can say is 'For f... sake just break down then we can fix you'
Intermittent faults don't you just love 'em.

Steve

[Brink]

Failure of fuel pump would kill engine and make difficult restart. Throttle pot would not.

Also the type of failure is consistant with a pot: i.e. sudden switch to rail and back. Just be glad it has not failled to the other rail, thus giving full stick! Change it.

[Liszt]

Also it could be an ecu fault which is a bugger to track.

[Graham]

We had a very similar problem with an mg rv8. went through the same swaping things out procedure...In this case it turned out to be damage in the wiring loom. everything was fine untill it got a bit hot and then the connections were breaking down. replaced a bit of wire and it was all fine..

so if you've checked everything else try continuity on the loom from the sensor plugs to the ecu plugs..

G

[V8 Disco]

Thanks all - appreciate the help.

I have a question about the throttle pot - I thought that I'd read on other similar posts of folks running with them disconnected? Can anyone please explain the individual function of TP and MAF on the fuelling?

Which one (I think I now know the answer from Brink's earlier post here re TP failure) tells the ECU the base info as to how wide the throttle is open? I guess the TP? In which case, what does the MAF do?

Thx again for the help!

Edited by V8 Disco on Sunday 18th March 11:00

[Trooper2]

TP- tells the ECU what driver demand is, where the throttle plate is positioned on it's axis.

MAF- measures the amount of air getting to the intake manifold, it returns a signal back to the ECU so it can make adjustments to the fueling needs of the engine, so that hopefully fuel and air are kept matched close to where they need to be.

[V8 Disco]

Thanks Trooper - any idea if it's possible to run the engine without the MAF? I read another internet article suggesting that diagnosis of a MAF fault without sophisticated test equipment could be crudely done by disconnecting - the article said that the ECU will use default values that are good enough for normal running. If the fault goes away, it's the MAF, if it stays, it's not.

If that's true, I think my fault cannot be the MAF, as the signal could not vary enough to create such a dramatic effect.

My thoughts are now focussing on the TP...

[Trooper2]

Yes you can disconnect to test the MAF and even if the MAF is shot the vehicle still runs fine or at least my 4.0 did.

Typically if the TP sensor has a worn spot on it's carbon track you will get a dead spot as you go through the throttle travel and the vehicle will get a sensation called "trailer hitching" like you are towing a trailer with a tow ball that's 3 sizes to small. A jerking sensation. The worn spot will occur at a place on the carbon track where you keep a steady speed like where you do your highway cruising.

It sounds like your problem is intermittent and that usually means something electrical. Maybe there is a chaffed wire,loose connection or corroded connection to the TP or some other sensor as mentioned by a poster above?

[V8 Disco]

The problem seems to be fixed - throttle pot, as per earlier post.

Thanks to everyone that has offered advice.

[Hard-Drive]

Hooray!! Does this mean next time I see you at work we can talk Land Rovers without you cursing and swearing and saying that you don't want to talk Land Rovers?

[V8 Disco]

Yes! - and one day, I might even be able to afford the petrol to drive it that far, so you can actually see it!