Alfa Romeo Forum banner

159 1.9 jtdm fuel consumption computer measures

324 8
hello all once again
i have an 159 1.9 jtdm, it is remapped with 180 horse power and egr deleted. i have 245/40 R18" wheels than the industry 205/55 R16". I noticed that i i never can drive the distance the computer was calculating and that i have a bigger consumption. so when the computer was calculating 6.8 lt/100km the real one was at least 8.5 lt/100km ( the kms that was calculating after zeroing the trip B and the diesel it consuming after fueling it back).
so im wondering how the computer is calculating the consumption and/or if i have any problem with my engine... this difference in consumption i suppose is very big one, is there any app that can calculate more accurate the consumption?
1 - 9 of 9 Posts

· Registered
Alfa 159 2.0 JTDm Lusso 6sp.
Joined
·
10,726 Posts
I dont have an answer....but that is odd. When I had mine tuned (map and then tuning box) the dash MPG was very optimistic....IE it said i was getting lots more MPG than I really was. Was the same in my old Grand Cherokee. I think this was caused by the car ECU not "knowing" that the extra fuel was being injected as part of the map. But yours seem to be the opposite thing....
 

· Registered
Joined
·
1,474 Posts
I dont have an answer....but that is odd. When I had mine tuned (map and then tuning box) the dash MPG was very optimistic....IE it said i was getting lots more MPG than I really was. Was the same in my old Grand Cherokee. I think this was caused by the car ECU not "knowing" that the extra fuel was being injected as part of the map. But yours seem to be the opposite thing....
No, his one has the same optimism as yours. With litres per 100 km the smaller the number the better. With MPG, the bigger the number the better. 'Better' meaning, using less fuel.

Best way really to calculate consumption is just do it yourself between tank fills and record the mileage travelled in that period.
 

· Registered
Joined
·
99 Posts
Discussion Starter · #4 ·
Thats the only safe way to calculate it for sure... But still the difference is way too much, this could mean there is an untrusted fuel left calculation too?
Does anyone know how is the computer working? I mean it measures the distance and the consumption of the reservoir? It measures the "injections" of fuel and the distance?
 

· Registered
Joined
·
527 Posts
Trip computer is very untrustful on short distance runs. Calculated fuel consumption would drop very slowly with each few kms you've passed on the motorway. But once you have stopped and idling, it would gain some 0,1 l/100km every few seconds!
 

· Registered
Joined
·
99 Posts
Discussion Starter · #7 ·
On remap method... Yes its the only way to explain it. And the only that i cannot find out! My tires circle is 3.2% bigger so in 100kms the real are 103.2 kms. The resistance though is way much bigger (245 vs 205)
 

· Registered
Joined
·
99 Posts
Discussion Starter · #8 ·
I still dont get it. Bigger tires= higher consumption ok. But still doesnt explain the difference between the computer and the real one. Driving with 5 passengers and packages is higher consumption than driving alone. This difference the computer should understand somehow and calculate it. Same thing with tires too. The computer doesnt calculate the amount of diesel it consumes in real? What it measures and calculate the consumption?
 

· Registered
Joined
·
527 Posts
Noone, except people who engineered and programmed this trip computer (or someone who reverse-engineered the formula from the firmware code) knows exactly how it works.
From my experience of ownership, it may be something like this:

( injection per stroke (mL) * RPM * time * "load factor" ) / (distance driven)



As your car is remapped, fuel injection values and some "load factor" were manipulated, and are not reporting actual realtime values to ECU. Hence, the difference.
 
1 - 9 of 9 Posts
Top