Ooops. Sorry - meant to post this question - D'oh!

[V8 Disco]

Hi Marcos folks.. I am normally on the Land Rover forum, but since this question is Rover 3.9 V8 Efi related, I'm hoping that there will be someone here who might be able to help.....

I have a LR Discovery with the 3.9 Efi (serpentine) and it's not well: The engine starts every time and runs sweetly, except for an intermittent, sudden drop of power. While driving the revs suddenly drop toward idle, then almost instantly 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 during this condition - I cannot help but think this is significant, as I think that 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 (think this is LR specific)
Fuel pump relay replaced - genuine Bosch
Plugs replaced - Bosch
Leads replaced - 8mm silicone
Dizzy cap replaced - Lucas
Ignition coil replaced - Intermotor
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? I have read that it's possible to run the engine with it disconnected? Same for MAF? Intermittent fuel pump failure maybe?

All help gratefully recieved!

[cazzer]

Does it backfire when it comes back?

[V8 Disco]

Interesting question -nope. I think I can see where you are going with this....

fuel but no spark, then spark = bang?
spark but no fuel, then fuel = continued running?

On the LR forum, the throttle pot has been suggested, also ECU ()

Edited by V8 Disco on Sunday 18th March 10:27

[Joe T]

Has it got a CAT, if so has anyone checked the Lambda Sensor?

[V8 Disco]

Yes, two O2 sensors and no..... not as far as I can tell (although it has been on the Land Rover ECU interrogation system 'testbook' which I imagine would show a code for a duff 02?)

Also, how does a duff 02 manifest itself? (other than throwing a MIL/CEL) surely the misfire would be more consistent?

Cheers for your suggestion!

[dr ivel]

If even just one of the lamda's isnt working / or has a loose connection the ECU will go into recovery mode and will work normally up to a max of around 17500 revs (or 50mph) and then abruptly cut back to idle.

It might also rev normally when stationary (and being tested)but as soon as you are on the move and the engine is warm it will cut out.

Sounds like your O2 sensors could be on the way out.

If so it will do this consistently and forever until you either fix the lamdas - or remove the cats, fit straight pipes, disconnect the lamdas and fit a CAT-free green resister to the ECU loom (like I did - works a treat).

I know this as I have one of the few Marcos 3.9s ever to be passed by a caravan on the M40

Edited by dr ivel on Monday 19th March 03:12

[V8 Disco]

Problem seems to be fixed - was throttle pot, intermittently sending wrong value.

Thanks to all that have offered advice!

[dr ivel]

Thanks for letting us know we were intermittently sending the wrong advice