2nd Gen Non-Engine/Transmission How to adjust the overhead console MPG?

[rweis]

I about have my 2nd gen running really well mechanically. Now I want to figuer out how to get the miles / gallon right in the overhead console so all the rest of the overhead console calculations will be correct concerning miles to go, average miles / gallon etc.



I think the mileage comes off the ring gear in the differential and that should be correct (or is there a way that it can be adjusted?).



I am thinking there must be a sensor input to the ECM that tells the ECM fuel flow so it can then do the calculations and send the data on to the overhead console.



Can that sensor associated with the fuel flow be adjusted somehow so it really is about in the correct range? Where is it anyway?, in the VP44? Is it based on rpm and assumes so much fuel per rpm?



Does anyone have any ideas?



It has to be some sort of an electrical signal from sensor X, to the ECM and converted and put on the CanBus so the overhead will display it.



IF we can intercept the signal and add or subtract resistance(?) so we can get the overhead to read reasonably correctly would be great.



Bob Weis

 

[Bob4x4]

The ECM rcvs inputs from sensors,and knowing fuel pulse widths,Injector flow #s rpm,map tables etc computes the #s. There are no flow or fuel psi sensors involved.



Bob

 

[WTBurke]

I too have played with this a little bit. I have changed tire size several times over the last 5 years. I purchased the SuperLift TrueSpeed tire size corrector. By observing my laptop based GPS I could dial in my speed dead-nuts on however; in making long trips I would use the mile-markers on Interstate 80 through Nevada for several hundreds of miles and made notes. I discovered there is slight errors in my Dodge system. By hand calculating fuel milage according to my odometer and taking the correction into the equation I found if I set my speed to check with the GPS my odometer was slightly off. Now if I correct my odometer according to the mile markers my speed would read a bit high. In all of this I found the inaccuracies to be just what they are. I have it set for correct speed at this time and by hand calculation, my fuel milage is off by 1MPG less than what my overhead reading gives me. I guess it's not too bad as several other members have posted some pretty large errors compared to what I am observing. I guess the Certified Speedometers that the Hiway State Troopers use have better electronics than we do. It's close but, not certifiable by strict standards.



You are right Bob Weis, blue printing that ring in the back pumpkin maybe the way to dial these in a bit better.



William

 

[WestTN]

The fuel consumption is calculated from stock engine parameters, not measured. The only Smarty tune that will report accurate mileage is the Stock tune. If you change fuel injectors the calculated value will be wrong. And tampering with the speed sensor will throw off the speedometer and odometer. It can also affect the anti-lock brake system. Then, there is the cruise control... .

 

[WTBurke]

okay.....

William

 

[rweis]

I should have mentioned that I have DD2's and that is what is throwing off the calculations (probably?). Tire size is stock tire size.



It may be too complicated to mess with. ABS works good, etc so I thought messing with the braking system components was not a good idea.



"fuel pulse widths,Injector flow #s rpm,map tables etc computes the #s. "



Maybe injector flow #s could be changed as you change injectors.



Rpms are the same to go the same speed.



I do not know about pulse width.



I would suspect that it would be less TPS (APPS) with bigger injectors than stock injectors to do the same thing.



Is it a TPS thing? Cruising along, TPS voltage is x. If you want to slow down the TPS voltage changes telling the VP44 to narrow the pulse width? If you want to go faster is it the TPS that tells the VP44 to widen the pulse width?, and you get more fuel?



Somewhere it assumes you have the stock injectors ie x amount of fuel per pulse at a given rpm and pulse width.



Don't know, maybe a wild goose chase.



Is this a possible Smarty idea? to be able to change injector flow rates say in increments of 25 hp? +25, +50, +75, +100, +125, +150, +175, +200 or 8 levels of injector flow rates?



Bob Weis

 

[Wyomingcowboy]

You guys make my head hurt with all of the technical talk, but I too am frustrated with this stuff. My hypertech tuner throws it off 3-6 mpg. Can't imagine what a smarty might do to it... ...

 

[brods]

The ECM rcvs inputs from sensors,and knowing fuel pulse widths,Injector flow #s rpm,map tables etc computes the #s. There are no flow or fuel psi sensors involved.



Bob

LOL, several hours of research last night... . and the answer was here all along! Thanks Bob.



So, somewhat oversimplified, the ECM just infers fuel consumption from the lookup tables? It does not monitor pulse width or rail pressure directly to come up with the fuel volume, it just assumes everything is working as its told to? That explains a lot. The overhead comes amazingly close given all the room for compounding errors.



So reasons for the ECM to have incorrect MPG numbers on a stock truck are as follows:



Rail pressure higher or lower than the ECM thinks it is (ie variances in rail pressure sensor accuracy).

Variances in injector flow rates from the ideal injector used for the lookup table data (undersized or partially blocked injector orifices, oversized or worn injector orifices, faster or slower response time than ideal injector).

Leaky injectors.

Speedometer inaccuracies.



Are these correct and are there any others?

 

[NoSeeUm]

It is a bit different for a 2nd Gen I would think, but more or less the same.



The VP44 is a positive displacement style pump. With the fueling solenoid closed 100% of the time the VP44 will pump a set volume of fuel per revolution. This volume is determined by the bore and stroke of the pump. From there its just math.



