When I bought this truck late last year I was getting 21.5 mpg hwy. in stock form with the exception I am running 265's. Now that I have done the mods listed in my signature, had a new FPR put on and the truck reflashed at the same time I am getting 18.5 mpg/Hwy. What the heck is going on?

The air filter has about 6K miles on it now, fuel filter has less than 2K on it. I am at a loss. Suggestions? If the air filter is partially dirty would that cause such a dramatic loss in mileage?

Maybe it's because you are putting gas in it :D

Are you hand calculating the fuel mileage? That will make a difference. Could also be the way it is being driven, also could be the fuel. You will usually drop a few MPGs from winter blend fuel. We are just now transferring out of winter blend.

The problem might be that you are running GAS in a truck that takes DIESEL or FUEL. ;)

Wow...didn't know we had such comedians on here!!!! Yes I am hand calculating but the digital truck computer is also showing a decrease. The 21.5mpg was during the winter and I doubt seriously here on the coast of California we ever truly had "winter" fuel.

I put on my flame retardant britches and PM`d you Teleskier... :D

Have you recalibrated your speedo for the increased tire size?

tire size was the same in all calculations. Also, RPMs were held at 1800 or so.

Also, RPMs were held at 1800 or so.

:confused: Ummm, what? the whole time?? I'm not even mad, that's amazing! :D

your speedo is off and therefore your mileage is off due to the increased tire size. If you recalibrate your speedo your mileage will increase to 19.5 to 20mpg. I have the same tires and I have done the math.

when i put my egr blocker on i noticed a decrease in "FUEL" mileage...was talkin to a teacher here at school about it and he said "well what do u think it is" and i said well since a egr puts exhaust gas back into the intake, its taking up space in the cylinder so the cylider is not full of 100% fresh air...so if you block off EGR then you get 100% air and the more air you put into the cylinder the more fuel that will be dumped in the cylinder to try and make a perfect air/fuel ratio...he said good thinking and then his phone rang...never gave me a straight answer lol so what i thought may or may not be right

when i put my egr blocker on i noticed a decrease in "FUEL" mileage...was talkin to a teacher here at school about it and he said "well what do u think it is" and i said well since a egr puts exhaust gas back into the intake, its taking up space in the cylinder so the cylider is not full of 100% fresh air...so if you block off EGR then you get 100% air and the more air you put into the cylinder the more fuel that will be dumped in the cylinder to try and make a perfect air/fuel ratio...he said good thinking and then his phone rang...never gave me a straight answer lol so what i thought may or may not be right

I dont know your teacher, so first establish that this is a diesel not a gasser. The truck does not read its AFR, therfore it will not adjust fuel for the additional fresh air.

So a PM from a member suggests the same thing. Blocked EGR = decreased "fuel" mileage. How is it explained then? Seems the only common link between us so far is the blocked EGR.

I dont know your teacher, so first establish that this is a diesel not a gasser. The truck does not read its AFR, therfore it will not adjust fuel for the additional fresh air.

hes one of the diesel techs here at school so yes we were on the same page as far as diesel goes...idk if i was right or wrong. i was tryin to think about whats going on inside the engine...if you get, lets say, 20 mpg then you put a egr blocker on and get 17mpg then something must be going on inside the engine that is needing the extra fuel...thats why i thought of the egr being blocked, more fresh air is being put into the cylinder thus more fuel being needed...im not the smartest guy in the world when it comes to diesels or gassers, thats why im going to school to learn but i was just tryin to think out the box on a situation like this.

:confused: Ummm, what? the whole time?? I'm not even mad, that's amazing! :D

+1 for the Anchorman reference.:D I'd love to get 18 mpg on the highway. I usually average 13-14 driving city.

I have an LBZ, but I too have noticed a decrease in mpg after blocking the egr.

-2-3mpg across the board.

It was probably the reflash that made the difference. Also, our trucks are set for 265's from factory 31.75". Noticed that my speedo was faster than my gps said it was. Looked at it when I had my PMT on the truck and it said 31.75" default. So, your speedo is now accurate with the 265's on. Best thing to do is EFI Live tune the truck and make it run at peak efficiency. I get 19.5 on 33's mixed driving. Not sure what highway is yet, but i'm thinking it's the reflash that dropped your mileage.

if so, where do I get it EFI live tuned?

