Andrew Patterson (qidane) wrote,
Andrew Patterson

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Duelfuel Cars

I had my car booked in for an annual service today. Nothing much wrong, just a stuck headlamp adjustor.

On the way to work this morning something odd happened. My car is the Dualfuel model of the Vectra. This means it has two fuel tanks, one petrol, one gas (LPG).

There is a small selector switch on the dash that chooses which fuel you are using. This decides which fuel it will run on, but there are a couple of overrides:

1 - The car has to start on petrol.
This means that if petrol is selected it starts as normal. If gas is selected it starts on petrol and runs for a short time then changes to gas. The time is about 10 seconds with a hot engine, 45 seconds with a cold engine, up to a few minutes on a freezing morning. This time warms the engine to ensure the LPG (L for liquid) does not freeze as it expands to gas and block its jets.

2 - The car can run on an empty gas tank.
This means that when the gas runs out it automatically switches to petrol. This normally happens without even a blip. The engine management light comes on for 1 second but I normally do not even notice.

Not a bad system, and the fuel is cheep (£0.39 per litre for LPG). There is however one confusing factor. The car only has one fuel gauge on the dash.

What has always happened ever since I got the car over a year ago was this. I always have the selector set to gas. When you start the car it starts on petrol and the fuel gauge jumps quickly to the amount for petrol. After a few seconds the gas system kicks in and the fuel gauge drifts sloooooowly to the correct gas value. (Mary Frost says that this is called filtering and the effect is to produce an average figure for the fuel level as it will be sloshing about in the tank. She works on airplane fuel systems.) When you run out of gas the system switches over to petrol but the fuel gauge stays on the gas level.

What happened today was that as I ran out of gas on the way to work and it cut over to petrol, the fuel gauge changed to petrol level quite quickly. Also the gas indicator light on the selector switch that normal glows a dim red started to brightly flash.

Ok, to everyone else this is not an event of note but you see when I first got the car this is how I thought it should work. When it did not I asked the garage and got a "no that is it working correctly".

This morning before they came to collect the car I phoned the garage and without saying what was up said something odd had happened and could I speak to someone who understood the LPG system. Bit of a pause and they got the local expert. I asked him to tell me what way it was meant to work. Turns out that what had just happened is what the car should have been doing all along. They are "Going to look at my relays"...
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