Just to throw in that max'd out, the number that gets often tossed around for the VP44 is 45 gph. And I am guessing that is at 3200 rpm. The VP44 is capable of 650 Hp give or take.



Realistically, lets say that roughly it takes 11 gph to make about 180 Hp. For the moment consider that, 11 gph is what the ECM thinks is 100% load.



Now lets say we are driving along about 60 mph, 1800 rpm and empty (approximately 44 Hp). The ECM might be at around 25% load. Straight up, that equates to about 2. 75 gph (11 gph x . 25) or 21 mpg by doing the math for speed.



When tuning or injector sizes gets altered the ECM still uses the same math algorithm. Like Bob4x4 kind of said "Garbage in = Garbage out" for the ECM computation.



I would think typically because that with a higher Hp tune, the engine is making more Hp for an existing ECM load percentage value. So the calculated value (overhead MPG) is frequently higher than actual.



So take another look at my example from above, but change some numbers. Lets say the ECM was at 23% vice 25% load for the same conditions. Which equates to 2. 53 vice 2. 75 gph and 24 vice 21 mpg. But in reality, the above numbers hold true and hence the error.



Sorry RWeis, I do go on sometimes...



Jim

 

[rweis]

Kind of what I thought - "When tuning or injector sizes gets altered the ECM still uses the same math algorithm"



I wonder if the Smarty would have access to that algorithm? and would it be changeable. Not precisely right on, but 10%, 25%, etc to get within say 5%.



Ah well, might be something I (we) can not do anything about.



I was thinking that maybe "tweaking" a voltage (or a ground) in the ECM might have some effect, but no idea of where to even start looking.



Bob Weis

 

[WTBurke]

With the SMARTY being added to my platform the milage calculation from the overhead console is now way off. Now I understand what all the guys were saying about the errors they were seeing. Oh well... back to hand calculations for me.



William

 

[NoSeeUm]

I wonder if the Smarty would have access to that algorithm? and would it be changeable. Not precisely right on, but 10%, 25%, etc to get within say 5%.

You must cut down on the Holiday Cheer... . A radical new VP44 Smarty development?



I would think that it would be fairly difficult or easy to do. At least that is the answer from allot of control software guru's. Probably much easier to reprogram on a 3rd Gen ECM, I would bet. At a minimum, a factor would need to be entered to the ECM via the downloader to compensate for different size injectors. The Smarty already does tire size and RPM. Call the bets off if a wire tap fueling box is in use.



In stock form, I am not sure how accurate mine was to begin with. It was nice watching a realtime MPG display for the first couple of months I owned the truck. These days I watch the compass / OAT display, for safety reasons.



Just guessing, but i suspect that the MPG calculation might come as a by-product of some core function running in the ECM. That just happens to glitz up the sales brochure and by not taking up too many CPU cycles for the combustion engineers to get uncomfortable.



As a suggestion, you might try a ScanGauge. I have one and have dinked with it some about realtime fuel economy (etc... ). But to be accurate, even the ScanGauge needs some mindly serious parameter tweeking. But I have not messed with it enough to achieve this myself, because once it gets unplugged it loses its mind. I got tired of re-entering the parameter specifications when I was still messing with the Smarty some. The ScanGauge uses fuel tank capacity and level drop rate for its calculations on fuel usage, I believe.



Maybe I'll mess with my ScanGauge some more if you are interested enough. Let me know.



Jim

 

[rweis]

NoSeeUm -



The ScanGuage looks pretty interesting. I especially like that it mearly plugs into the truck. Are there any functions that you find it does not do on the CTD?, or does not do well? Looks WAY more versitile than the overhead (of course the overhead was never designed to the extent of the ScanGuage).



Hmmmmm, now how to mount the ScanGuage to the overhead console. The cable that came with it probably not long enough, but I would bet they may be able to make a longer cable. There is probably a cable with enough conductors in the cable at one of the major national electronics parts houses like MilesTek.



Could finally be an answer to the overhead, of course there is a cost involved



Definitely interested!



Bob Weis

 

[NoSeeUm]

Merry Christmas to you, Merry Christmas to you!

NoSeeUm -



The ScanGuage looks pretty interesting. I especially like that it mearly plugs into the truck. Are there any functions that you find it does not do on the CTD?, or does not do well? Looks WAY more versitile than the overhead (of course the overhead was never designed to the extent of the ScanGuage).



Bob Weis

It does not do ignition... .



Other than that, as I recall, the MAP either only goes to about 5 psi or the ECM does not care what it is if boost is higher than that. The MAP reads out in inHg. Maybe fuel pressure does not work, for obvious reasons.



The engine code reader function / reset has been useful at times. It also has some nifty other features, like max rpm, max speed, average speed and distance traveled per day, per trip etc... .



It does TPS and Engine Load % which are neat to watch sometimes. I bought it mostly to monitor IAT.



I mounted mine on the dash under the radio using velcrow strips. The cord is not overly intrusive, but makes it lose that OEM look. Everyone seems to think it is another hotrod box I have stacked in the cab.



Mounted in the overhead would be really tight, because you can change the display to about a bazillion different colors.



Jim