3.5 V8 with 4.2 Efi system, problem with high altitude (pantelis)

People Hi,

a good friend has modified his 3.5 V8 carburated engine and fitted the Efi
system from a 4.2.
A piper cam was also used and the engine has received a full rebuild.
The Efi system was originaly fitted with "ë" (lambda) sensors and catalysts
and had the "green" resistance on the wiring loom just before the ECU.  The
lambda sensors and catalysts were NOT fitted and the resistance was changed
to the "blue" one.
The airflow is a 5AM and the system is using the standard Range Rover barrel
type air filter housing with a standard paper air filter.  No snorkel is
fitted.

They have also fitted an RPi sourced higher output electronic ignition
amplifier.

Then the car has also received an LPG system (last generation with ECU and
separate LPG feeds per injector on the V8)

The car behaves like a dream when used at sea level and above but during an
offroading excursion this week when they have reached an altitude of around
1500 to 1700 meters above sea level the problems have started.

Erratic idle with the V8 dying.  The engine also lost all its power and the
car could be only driven with low gear first and second.  Any gear above
that would result in total loss of oomph and eventually the engine would
die.

They have tried to revert back to petrol (instead of LPG) but the symptoms
were exactly the same.

When they have returned to lower altitude the engine behaved once more like
a dream without any of the above symptoms.

I would be grateful if you could provide you input and comment/suggestions
on how to rectify this problem.

Take care
Pantelis Giamarellos
www.lrcg.gr