Mass Fuel Desired vs. MPG or L/100Km:

First, Mass Fuel Desired (MFD) is a simple number that represents a command from the computer to tell how much fuel should be applied every time the injector fires. Fuel applied per cycle is a combination of the Injector Control Pressure (ICP) plus the Fuel Injector Pulse Width (FIPW). Low ICP with a short pulse width is what we see at idle. High ICP with a longer pulse width is what we "feel" in permagrin mode. MFD is, in essence, throttle... which we can see with a scan tool in cruise control, or by your right foot.

Now... If you drive along at 50 MPH with a low MFD and a high RPM, this could possibly use the same fuel quantity per hour as a high MFD at a low RPM. Estimating your fuel economy is a matter of combining vehicle speed, RPM, and MFD. We mere mortals could never track all of this with fluctuating vehicle speed, engine speed, and MFD... but we can watch the MFD in Cruise Control and get an idea of what helps/hurts our economy.

Say you are driving along at 65 MPH (with all due respect to our metric friends) on the flat with zero wind, 1900 RPM, and your MFD is 20. Wind or a grade will change the MFD, but not the MPH or RPM.



Now... say you want to learn what happens to MPG when you inflate/deflate your tires 10 PSI. Simple. Get your MFD at a set speed/RPM on the flat with no wind. Try your tire experiment and repeat your run with an identical speed/RPM. You will not only see IF it helps/hurts... but by how much.

Theoretical example:

• 60 PSI air = MFD 20
• 50 PSI air = MFD 22

In this case, deflating your tires by 10 PSI would do the same to your economy by 10%.

Increasing your speed gets more complicated:

• MFD 20 @ 1900 RPM = 65 MPH
• MFD 23 @ 2200 RPM = 75 MPH

Your MFD went up 15% and your speed went up about 13%. Close to a wash, right? Nope. Your RPMs went up over 6% (more cycles per second AND more fuel per cycle). Finger-dancing on the calculator with this theoretical exercise lost 6.5% fuel economy... but I think reality would show a bigger loss than what I have shown here.

You are more than welcome to hit me with the numbers on your vehicles and we can do the math together. All we need is engine speed, vehicle speed, and MFD... then change just one thing (like speed, or which gear you're in) and drive the same run. Again, this works best on the flat with no wind... unless the road or the wind is the only thing that changes. Why this boring exercise in math? You're looking for a way to get better MPG. G'head... tell me I'm wrong.

 

I definately need to get more towing data. After the cup party, I hope to get more on a planned tow in July. For now, take a look at this section of data while hauling at 16K GCW in 80t over Steven's Pass, WA. There are two stretches of road that have a good incline, separated by a hairpin turn where I slow down - you can see this in the data. While the climb went well, I had to modulate EGT's with the pedal, and I had a bit more smoke than I would have liked - I think due to the HPOP not being able to keep up. Green line is IPR, Red line is ICP, Blue line is RPM. You can see the IPR is asking for over 97%, but the HPOP struggles, then climbs a bit. Not sure why it recovers. RPM is pretty steady.



This last weekend, I ran in a modified 40T, but did not have the laptop with me (stayed unplugged on this campout). Had the boat on this time, for the full 19K GCW. Switched between Rich's modified 40T and a modified 60T I bought from DP that adjusted shift strategies. Both worked great, and supprisingly felt about the same in terms of power. I'm told that Rich's 40t has been juiced up a bit, similar to a 60t. EGTs would not go over 1200 degrees for both, and smoke was minimal. I'm sorry I did not get this data, as I'd like to see how the HPOP and IPR were doing, but I'm betting the HPOP was more than able to keep up at this power level.

I have a "70t" custom tune I'm going to try next time, to try to use up all the EGT I can, be more gentle on the HPOP, and have max safe power with this stock injector/HPOP/turbo setup I have now. 80t is too much, but 60t appears to leave some on the table.

