Lpg won't switch after engine change

[Riddlemethis]

I've swapped out engines in my Grand Cherokee. All seems good but when I switch from petrol to lpg, it kinda stutters then cuts back to petrol. Now, when it had engine change it had practically nothing in the lpg tank (was on red light) but did switch - I know this because I tried it the last time it ran on old engine. That was 4 weeks ago. Is it possible that what little gas was left has evaporated?
It's an OMVL dream.
Thanks in advance.

[Gilbertd]

It won't have evaporated but you may have a tiny leak that has allowed what little was left to seep away. If you've had any pipes disconnected you will have lost the content of those too and that may have been just enough for it to run before and as it isn't there any more, it won't. Bung a few litres in to see if that cures the problem. That fact that it is switching shows there's not much wrong.

[allante]

After my engine swap the 2 hoses that go to the pressure/vacuum sensor were put on the wrong way around which caused similar problems.

[john_d]

gas ecu lost its settings?

[Riddlemethis]

Where would I find the vacuum/pressure
sensor and how likly would it be that the ECU has lost its settings…battery was disconnected for about 4 weeks

[Gilbertd]

Unlikely that the gas ECU has lost it's settings as it is trying to switch, with default settings it would probably not switch. If it is an older Dream, the vac sensor is in the ECU. Does it have a thin tube going into it. Assuming you have now tried putting more gas into it, then it is likely that either the wire to the tank solenoid has been dislodged or the solenoid has gummed up through lack of use while the car was off the road. Do you have gas getting to the front end?

[Riddlemethis]

Ok well, I get the car back from MOT in morning. I'll fill it up and see what happens. Im hoping somebody could guide me through if it isn't low-gas related. The system I have is the one without a vacuum hose going to the ECU, so I think I'm right in saying that it's a newer version. It was done in 2010.
It has six individual solinoid a in blocks 2 blocks of 3 so I assume these are the injector solinoids.
Thanks for help so far

[Gilbertd]

It has six individual solinoid a in blocks 2 blocks of 3 so I assume these are the injector solinoids.

I suppose you could call them solenoids but normally they would be referred to as the injectors. There are safety shut off solenoid valves at the tank and at the front end. The front end one will incorporate a filter bowl and will be in the pipe that comes from the tank just before the reducer/vaporiser/regulator (all the same thing but referred to by different names by different people). I would suspect that one or the other is either sticky from not being used and isn't opening or you have dislodged the wiring from the one at the front while you have been changing the engine.

[Riddlemethis]

OK thanks for the replies. Because it would switch over but immediately cut off. I checked and double checked the wiring that I disturbed and all connections looked good…then the missus said you 'have plugged it in haven't you' course I have!
Then I remembered. The connector for the LPG ECU going on the batt terminal wasn't tightened as I left it loose to do later. All ok now! doh!