3500HD Diesel MPG gone south quick

[mwool]

I have about 9K miles on truck. Currently MPG is having trouble averaging 3-9MPG on gauge recently when not pulling trailer and both highway and town driving(use to be 16-19). Have been mostly highway and pulling 5K trailer. Do not think this is related but about 2K miles ago changed oil to Royal Purple CJ-4 and for the first tank of fuel my MGP went up a good 30%. Then put in brand X fuel and took big hit downward so I blamed the fuel. I do use the gray bottle fuel additive from Walmart every tank fill up. Ran tank down to 3 gal. left and went back to fuel I normally use. First half of this tank has been good MPG but now dropping again to around 8-3 MPG not pulling trailer. (even after 30+ minutes highway driving still going down)


One more item, The last time I saw it this bad the Banks 6 gun had gone bad. I am going to take it out as soon as I can to see if it is bad again. But does anyone have any other ideas??? BTW- fuel filter still has 50% left. Air filter is Banks and shows good.

[More Power]

Hmmm... Is the fuel mileage really 3-9 mpg (hand calculated)?

Yes, remove any performance product before suspecting something else.

Jim

[mwool]

The 3-9 MPG was from the gauge. When I hand calculate after a fill up the calculated is consistently 1.5 MPG less than the gauge.


First I did not remove the banks but only the fuse for the Banks. According to Banks only the fuse has to be removed. From my past unit that went bad. When bad the Banks gave the same problem with or without the fuse. My performance then did not improve until I removed the unit.


However, this time, driving the same road and same speeds, with or without the fuse I get the same response. I then added fuel additive from Walmart and shortly afterward my MPG began to climb and act normal again. It was up to 19.2 on the gauge before I got back into traffic.


Almost seems like bad fuel again but could it be something else?

[More Power]

Could it be that the low fuel economy you're referring to is being reported during the time(s) that the LMM is in the middle of an "active DPF regeneration"?

Jim

[mwool]

I was watching for DPF. Strange as I had completed about 400 miles pulling a trailer and most was highway driving. The next day I began to notice the dropping taking place. So I began to watch for DPF signs. If it was DPF, it keep starting and stopping. I have read the GMC site and here as to what can cause the DPF to stop. At one point I took the truck out and drove highway speeds for about 30 minutes and it still dropped after that.


I do not recall the GMC site but I believe it talked about an air intake under the truck for the DFP system to add air for the burn. I bring this up as now that the rain and mud have again stopped, I do not see the dropping taking place (maybe I am just between DPF cycles). Have you heard of of know of any air intake for DPF?


Also, I have the Banks air scoop but can not see how this might be weather related if weather is the case.

[More Power]

The ECM will command the injection system to inject fuel during the exhaust stroke (late injection event) when regeneration is required. This provides a fuel source necessary to bring the Diesel Oxidation Converter (DOC) and Diesel Particulate Filter (DPF) up to regeneration temperature. This increases DOC/DPF temperature to 500°C (932°F) or higher. Simultaneously, and if necessary, the computer-controlled air intake valve reduces the amount of intake air, further contributing to the heat production. And finally, when necessary, the variable vane turbocharger is used to restrict exhaust flow and increase load the engine during regeneration. All of these factors work together to build heat in the exhaust system, and could (likely will) affect short-term fuel consumption.

The "air intake valve" is a computer-controlled valve located just upstream of the intake manifold. This operates similarly to a butterfly valve in a carburetor on a gas engine, in that it restricts airflow into the engine.

DPF temps must remain with a range of about 900 degrees F and 1400 degrees F for a regeneration to occur. Generally, running at freeway speeds provides the necessary conditions - as does towing, without requiring additional fuel to be injected. And yes, if the regen cycle is interrupted, it will pick up where it left off once the conditions are met.

Jim

[mwool]

OK,*calculated*came*in*at*10.03*MPG.*After*your*de scription*and*looking*at*how*the*Banks*is*installe d,*I*have*removed*it*(not*just*disable*it*by*remov ing*the*fuse).*pulling*trailer*right*now*(wife*dri ving)*and*MPG*has*improved*and*we*seem*to*be*at*le ast*2*MPG*better*than*past*four*trips.*will*still* be*couple*of*weeks*before*I*know*for*sure.*

[Mark Rinker]

Must be *bumpy*? :D