Hello All
I have had my 1994 Soverign x300 4.0 for eight years. I go it with only 9K on the clock and imediately converted it to LPG.
I have done 41k in it and only had one problem which started about 2 years ago.
The change over to Gas had allways been undetectable but I started to notice in very cold weather it would hesitate when changing to gas.
I could find nothing wrong no engine fault codes and the gas system all appeared normal.
The next development was a smell of gas in the boot. Lots of soapy water but nothing was found.
Recently the car has developed a very lumpy tick over on gas and slightly uneaven on petrol.
The engine then showed a Lambda sensor heater failure and the fuel trims were way out. The sensor heater was open circuit. Replacing the sensor has cured the lumpy tick over and the changeover to gas is again undetectable.
I think the conclusion is that the Lambda sensor has been failing over the last two years but not enough for the ECU to flag a fault code. I also suspect that the gas smell has been produced by un-burnt gas getting from the exhaust into the boot vents due to the engine running very rich.
It is known that LPG is sensitive to ignition faults it must also be sensitive to Lambda sensor faults.
I read lots of posts when trying to fix faults so I hope this helps somebody.

 

I've just (at the weekend) fixed the same thing - minus the smell of gas!

It's interesting to look at the output from the lambdas when running on gas, compared with when running on petrol - you can see the delay in response, making it obvious why you want to get your gas map right!