Why such big diff from diesel DIC mpg & hand calculation?

Babyhauler · August 31, 2015

I have had 04 2500HD Duramax diesel for about 5 months. 200k mi. Daily work driver and tow vehicle. Is any other Duramax owner experiencing this? Have you technicians run across this at dealership?

The question is: Does anyone know why mpg calculation by hand is 4-5 mpg lower than what DIC is showing? This is not about my mpg, but why is the computer showing much less fuel used than actual. My other trucks and Tahoes were usually only about 6 tenths off, not 5 mpg. A consistent example would be a fill up of 20.9 gal, while the DIC shows 14.38 gal used. (11 mpg vs 16 mpg). Over 6 gal difference. DIC reset with each fill up.

Where does the computer measure the amount of fuel used? Pump revolutions or a flow meter somewhere? Is it dumping excess fuel because it thinks it is "leaning out" under power?

This has been consistent in town towing or on the road. Every fill up. I try to park level when fueling & fill to the same spot in the tank hose.

It has needed injectors and they were just replaced. Same results, but mileage was better.

So technicians, do you know the answer or can you look it up in GM's problem solving database?

Thanks for your help.

2500HD 4 x 4, Crew, MBRP exhaust, cat delete, Bully Dog CAI, Edge tuner w/ Attitude, 265/75-16 BFG T/A's

Doug_Scott · August 31, 2015

With electronic injectors the ecm can use the duty cycle of the injector to calculate how much fuel is being injected. If you change the size of injector I can see how that would alter what the ecm thinks it is providing.

You will get a better view of the mileage by not resetting it at fill up. Run 10 tanks of fuel through it without resetting the DIC and see what the avg MPG the DIC provides.

yareelohim · August 31, 2015

What size tires are you running and are they larger, smaller or same as factory tire specs?

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Babyhauler · September 1, 2015

Thanks for the replies.

Trying to find the inaccuracy of the fuel used vs what is actually used.

Not worried about mileage right now, just why it says 15 gal used and actually used 20 gallons as an example.

Some part in there is not reading right or squirting right or pumping right.

Tire size does not matter in amount of fuel used. Stock was 245/75-16, now have 265/75-16 BFG T/As. Last line of op.

Thanks.

Chris · September 1, 2015

If you want hand calculated accuracy, but digital, there is an app called FuelLog. At least on Android. You enter odometer reading, fuel price, and amount filled. It will actually keep track of everything. Mine doesn't have a lot of data because i have a new phone.

As for the accuracy of the DIC, I've seen them go from .5 to 3 MPG off. After the tires start wearing the circumference changes, air density can change with temperature, and a whole lot of other variables that can factor in to it.

Here is what the app looks like if you want to try. It was free, but i got the paid version. I love it. It will also let you track maintenance costs.

yareelohim · September 1, 2015

Tire size does not matter in amount of fuel used. Stock was 245/75-16, now have 265/75-16 BFG T/As. Last line of op.

Thanks.

In your case the difference in tires may not be the cause but tire size affects the amount of fuel registered in the DIC vs actual used.

If you increase to 35" or greater tires and don't lower your gears to match the change, then your odometer will register less miles than you are actually driving. You can drive 200 miles and the odometer will only register 150 miles. The DIC averages the fuel used with the distance traveled. With 37" tires and 4.10 gears, you would see 4- 5 mpg less on the DIC than actual.

That unfortunately may no be the case for you though.

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Doug_Scott · September 1, 2015

Tire size does not matter in amount of fuel used. Stock was 245/75-16, now have 265/75-16 BFG T/As. Last line of op.

yareelohim is correct and in one context you are also correct. Another way to explain it is that while the ECM will still measure the amount of fuel used the same way, it will also measure distance traveled the same way. Change tire diameter, and the distance traveled per revolution will change. ECM has no idea you changed tire diameter.

All that being said, the difference is a huge 4%. And with mathematics being a pure logic, your mileage will also change by 4%, provided the load on the engine is the exact same. But, it is not going to be the same. Mileage will change slightly though.

yareelohim · September 2, 2015

I just thought of something that could also have this affect. Is it possible the gears were changed in your truck? Lower or taller gears with stock diameter tires will create a false DIC reading.

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Babyhauler · September 2, 2015

I do not believe gears have been changed. I can contact previous owner & ask.

The gears and tire size would matter on mpg.

But, the question remains: why is the gallons used 30% off? If the engine was in a boat or running a generator, the fuel used is not being read correctly vs what it is using. What little part does GM have in there somewhere that sends the "fuel used" number to the computer? What makes mine & others read the wrong amount?

Thanks and ask around.

Doug_Scott · September 2, 2015

I do not believe gears have been changed. I can contact previous owner & ask.

The gears and tire size would matter on mpg.

But, the question remains: why is the gallons used 30% off? If the engine was in a boat or running a generator, the fuel used is not being read correctly vs what it is using. What little part does GM have in there somewhere that sends the "fuel used" number to the computer? What makes mine & others read the wrong amount?

Thanks and ask around.

I know it has already been said, but, you cannot think that the fuel used or DIC mileage calculations are even close to reality if you reset it at every fill up. If you reset the DIC on your next fillup, try to get a highway as soon as you can and then start trying different scenarios and watch your mpg number jump around. You can go from near mpg to over 50mpg just by starting from a dead stop, and run the truck up to 100mph. Then let off the gas pedal and watch mileage climb.

Babyhauler · September 4, 2015

Wow.

My Duramax is running a water pump in a remote location. I have the DIC set up on a mobile telemetry cell phone wifi so I can monitor it remotely. It happens to have a 26 gallon tank. It runs a little each day. I make the 3 hour drive on friday to check it in person. The fuel tank is almost empty and I did not take any fuel because the sensors said it had used 18 gallons instead of the 24 it used. The fuel truck will be there Monday. I drive a 4 hour round trip to get extra fuel for it because it has a faulty sensor. On Monday, I call my GM Duramax Water Pump Co. I say, I need to "ask the GM Technician" a question about my fuel sensor. That's really why I need an answer for this, so the villagers wont go with out water.

I may be wrong, but I think that fuel used is just that, fuel used. Maybe a Bosch or Isuzu or GM engineer knows how it works and will tell us because I really want to know.

No math calculations based on speed or coasting or gears or tires or loaded or empty. Just how much fuel goes by a little sensor somewhere. Then everything is figured off of that.

I know the avg mpg varies as you drive, It changes a lot at first due to the math, but calms down as the miles driven and fuel used increases.

My 04 has a DIC business calculator setting that I reset when I bought truck. The mpg does not change much. The 6 months total of fuel used goes up a little each tank. I could manually add up my hand recorded gallons used and compare it to the Business setting gal used. No way. MPG does not matter now since I am running a water pump.

I guess the Fuel Injector Control Module might be the magic part. Does this part read fuel usage?

Thanks again and the search continues.

Doug_Scott · September 4, 2015

This is beginning to sound like you are asking for information on how the DIC calculates fuel used on your daily driver, then you apply the suggestions to a stationary engine. You could have saved a lot of time if you had said this was a stationary engine. I know it should not matter if you are only looking at the one item, fuel used(or fuel left as well).

There are only so many ways that the pcm can calculate fuel used. One way is to simply use the duty cycle of the injector, along with the engine speed, aAnother way is to use the fuel level gauge, along with duty cycle and rpm.

Have you considered using a custom or homemade tool to measure the fuel flow from the tank, and also the fuel flow in the return line (if so equipped)?