? About Fuel Gauge and SCT - Diesel Truck Forum - TheDieselGarage.com
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post #1 of 5 (permalink) Old 03-24-2010, 02:22 PM Thread Starter
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? About Fuel Gauge and SCT

When I was stock the fuel gauge would be pretty accurate as far as miles until empty. I could normally get down to 50 miles until empty then refill. The miles would on the counter would match fairly well. For ex 100 miles to empty and I could go 50 miles (actual driving miles) and then the miles to empty would be 50 or there about. Now with the sct on the vehicle and driving habits the same (pretty much) My miles to empty go from 150 miles to 40 miles to empty on sometimes 40 miles (actual driving miles). Last week I got in the truck and it said I had 123 miles to empty and the gauge was between 1/4 and 1/2. I went 38 miles and the gauge said 40 miles to empty. I was 10 miles to the fuel station and when I made it there the gauge said 20 miles to empty....I turned the vehicle off, fueled up then left. About a mile down the road the truck ran BAD, Rough, Shaking then as I was pulling to the side of the road it was fine. No problems since then other the gauge being off as usual. Another example was I had a 1/4 tank put 15 gallons in it then it showed almost 3/4 tank and like 400 miles to empty. As soon as I started driving the miles to empty rapidly fell from 400 to 200 (probably where it should have been) while driving.

Have any of you noticed this with the SCT or am I having some other problems such as fuel sender problems? Please help and thanks if it you made it this far in my post

2006 F250 6.0 Crew Cab King Ranch 4x4
Edge Insight
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post #2 of 5 (permalink) Old 03-25-2010, 01:21 AM
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to the op i know i posed this in a different forum but some might find it informative.

Originally Posted by SpartanDieselTech View Post
The PCM also calculates an MFDES, (mass fuel desired each stroke) off of pedal position and other various sensors. It converts this MFDES into a VFDES, (Volume of fuel desired each stroke) which is the amount of cubic millimeters of fuel delivered to each cylinder on each firing event (found my multiplying the MFDES by an estimated fuel density). By keeping a counter of firing events off of the CKP and CMP sensors, the computer knows how many firing events have been completed, and calculates a total of how many total mm^3 of fuel have been delivered. This calculation is converted from mm^3 (which a cubic millimeter is .001 milliliters) to imperical gallons, and the milage traveled in the given distance is divided by this to come up with the estimated MPG.

The reason yours always reads high is because your actual VFDES is much higher than the commanded VFDES from the PCM, thanks to the Banks unit intercepting it and raising it before it reaches the FICM on the valve cover to allow more fuel delivery- therefore more fuel is actually being delivered to the cylinder than the PCM is aware of.

The PCM knows the capacity of the tank, and the percentage fuel that is read by the tank sending unit, and calculates how much volume of fuel is left in the tank, and multiplies this by its estimated fuel economy value to get your "Miles to Empty" estimation read by the overhead meter.

The reason it is not always accurate is because it is impossible to get an EXACT fuel delivery calculation. Alot of people gripe and complain (on stock trucks) about the accuracy of the gauge, but considering how it must be calculated, I think it does a very fine job. When you calculate by hand, you are making a single calculation and a small percentage error poses very little threat to an accurate figure. However, seeing that the computer recalculates on every single firing cycle, it has ALOT of chances to be off a very minute amount. As you can imagine, a very, very small error multipled thousands and thousands of times can total up to a pretty significant amount. I highly doubt any human being could make that many thousand hand calculations and come out as accurately as it does, so people shouldn't rag it so hard

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post #3 of 5 (permalink) Old 03-25-2010, 07:25 AM
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The mileage thing is always messed up by tuners.

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post #4 of 5 (permalink) Old 03-29-2010, 01:32 PM
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this problem left me out of fuel a couple times until I got used to it. Now I start looking to fill up when it gets to a 1/4 of a tank. not a good thing to run these trucks out of fuel or on low fuel pressure

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post #5 of 5 (permalink) Old 03-29-2010, 11:12 PM Thread Starter
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What scares me is that I am about to do a 600 mile trip while towing my 16,000 5th wheel. Yes I will have Matts tow tune (not the tow pig, just a tranny tune and turbo brake) but now I am afraid to get below a half a tank with the damn thing. I guess I will have a couple of 5 gallon tanks with diesel in them ready to go.

2006 F250 6.0 Crew Cab King Ranch 4x4
Edge Insight
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