I replied to your email Glen.
Just so everyone can benefit from the diagnosis, this was Simons reply.
Will need to get lambda to flick while running on petrol before being able to get the best results from any LPG system (in fact in order to get the best results when running on petrol too, but petrol is far more forgiving regards mixture than LPG so in case of incorrect mixture a vehicle is more likely to drive more normally on petrol than on LPG.
When running on either fuel, following engine warm up (so not just after starting the car but when engine is fully warmed, and perhaps even this will take a bit of a drive in an old Jag), lambda voltage should flick between 0 and 1 volts, or between 0.8 and 1.6 volts, or between 0 and 5 volts continuously with the engine running. The voltage range depends on type of probe used on the vehicle, the correct type to suit the vehicle must be fitted.
We don't know which wire on your probes is the signal wire but it is easy enough to connect to each wire in turn to check for a voltage flick. To check the voltage while driving it is possible to run a wire into the car interior from the probe wiring, drive the car with the multimeter on the passenger seat..
Historical OBD error codes have the potential to make the petrol ECU ignore the lambda probes, i.e. to make the engine run in limp home mode... Disconnecting the battery overnight should make an old (pre OBD2) ECU forget any historical error codes, so should make the ECU start to respond to lambda signal.
If disconnecting the battery makes the lambda start to flick as it should when running on petrol, but then when you switch to LPG we find that lambda stops flicking as it should, then this will point to a problem with the LPG system (perhaps just calibration). This is why I said only run the vehicle on petrol after disconnecting the battery.
We don't know if your Jag has been ignoring lambda signal due to an historical code, or if your lambda probe(s) are broken, or if there is something else wrong with your car (MAF sensor, coolant temp sensor telling the ECU the engine is too cold for the lambda probes to work properly), vacuum leak, throttle position sensor, etc. Lambda probes are the usual suspect, particularly if any coolant has found it's way into the exhaust at some point.
If the petrol ECU is running limp home / open loop mode the lambda signal won't flick like it should, the signal will be much more constant and may even be stuck at one extreme of the probe's signal voltage range. It is still possible to check the probe function: With the engine warmed up and idling - if the probe voltage reflects a lean mixture we can introduce more fuel by squirting propane / butane gas into the air filter to make the engine run richer, enough gas will cause the engine to go very rich, so this should make the probe signal reflect the rich mixture. If the probe voltage reflects a rich mixture we can allow false air to enter the engine by e.g. pulling a vacuum pipe from the inlet manifold. In either case we would be trying to confirm that the non flicking lambda signal wasn't due to a faulty probe.
Me now....I am going to test the lambdas again as last time (see first page) I didn't know that the engine had to be hot.