As the weather has gotten colder I've noticed that the engine runs very rough when switching to LPG. Running rough is probably an understatement as it can't accelerate/rev at all when moving and immediately stalls at idle. The colder the temperature is the worse this is. Once the engine is fully warmed up it switches over fine and is generally fine the rest of the day.

Initially I assumed I had the changeover temperature set too low. I increased it to 65 degrees for a test but it was still no good. I originally had it set to 35 degrees - does that sound reasonable for the future?

It turns out that when it's cold the LPG pressure is being regulated around 3bar. Once the reducer warms up the pressure drops back down to the usual 1.3bar. I'm assuming something inside the reducer is gummed up (the spring?) so it can't regulate pressure. It's a Tomasetto AT13 XP Antartic. Does this sound like something a rebuild kit would cure or is it new reducer time?

It's probably due to a combination of problems...

We've been talking about dirty LPG recently, contains dissolved oil ('heavy ends') which comes out of the LPG during evaporation inside the reducer, flows along with gas vapour to injectors, when cold turns into a sticky tar like substance that can prevent or slow the speed of injectors opening https://lpgforum.co.uk/viewtopic.php?t=19640

The reducer could be another problem. Maybe also due to heavy ends, maybe not... I've known quite a few Antarctic reducers have the problem of slightly sticking open gas inlet which sees pressure rise to 3 bar when there is no gas being drawn off (when the gas inlet port should be closed). The gas inlet port could be sticking open due to heavy ends. Or if you haven't monitored the gas pressure reading before the gas injectors start firing for a while, maybe it's been 3 bar before the injectors for a long time but still switched to gas OK until recently in which case we couldn't attribute all your recent problems just to the high gas pressure (although it would never help).

Don't know what ECU you're using but another problem can be that if you have set a 1.3 bar reference pressure we'd expect the ECU's default (none adjustable on some ECUs) pressure correction to compensate Ginj (gas injector pulse length) by around negative 50% if the pressure reading is 3bar, the default ECU settings are based on physics but don't take into account that the higher pressure will also have the effect of slowing the speed of injectors opening, so at such a high pressure (that will make a big difference to speed of injectors opening) the negative 50% is vastly excessive and has the effect of leaning the mixture due to too short Ginj. If the over-pressure situation is momentary (say the pressure decreases rapidly as soon as injectors start opening) it can be better to dial in a positive (obviously as opposed to negative but I mention it to highlight the point) pressure compensation for pressure readings above the usual range that will be seen during normal engine operation to prevent the over-pressure from leaning the mixture and also to give the injectors a longer pulse to give them better chance of opening and bleeding some of that pressure when they are first pulsed during changeover.. If possible I'd go with the standard pressure corrections up to 1.8 bar, then a zero correction to 2bar, then a positive correction up to around +15% at 3bar. Heh, my +15% correction for +3bar will be around a 65% longer pulse than the ECU would give with standard pressure correction. At cold changeover there'll also be some negative Ginj correction because besides pressure cold gas is denser than hot gas. That said, some ECU's also have a default minimum injector pulse length for the type of injectors you've selected in software (and some others allow a minimum pulse length to be defined by the installer). What ECU are you using?

Could be one I converted, I don't remember whether I did yours or not lol.

It's an LPGTech ECU.

When I was looking thiss morning the pressure was 1.3bar from overnight. As soon as the solenoid opens the pressure increases to 3bar. Even with the injectors firing the pressure doesn't reduce (after a few seconds whilst driving I have to switch back to petrol so the engine doesn't die). When I switch it over again 15 minutes later the injectors start firing and the pressure reduces quickly to the regular ~1.3bar value.

It certainly sounds like the same symptoms as you describe with a sticking gas inlet that frees up when the reducer is warm. What's the solution to that? Rebuild kit or replacement?

You didn't convert it but you did the tuning in on it a few years back.

Clean the injectors as discussed on LPGforum for sure.

I'd be interested to see if cleaning the reducer will cure the over pressure after solenoids open and before LPG injectors start bleeding pressure... But you might find it easier or prefer to just change the reducer.

If I calibrated the LPGTech system I'll have done it in 'standard mode' (as opposed to 'tech mode'), Standard Mode allows more control than Tech Mode (Tech mode is more for installers who try to get away with fit and forget and not do a proper job of calbration). You can adjust the pressure correction with LPG Tech, click Map, Correction, and find Gas Pressure Correction under the Others tab. Right click on a pressure and you can set a deviation from the standard pressure correction (deviation because your input figures are in addition to the underlying calculated pressure correction figures). The figures I mentioned above are absolute, to get to the absolute figures by entering deviations we have to enter figures of +16% at 1.8 bar and +43% at 2.5 bar, the scale only goes up to 2.5bar. You can also set a minimum injector pulse length in Settings, click on the 3 dots beside Injector Type, change Minimum Gas Injector Time to Manual and set a figure such as 2.3ms.

If you want you could email me your setup file (I'll have a copy but would take time to find it as it'll be on an old hard drive from an old Laptop), I'll modify it and email it back to you. It might help but I doubt it will cure the problems in itself.