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My Petrol Gauge has lost the plot? (LPG Conversion Installed)

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David El   
Mon Oct 13 2008, 05:59pm
Hi there,

I've recently had an LPG conversion (by recently you can read a week and a half ago when it was completed). Wonderful thing, and made my already awesome C4 even better.

It is accurately reporting the new MPG - it's at 90% of what it was. How on earth it is measuring this accurately, I don't know, but it is.

The problem is that the more I drive the lower the petrol gauge is going... as if petrol was still being used (it's not). Has anyone else had this problem or can someone explain how the digital petrol gauge works and if there's some way I can reset it to an accurate level or make it behave itself?
Mon Oct 13 2008, 06:59pm

Member No: #82
Joined: Jan 22 2007
Location: Hoylake
Hello, David, and welcome to C4Owners!

It's clear that your new LPG fuel tank has no dedicated link to the car's BSI, and that MPG is being measured by via fuel flowing into the engine (and the distance travelled and the time the engine is switched on) so no surprises there.

The amount of fuel remaining displayed on the dash is calculated from the fuel used, and when the engine is switched off the level in the tank noted. When the engine is switched on again, the level in the tank is compared with the noted level and adjustments made.

I do know that the fuel system computer has to be set using reference points of an empty and a full tank, and that the system is remarkably accurate.

It'll be interesting to see what happens when you have to refill the petrol tank.
David El   
Tue Oct 14 2008, 01:42am
I have had to fill the petrol tank once because the petrol light indicates. At that point it shows the correct amount. However, I now realise (and thought that more petrol than I paid for had gone in) that the gauge drops constantly when driving.

It's clear from the PDF that what resets the fuel level is putting in more fuel - does anyone know of a way that C4 can be reconfigured to recalculate the fuel level once every 100 miles, or when it's off by 20%, or every 3rd time the car is turned on (or whatever)?
David El   
Fri Oct 17 2008, 03:26am
Fri Oct 17 2008, 03:46am

Member No: #90
Joined: Jan 29 2007
Location: Bristol
I very much doubt it...that sort of logic would be programmed into the cars management system. As you use the car however you become acustomed to the amount of fule it uses...for mine is approx 30miles per nugget on the gauge and 160miles is about just above half a tank of town driving...a bit less consumption for out of town driving.

So I am sure I understand your question...are you wanting the fuel gauge to show the amount of petrol in the tank, Autogas in the tank or both?
Fri Oct 17 2008, 04:15am

Member No: #82
Joined: Jan 22 2007
Location: Hoylake
You need the gauge on the LPG tank to be linked to the car's BSI. As it stands, the fuel entering the engine is assumed to have come from the petrol tank but it hasn't.

You'll need two fuel tracking systems, each of which indicates the remaining fuel/distance in the corresponding tank and fuel consumption.

This info would need to be combined to give a total distance remaining from all the fuel in the car as well as overall mpg.

You're going to need some serious soft- and hard-ware engineering for that.
David El   
Fri Oct 17 2008, 04:27am
That's not what I want:

AT the moment, the LPG guage is correct and working perfectly
The Mileage is also correct, no matter whether I use the LPG or the petrol, it gets it right

HOWEVER, the petrol guage is measured at fill up, and then deducts due to the mileage, no matter whether LPG or Petrol is going into the car.

The reason why this happens is because the C4 only resets the guage if the level in the petrol tank is different from when the car was last turned off: i.e. if it's been filled up.

What I want is it to reset the gauge in any (and i only need one of the three, and each is fine by me) of the following situations:
1) Every time the car is turned on
2) If the level is the tnak is 5% different from what the C4 thinks it is (rather than 5% different from the last time it was turned off)
3) Every X days

It is entirely a software matter, as the hardware is all working perfectly. The only thing I want to change is the time and/or frequency at which the C4 measures the amount in the tank - I just need to know how.

If anyone needs reference, it's the older (138bhp) version of the 2005 C4 VTR+ 2.0 petrol.
Diesel Dog   
Fri Oct 17 2008, 05:38am
I may be dumb here but is the car actually using any petrol at all, when i had a LPG monster i used to put in about £5 worth of petrol every 6-8 week's. That was just for start up purposes.
David El   
Fri Oct 17 2008, 05:51am
Yes it uses petrol just to start: that's my point, it shouldn't be going down. I can't judge when I need to put petrol in (should fill up to about 3/8 full when it hits 1/4 full) if it's showing empty with a half full tank in!
Fri Oct 17 2008, 06:46am

Member No: #90
Joined: Jan 29 2007
Location: Bristol
As John mentions the way in which the fuel usage is calculated is built into the ECU Hard/Software and I'd imagine its serious stuff to alter.
Fri Oct 17 2008, 07:40am

Member No: #82
Joined: Jan 22 2007
Location: Hoylake
The level of fuel in your LPG tank is not connected to car's BSI. That's really what this is all about.

How that can be done is presumably possible but well beyond me, and will certainly require purpose-made devices and a major re-configuration of the car's principles of operation to give the options for dual-fuel info in addtion to the default single fuel system.

A top LPG installer may have the necessary parts and skill, but I'm only guessing now.

Fuel is measured as it enters the injectors - it does not distinguish between the two sources. A calcuation is made and the result is shown on the dash's LCD display as the fuel remaining in the petrol tank - as there are no links to the LPG tank's fuel gauge.

After the car has been switched off and restarted, a comparison is made between the calculated and actual fuel consumed in the petrol tank. Normally, any difference is negligble unless more fuel has been put into the petrol tank - which is probably what the BSI will conclude in your case, even though no or little petrol has actually been used.

Consequently the info displayed will be incorrect as there is no reference to the LPG used.
Fri Oct 17 2008, 07:57am

Member No: #1
Joined: Aug 07 2006
Location: Northumberland
A Citroen Dealer, or anyone with access to Lexia/Proxia can check and set the BSI for:

Menu : fuel gauge low – oil level gauge low

Type of fuel

Oil level sensor option

Tank capacity

Resistance of gauge, tank empty.

Resistance of gauge, tank full

Fuel gauge law - alert level

Fuel gauge law - selection of the vehicle

If there are issues on the 'Petrol' side then I suggest you speak to your dealers on it and have the system checked out.

LPG side, well BJD seems to have that covered off.
Diesel Dog   
Fri Oct 17 2008, 09:22am
Does a LPG conversion cancel any warranty you think??
Fri Oct 17 2008, 09:25am
Member No: #209
Joined: Apr 05 2007
Location: West London
Would imagine that's a big YES
Fri Oct 17 2008, 09:38am

Member No: #1
Joined: Aug 07 2006
Location: Northumberland
OK having a bit more time to consider this and had a discussion with an ex Citroen Tech

Modern engine technology is complex to meet Euro emission standards and things are monitored in a far more complex manner.

The C4 BSI looks at fuel consumption via the injector measurements for fuel and air passing through.

It doesn't matter what the fuel is so in this case it's LPG. But the car is measuring the fuel flow and doesn't know where it's come from as the fuel feed changes from petrol to LPG before the injector monitoring system so can't differentiate.

Obviously the BSI thinks that the petrol is going down, when in fact it isn't (as you already know)

How you will ever overcome this well..........

We don't have an answer, but will be very happy if you do find out and post it back for future reference.
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