I apparently spoke too soon...
The new lambda sensors were fitted yesterday, and the car ran perfectly from cold, felt nice and crisp, and the orange (!) symbol on the dash didn't illuminate, even under full throttle.
As soon as the car was up to temperature however, the light appeared and it went into limp mode again.
Using the ECUmate with the car running and fault codes cleared, when revving the engine in neutral it tells me that lambda A is "ok" throughout the rev range, however at a little above idle lambda B tells me that it is "lean". I assume that lambda B is the one reading from the right, o/s bank, and that the lean conditions are what's causing the misfire putting the car into limp mode?
So...what would be the possible cause and where do I start fixing it?
In addition to all of this, on the way home from having the new lambdas fitted, I lost forward drive, leaving me stranded until the RAC could pick me up and take me back to the garage. I've still got reverse, but none of the forward gears do anything. Am I looking at worn out forward clutch packs and therefore a replacement gearbox or a rebuild?
I'm not having much luck at the moment...
I feel your pain! Range Rovers can be fickle and cruel. I'm afraid I can't help with gearbox issues, gear boxes scare me! Now, the engine issue - so you have ignition sorted, throttle pot is now doing its thing and the engine runs fine when cold. IIRC the ecu relies on the MAF when cold then switches to lambda once warm to control the fuel trim. It sounds like the ecu is working O.K. and switches to lambda control so the issue is something else.
The indication of a lean condition suggests the efi system itself is probably O.K. because failures in the efi sensors normally result in a rich condition so what are the possible culprits?
I think the first thing I would look for is a possible inlet air leak. Tracking down an inlet leak can be a bit hit and miss, you could try having the engine at idle and then cupping a hand over the main air inlet (the big pipe on the air box that holds the air filter), in theory the engine should stop but as the air is restricted, you might be able to hear air being drawn in somewhere else. The other method is to spray water around the various hoses and joints in the inlet system and where there is a leak you should hear a change in the engine idle.
To save too much heartache it might be more sensible to bite the bullet and remove the plenum, ram housing and inlet manifold. This has a couple of potential advantages - first you can clean all of the inlet system, second you can check the various hoses associated with the inlet side so any split or perished hoses can be identified, third you can check and set the throttle butterfly and lastly, you can seal the ram housing and plenum when you put it all back together. It should take a morning to remove, clean and reassemble the plenum and ram housing.
If you go the route of removing the plenum etc, it isn't much more work to pop the inlet manifold off, if the manifold isn't properly sealed the effect is essentially an induction air leak, it will also give you the chance to check for the common coolant leaks associated with the valley gasket that you can't always see with everything in place. Luckily, if the valley gasket is in need of replacement the cylinder heads don't need to come off so it is a simple case of undoing the bolts and putting in a new valley gasket but be sure to use hylomar or similar gasket sealant as appropriate.
If there isn't an air leak, I might be tempted to say it could be a worn camshaft, if the inlet valves aren't able to open fully then arguably it might show as a lean condition.
A compression test might also be a useful thing to do.
So perhaps the best way to proceed would be first do a compression test (do dry and wet). Then remove plenum and ram housing and thoroughly clean, by the way, if you find a loose trumpet or two on the ram housing don't panick, you can re-seat them and hold them in place with rtv silicon. With the plenum and ram housing off, have a careful look at the inlet manifold, check there are no loose bolts and look for any evidence of coolant leaks where the inlet manifold meets the valley gasket, replacement of the valley gasket might be a good choice.
With all that done, put it all back together and fire it up!
Hope that helps, good luck.