I'll load both modified 40t and 70t and take the data and get it back here in July for your review, 40t (really close to 60t) on one leg of the hill, and 70t on the other.

 

my picture stinks! Guess I need a little help. I've saved it to my gallery, making sure it's no bigger than 1024 x something, and the size of the jpeg is only about 124K, but it's being scrunched to about 33k by the forum, making it small and fuzzy. Hmmm...

 

I looked at the details of your picture and it's located in something called "Galleries". I use my photo album for my images.

 

My current HPOP, tuning, and sticks wouldn't ask for more IPR than about 50 %, and my ICP can hang around 3000 PSI all day. When I had the stock sticks, it asked for a little more work from the HPOP... like about 60% IPR at WOT.

With your microscopic graph showing about 1800 PSI and 100% IPR, either your tune is too hot for the climb... or your HPOP is not up to this new task.

Stock injectors, stock HPOP, stock tune.



Stock injectors, stock HPOP, tuned.



For those asking "What's happening?".... the tune is asking for more oil than the system can deliver. When I let up off the throttle, my Powertrain Control Module was asking for everything the HPOP had, but it can't back off the demand quickly enough... so the Injector Control Pressure spiked to about 3600 PSI (hard on the ware). This can be brought about by a weak HPOP, tired injectors, internal oil leaks, a hot tune, a bad IPR, a bad ICP, or maybe something I'm overlooking at this moment.

 

Right. The photo album I use is "Jeff's Truck". It's located in "Edit Galleries" on my user control panel (user CP). It says it's an album, but maybe there's a way to access it separate from the gallery - I dont know.

 

Thanks, Rich. I missed the obvious, and should have used the "Albums" link next to "User CP". Picture comes up good now. I'll start moving my pics to this "Jeff BWST" album.

Looks like i get that same PCM suprised spike in ICP when I let off the pedal at the hairpin. A good spank to the HPOP it probably does not care for.

 

I did more research on my Computer Data Link issue. The pin that isn't hooked up is circuit 693, is this my issue? My AE wont pull : number of misfire, misfire diagnostic, O2 sensor, or perform a buzz test. My trucks vin ends in 00380 so it is a very early 99.
Thanks in advance.

 

Much of what you list is not on our diesels, but the buzz test should work with KOEO. Circuit 693? I'll try to find a reference to that.

 

The diagram reads.
Pin-3, Cuircut -693 (OR), cuircut function- UBP diagnostic.
I assume the OR is orange.
The two wires hanging are red white stripe, light blue.
That's all the info I have
Thanks Tugly

 

Ah... I'm on it now. There are multiple networks in our vehicles, depending on the model and the year. The highest number of networks I can find are 4 - ISO, SCP, ACP, and {drum roll} UBP. Would you like to know what modules are on the UBP "bus"? Air Suspension module (automatic), Driver Seat module, Electronic Automatic Temperature Control module (for the cab), and the Vehicle Security Module. The print shows a UBP connection to the Instrument cluster, but it also shows the Instrument cluster is on two buses - UBP and SCP. The Instrument Cluster is where you get speed readings (through the SCP bus) and maybe a few other PIDs. PATS is also on the SCP bus.

My truck doesn't even have a pin 3 connection on the print or on the OBDII port, and I get all the data I've been looking for.

 

Thanks for your help, so I am straight I don't need #3 pin. Guess I will contact AE make arangements to have my unit looked at.
I am going to reread this post later I will give you readings on a less than perfect truck for class.

 

Mass Fuel Desired. What happened?

I gave it a little time and the whole subject made a resounding thud during the face-plant. This is big, guys... it's for troubleshooting your MPG or L/100km. I will guess that I failed at explaining it in a way that isn't boring. If I get a little feedback on this, I'll know how best to help you guys get better fuel economy.

 

I watch my mass fuel desired all the time, it usually stays between 16 - 18 with cruise set at 60.

 

Excellent! Do you know what your economy is on a long stretch of driving at that speed? I know what mine is... over 19 MPG.