It was probably the reflash that made the difference. Also, our trucks are set for 265's from factory 31.75". Noticed that my speedo was faster than my gps said it was. Looked at it when I had my PMT on the truck and it said 31.75" default. So, your speedo is now accurate with the 265's on. Best thing to do is EFI Live tune the truck and make it run at peak efficiency. I get 19.5 on 33's mixed driving. Not sure what highway is yet, but i'm thinking it's the reflash that dropped your mileage.

That's what my guess would be too. I would like to get mine udpated, but am afraid of what it will do to my mileage. I'm original production stock with 285's and just got 17 on a road trip with a small utility trailer in tow. If I stay out of it I can get 19, and if I drove 60 I've gotten as high as 21. That's all with gps miles and fuel receipts.

so another question...can you go back to the original settings somehow before it was reflashed?

so another question...can you go back to the original settings somehow before it was reflashed?

Yes. You just need a stock operating system tune that matches the one you have now. Anyone who has EiLive can do this very easily.

Kodiak,

Does your post mean yes...I can get the stock flash settings back that I had before I reflashed it? And where can I find people who have EFI live?

Yes you can put your stock tune back in. 99% of all people just buy Efilive from a vendor. It is a "tuner" that plugs into you OBD II under the dash. If there is someone in your area, they can use one of their licenses to put a program in your truck....but you will have to have them put the stock tune back in with their Efilive tuner.

Just do a quick search on "Efilive" there is tons of info for you to look at. It has a learning curve and you will need a laptop computer to do the tunes. Its not for everyone because its not like a chip. In other words, you don't just plug it in and are finished with it.

It is more like a tuning tool that people use to make custom tunes that fit the way people use their trucks. You can make mileage tunes, pulling tunes, race tunes, ETC. Its completely customizable.

Where are you located?

I am in Eureka, California, about an hour south of the Oregon border on the coast. I am 3 hours from Redding, Calif. and Medford Oregon and 6 hours north of San Fran, 5 from Sacramento. I assume you are asking to see if I am close to someone you know with EFI live. Thanks

Yeah, i'm asking for everyone. Someone with EFI is close to you.

hes one of the diesel techs here at school so yes we were on the same page as far as diesel goes...idk if i was right or wrong.

I am not trying to beat up on you. Good luck with the school. Modern gas ECMs use the O2 sensor pre cat to determine AFR and then tweak the tuning tables for desired under "cruise" loads.

Your duramax ECM with the blocker plate is doing things a little different. If you remove the EGR i.e. block it and do not adjust the tuning, the ECM has been programmed to calculate with the total air including EGR when EGR is active, therefore, your ECM calcs are off some when the ECM thinks you are getting EGR into the intake stream. I hope this clears it up some. I didnt want to get into all the tuning and scaling.

I am not trying to beat up on you. Good luck with the school. Modern gas ECMs use the O2 sensor pre cat to determine AFR and then tweak the tuning tables for desired under "cruise" loads.

Your duramax ECM with the blocker plate is doing things a little different. If you remove the EGR i.e. block it and do not adjust the tuning, the ECM has been programmed to calculate with the total air including EGR when EGR is active, therefore, your ECM calcs are off some when the ECM thinks you are getting EGR into the intake stream. I hope this clears it up some. I didnt want to get into all the tuning and scaling.


Would there be any reason for less mpg?

Would there be any reason for less mpg?

The fueling and boost tables change between EGR Off and EGR On. I believe it's less boost on a little more fuel when it is on. This compansates for the exhaust dilluting the oxygen supply. I think timing also changes a little. So the ecm still thinks that the EGR is still working and it will make the desired changes. Plus the stick doesn't give the same readings a functioning EGR does, and this would throw it into different cells in the tables. I could be wrong, but I think that is pretty close.

I checked with my local dealer yesterday and they say they cannot put back in the stock computer information that was present before the reflash. Wouldn't you think these "codes" or information in stock form would be available from GM to the dealer somewhere, somehow?

They always update/upgraede and don't go backwards. EFI live it.