How does the dash get the mpg generated?

StewartIngram · 14th December 2023, 17:57

Quick question - how is the actual mpg display generated for the dash display?
I noticed today whilst driving with the display showing, that it is constantly changing. I would expect this, but actually not expect it on my car (I'll explain why in a moment).
I'm guessing that it takes a reading from the speedo supply (for mileage) and something from the fuel system (but where?) for the fuel used, and compares the two to calculate the mpg.
My car is lpg converted (starts on petrol), so I would expect when running on gas that there would be no petrol used as the petrol injectors are turned off, but the display I saw today still worked. Any ideas?

Matt_75 · 14th December 2023, 23:18

Extract from Rave:

SteveThackery · 14th December 2023, 23:24

Quote:

Originally Posted by Matt_75

Extract from Rave:

So, Stewart, I imagine the ECU still "thinks" it is injecting petrol. I assume it is still sending pulses to the injectors, even though they aren't injecting any petrol. So it is showing the MPG as calculated from the pulse durations and the distance travelled.

It has no way of knowing how much LPG is being injected. I imagine it doesn't even know the engine is running on LPG.

This is just my guess. It'll be interesting to hear from people who know for sure how it works.

StewartIngram · 15th December 2023, 15:15

Thanks for that useful reply, I fully understand it now.
The car was obviously built as you have shown.
I have used a professional kit, diy installed and certified afterwards.
There is a gas ecu fitted, literally, on top of the petrol ecu. the 6 injector o/p wires (one for each cylinder) are cut and redirected via the gas ecu. They are fed through relay contacts, and have 2 outputs for each of the 6 inputs. When on petrol, they are fed straight out to the petrol injectors as normal.
When the system switches over to gas, the 6 o/ps are switched over by the relays, and then feed the gas injectors instead. So the petrol injectors now do not function**.
In addition, the gas ecu has been calibrated after install to change the length of time the injectors are fired, as gas burns slower than petrol.
This last week I've had to change my thermostat, which had developed a water leak. This of course involved some stripdown, in particular the front fuel rail. Re-assembly as usual gave me some problems with O-rings, and unwittingly I put £30 of petrol in, only to find it all gone about 10 miles later (and when running on gas). After doing the repairs, I drove back to the garage to fill up, keeping the mpg display on, and was surprised to see it function whilst on gas. (I was checking if I was losing petrol from another leak, but all was ok). Hence my query of how it is derived,
**This has raised another point for me. As the petrol pump is still on with gas, I may try a rewire to stop that when on gas (as indeed the fuel line for gas is switched off when on petrol) purely as an added safety function.

SteveThackery · 15th December 2023, 19:57

Thanks, that's really interesting - I've never had cause to investigate an LPG car. As you say, the original ECU has no idea the car is running on gas, so happily reports a fictional miles per gallon.

14th December 2023, 17:57	#1
StewartIngram Gets stuck in Rover 75 Connoisseur 2.5 V6 auto Join Date: Jan 2008 Location: Chatteris, Cambridgeshire Posts: 903 Thanks: 320 Thanked 159 Times in 127 Posts	How does the dash get the mpg generated? Quick question - how is the actual mpg display generated for the dash display? I noticed today whilst driving with the display showing, that it is constantly changing. I would expect this, but actually not expect it on my car (I'll explain why in a moment). I'm guessing that it takes a reading from the speedo supply (for mileage) and something from the fuel system (but where?) for the fuel used, and compares the two to calculate the mpg. My car is lpg converted (starts on petrol), so I would expect when running on gas that there would be no petrol used as the petrol injectors are turned off, but the display I saw today still worked. Any ideas?

14th December 2023, 23:18	#2
Matt_75 Gets stuck in 75 Saloon Join Date: Apr 2022 Location: North Somerset Posts: 548 Thanks: 13 Thanked 20 Times in 8 Posts	Extract from Rave: __________________ Matt 2001 Rover 75 Connoisseur SE 2.5 (BRG) 1999 Rover 75 Club 2.5 (Dorchester Red) 1973 Triumph Dolomite (Honeysuckle)

15th December 2023, 15:15	#4
StewartIngram Gets stuck in Rover 75 Connoisseur 2.5 V6 auto Join Date: Jan 2008 Location: Chatteris, Cambridgeshire Posts: 903 Thanks: 320 Thanked 159 Times in 127 Posts	Thanks for that useful reply, I fully understand it now. The car was obviously built as you have shown. I have used a professional kit, diy installed and certified afterwards. There is a gas ecu fitted, literally, on top of the petrol ecu. the 6 injector o/p wires (one for each cylinder) are cut and redirected via the gas ecu. They are fed through relay contacts, and have 2 outputs for each of the 6 inputs. When on petrol, they are fed straight out to the petrol injectors as normal. When the system switches over to gas, the 6 o/ps are switched over by the relays, and then feed the gas injectors instead. So the petrol injectors now do not function. In addition, the gas ecu has been calibrated after install to change the length of time the injectors are fired, as gas burns slower than petrol. This last week I've had to change my thermostat, which had developed a water leak. This of course involved some stripdown, in particular the front fuel rail. Re-assembly as usual gave me some problems with O-rings, and unwittingly I put £30 of petrol in, only to find it all gone about 10 miles later (and when running on gas). After doing the repairs, I drove back to the garage to fill up, keeping the mpg display on, and was surprised to see it function whilst on gas. (I was checking if I was losing petrol from another leak, but all was ok). Hence my query of how it is derived, This has raised another point for me. As the petrol pump is still on with gas, I may try a rewire to stop that when on gas (as indeed the fuel line for gas is switched off when on petrol) purely as an added safety function. Last edited by StewartIngram; 16th December 2023 at 11:17..

15th December 2023, 19:57	#5
SteveThackery Avid contributor Rover 75 Connoisseur SE saloon, 2.5 KV6 automatic, 2002, White Gold Join Date: Oct 2023 Location: Nottingham Posts: 191 Thanks: 0 Thanked 0 Times in 0 Posts	Thanks, that's really interesting - I've never had cause to investigate an LPG car. As you say, the original ECU has no idea the car is running on gas, so happily reports a fictional miles per gallon. __________________ If it ain't broke, keep fixing it until it is.