Immobiliser issue?
 

I don’t know if this is actually in immobiliser issue or not but I’ve now had 2 instances of the car struggling to start. In both cases the car had been switched off when at normal operating temperature and then an hour or so later, when I try to start it, it struggles to stay running. The tick over is very slow so it barely keeps running and it’s reluctant to rev.

In both instances I switched it off and tried to start it again and it was the same. I get no immobilised warning light (the key symbol) and all the dash lights look ok but I suspected an immobiliser issue because the first time it happened I was loading the car with luggage and it passive armed itself so I thought maybe the ECU was immobilised (although the engine did start and run, albeit badly). Also, the car runs lpg and when you switch on the ignition the led lpg gauge normally illuminates but this time it didn’t. I thought maybe the ignition feed was isolated and so the lpg ECU had no power and hence the gauge didn’t light up. The power wiring for the lpg ecu isn’t exactly obvious, it has a direct earth and live off the battery (both of which were ok when tested with a meter but without a load of dismantling, I couldn’t trace an ignition feed and there was no sign of any wiring that had been added in which would give a clue where the ignition feed is taken from. I was pulling off trim panels and checking fuses and generally fiddling and I tried locking and unlocking the car a few times and eventually, for no obvious reason, it started and ran fine.

Last night it did the same thing, so I just locked it, left it alone and a couple of hours later, it started first time. So far it’s been a minor inconvenience but I know it’ll be back to cause more bother later. Can I discount the immobiliser as the cause as there’s no warning light (and I did a code scan the first time and got no errors). I think having 2 ecu’s makes things more complicated and this hasn’t helped with my long running misfire issue which for the time being is ok on lpg but not great on petrol. The car is a 2000 S6 40V, when it’s running right it’s brilliant but these constant niggling problems are starting to annoy me.

If its an immobiliser problem, the engine will start perfectly, then cut out abruptly after a second or so with the immobiliser light flashing - You dont seem to have this.

In the past I've seen LPG systems installed with the ignition feed taken out of the plenum chamber - Big plastic box on passenger rear side of engine bay housing all the ECU's.

Sequential LPG systems sit in the way of the petrol injectors, so if the LPG system is losing power, it will knock all fuel supply out.

I think you're on the right lines by investigating that further......

Cheers for that. I've just been reading threads oin the immobiliser issue and like you said, it looks like that isn't the cause.

The lpg system has a seperate harness that breaks into the petrol injectors, I think this is so the lpg ecu can stop the injectors firing and also make the petrol ecu think that it's actually still running on petrol.

I'll have a look at tracing the wiring back from the lpg ecu connector that's in the plenum on the passenger side - it will have to get an ignition feed from somewhere. However, what if the ecu got it's main power directly from the battery and took an additional switching ignition feed from somewhere else - like a coil pack? From memory coil pack 8 has had one of the wires broken into and I think that goes to the ecu - this looks like a good starting point.