Poor LPG MPG..

[RustyNut]

Evening all.

Is anyone knowledgeable on LPG systems? I bought a B5 A4 2.8q Auto recently which has a Stag LPG system fitted and I thought I would be getting manageable MPG figures based on the cost of LPG. It's a lovely car and nice to drive etc but i'm getting at best 10 - 12mpg around town driving sensibly. It does get better on the motorway but it's costing me more to run than I thought… There's no leaks as far as I know (can't smell any gas in or out of the car) so was wondering if maybe the system was flawed in some way causing the poor mpg figures? What I need to know is whether this is normal or not? Would rather fix the problem if there is one as it suits our needs perfectly. I did initially want the 2.5TDi Q but couldn't find a decent one at the time and this came up with just over 100k on the clock so seemed to be a good alternative.
Any help or advice would be welcomed.

Cheers

Russ

[prj]

Needs to be tuned properly.
And if you want best MPG map switching needs to be added to the ECU with the correct ignition advance values based on when the LPG system is active.

But first step is definitely to tune the LPG system properly with a wideband O2 and not the self-calibration **** the fitters do.

[RustyNut]

Thanks pRJ. So what sort of figures should I realistically see if the car was tuned properly? I'm basically getting around 140 -150 miles from a 60 litre tank. Even around town I was hoping for at least over the 200 mark? It was checked over by a local reputable LPG installer but i'm not sure what method he used to calibrate the system.

[prj]

None of the LPG installers have any idea about tuning engines.
They just push some built in buttons in the software.

I'd expect you to get better mileage than that, how much better I do not know, but I know that I had better mileage than you do out of your car out of an Audi V8 around town.
So I guess that tells you a little about it.

You need a wideband controller, a lead for your LPG system and the software (which is usually free) and then do lots of datalogging and dial in the correction map.
This will get you fairly close.

After that - what I did (because I can) was re-calibrate the stock ECU with map-switching to automatically advance the ignition (LPG likes lots more advance) when the system is turned on, and then retard it when the system is switched back to gasoline.

The MPG will never be as good as on petrol, but it should not be much more than 25% worse either, or something is not running right.

For kicks and giggles ask your "reputable fitter" what kind of lambda sensor he used to calibrate the system or whether he even knows what a wideband is

[RustyNut]

I take it this is a service you could offer? For a fee of course. I've been advised to contact you to remap my car when the RS2 conversion is done so might be able to kill two birds?

[prj]

If I ever get myself to UK, sure