PCM Gas Mileage?
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PCM Gas Mileage? My Turbo 02 Z car was tuned using 35lb injectors in the PCM where I really have 42's and my gas mileage went up 3mph, but, the tune was way rich.
Does anyone know how the PCM figures out gas mileage?
Thanks!
Does anyone know how the PCM figures out gas mileage?
Thanks!
Last edited by rosulekg; May 29, 2007 at 04:39 PM.
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From: England UK
Are you comparing injectors like for like?
What I mean the flow numbers of the #35lb was it running at 43psi? and are the 42lb @ 43 psi?
If they are both measured at 43 PSI then I guess you are running richer than they tune was, its better than lean. What do your fuel trims look like, they may be taking away enougth fuel to make it run almost right as there is +/- 25% in the fuel trims.
3mpg, what are you reading that from? is it on the dash or are you working out from filling up and counting the miles.
What I mean the flow numbers of the #35lb was it running at 43psi? and are the 42lb @ 43 psi?
If they are both measured at 43 PSI then I guess you are running richer than they tune was, its better than lean. What do your fuel trims look like, they may be taking away enougth fuel to make it run almost right as there is +/- 25% in the fuel trims.
3mpg, what are you reading that from? is it on the dash or are you working out from filling up and counting the miles.
TECH Resident
Joined: Aug 2003
Posts: 970
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From: England UK
okay that would make sense.
Bascially the PCM knows how many grams per second of fuel its putting into the engine, it knows the speed so it works out the MPG from that.
The PCM thinks 36lb's are installed by looking at the injector flow rate table.
If you car is running in closed loop, which as default it is, so if your running rich/lean it will self calibrate.
You have put in 42lb'ers, and you PCM is now putting in less fuel, the fuel trims will be negative. Now the injector pulses are much smaller. As the injector flow rate table is still the same, it thinks its putting in a fraction of 36lbs fuel. So the PCM thinks its delivering less fuel, therefore MPG will go up, infact the car is deliverying the same amount of fuel as it has adjusted the fuel trims. The error is in not changing the IFR table. You havn't gained MPG, it will be the same as before.
Hope this helps, hope I explained it well, if not let me know.
Bascially the PCM knows how many grams per second of fuel its putting into the engine, it knows the speed so it works out the MPG from that.
The PCM thinks 36lb's are installed by looking at the injector flow rate table.
If you car is running in closed loop, which as default it is, so if your running rich/lean it will self calibrate.
You have put in 42lb'ers, and you PCM is now putting in less fuel, the fuel trims will be negative. Now the injector pulses are much smaller. As the injector flow rate table is still the same, it thinks its putting in a fraction of 36lbs fuel. So the PCM thinks its delivering less fuel, therefore MPG will go up, infact the car is deliverying the same amount of fuel as it has adjusted the fuel trims. The error is in not changing the IFR table. You havn't gained MPG, it will be the same as before.
Hope this helps, hope I explained it well, if not let me know.
