RomRaider

A friend has a 2007 Legacy with EJ204 (NA, intake AVCS) and an LPG conversion (Liquefied Petroleum Gas). I helped him service (change filters) the LPG system as it hasn't been done in a while and he says it made a difference in that the car is running better / smoother on LPG. It's also running fine on petrol.

However, the change-over from LPG to petrol and vice-versa is very bad, the engine almost stalls in the process. Since I have a very similar car with very similar LPG set-up (one year older) I know the change-over can (and should) be very smooth and almost unnoticeable.

LPG ECU essentially works as a piggy-back controller, which takes over fuel injector signals from the OEM ECU and then controls it's own set of LPG injectors accordingly while at the same time simulating petrol injectors to the OEM ECU to keep it happy.

So I used RomRaider Logger to see if there's anything weird going on and the first thing I noticed was the relatively high A/F learning (+12) when the LPG system is turned off. After switching over to LPG, the engine almost stalls and I see A/F correction shooting up immediately until A/F learning follows-up and goes up to +30 - but after that the engine seems to run fine. Changing back to petrol feels the same - the engine runs really rough for a few seconds and when all 4 cylinders are back on petrol it smooths out (A/F learning goes back to ~ +12).

The diagnostic software for the LPG system allows me to switch-over individual cylinders, so I tried to do that and noticed that when I change cylinders 1 and / or 3 (the right-hand bank) it all works fine, switch-over is barely noticeable and A/F learning stays almost the same (it increases slightly from +12 to +16).

But when I do the same for cylinders 2 and / or 4 (left-hand bank), the engine starts running rough, almost stalls and A/F correction goes negative - if I wait it out, A/F learning eventually drops to about 0.

I inspected all LPG components (filters, hoses, injectors) and I couldn't find a problem. I couldn't properly flow test injectors, but using compressed air at ~30psi all 4 seemed to let out about the same amount when opened.

Summary:

• engine running smooth with all 4 cylinders on petrol, A/F learning is ~ +12.5
• engine running smooth with all 4 cylinders on LPG, A/F learning is ~ +30
• engine running smooth if left bank is on petrol, right bank is on LPG, A/F learning is ~ +16
• engine running rough if left bank is on LPG, right bank in on petrol, A/F learning is ~ 0

Any ideas what might be causing this?

Try swapping the 2 & 4 injectors with the 1 & 3 injectors and see if the problem follows the injectors. You could also try the same thing with spark plugs, too. They're probably harder to reach, though, and they seem less likely to be at fault.

If the engine has AVCS, try logging the cam angle and see if it's different in the 2/4 bank than in the 1/3 bank. I'd expect a CEL if it's way off, but I haven't got any other ideas.