I recently purchased a 1990 XJ6/XJ40 4.0 Sovereign (with history and lots of work done over the past 30 years), it has ran perfectly on petrol (99 octane) since owning and I've done around 250 miles. It has a, "bedini", LPG system that had been refitted in 2006 and I had found an old receipt saying the previous owner had filled up on LPG April this year. I thought this was a good indicator it was still working but just empty as when I picked the car up it was only running on petrol. On a quick visual inspection it all seemed fine and nothing seemed in bad shape at all, when re-filling the LPG everything seemed fine and I put 25L in.

About 5 miles up the road I switched to LPG when parked up, the indicator flashed green, the engine stuttered and once the green light stopped flashing it seemed to idle fine, four dots showing LPG level then showed only one, correctly showing I had a quarter of a tank. Within minutes of switching a loud bang happened and I started to lose power, lots of misfires engine revving etc, I switched back to petrol on the control panel and it didn't change much. I had 5 miles to get home and it gradually but rather quickly started to get worse, stuttering and stalling if I slowed down too much. I had to keep the revs up and just as I pulled into my house it just stopped.

Upon looking at the engine, the only thing I noticed after it wouldn't idle correctly was a 5cm wide hole but thing had appeared where the rubber bellows are between the air intake and the engine right at the top so easy to see, I thought this was an obvious culprit as the MAF wouldn't know whats going on, perhaps bad air /fuel mixture regardless LPG/Petrol. However, upon a, "quick", fix of using silicone gasket repair glue and a small bit of repairing adhesive tape and clamping all that down with the jubilee clip, things seemed a hell of a lot better. This was until it immediately bounced off 500rpm up to 650 like it was struggling to idle, if I switched from Petrol to Gas, the revs to back to 750/800 but everything is shaking the car and it doesnt feel quite right. I drove it up the road to see if a makeshift repair would do the job until I find another bellow or some hose to fit, but the misfire and hard stuttering was still present, if I rev it up it seemed fine but wont change down and rev hard (in sport mode) anymore, as if it was in a sort of limp mode.

I know my, "fix", wasn't great but with other cars this has done the trick to keep it running somewhat smoothly since I needed to drive it. As I sat stationary seeing how this worked I got the revs up to 3k after warming the engine to see if I run off only petrol it would burn out any residue of Gas, not sure my thinking on this but I tried it, to my surprise a large bang was heard again and smoke came back through the air intake.. This made me slightly worried that something has cooked valve wise on one of the cylinders. It now sounded like it was running on only 5cly at idle but when you rev it, it sounds back to a smooth 6cly minus the drops of power/misfires.

I tightened everything back up and looked at the leads etc and I know they have been changed around 3 years ago, the car drove fine and then revved fine but the bad idle was still there on petrol 5/600rpm bouncing. Gas was still giving me a better idle but lacked much power etc compared to petrol. Finally, on the way back of my 1 mile circle, it went bang again and barely limped back stuttering misfiring, backfiring the lot.. What has happened.

- I can't find any vacuum leaks but there could still be a small one, I will be dismantling it all and trying to replace parts that could cause bade airflow

- I'm thinking a sensor has gone kaput, crank angle or lambda sensor but I don't know much about these engines.

- There is a large hole in one of the exhausts but the back-pressure I'm sure wouldn't cause that much of an issue its just loud.

It feels like if I never bothered to run it this old LPG system and kept to petrol I may have never seen an issue but this has ruined my car after only 1 day and I'm gutted..

Thanks, I have probably forgot things that have happened so I'm sure someone will point out something stupid.

 

What you described is an LPG backfire. You probably broke the rubber and/or elbow plastics, possibly air filter box too, especially when you done it twice. Smoke was from air filter getting destroyed, that's definitely toast now. Check for blown up vacuum lines going into intake manifold, mine used to come off randomly during backfire. Remove every single piece of intake going to the throttle body and inspect/replace any broken parts. Normally during backfire, the rubber boot comes off at the bottom and you have a major air leak, hence poor running, but all parts can be damaged, that's why a good set up should have a backfire valve. Yours clearly doesn't have one.
Do not attempt to run it on LPG again, its clearly set up incorrectly and/or you have ignition issues. The LPG is generally super simple and reliable on these cars but only when set up correctly and only if your ignition parts are in tip top condition, yours clearly need to be serviced. I've been running mine for close to 10 years now. Makes running a Jag cheap as running a Nissan Micra. Its amazing especially now with lots of imbeciles panic buying fuel and causing fuel shortages.
If you decide to get it serviced yourself start with getting a cable and an interface for that LPG system, your reducer is also most likely shot by now and will either need replacing or at least the seals and gaskets replaced.