A Collection of Comments About Fuel Mileage
Updated February 28, 2007
I've started to notice that some trucks just get better mileage. I've been
on the road caravanning with trucks that are exactly the same as mine (98 wagon
with GSA's loaded the same) traveling the same speed and fueling up
together. One truck would get 14.5 while one would get 13.
I'm thinking it might have to do with the resistor in the FSD module (on
turbo diesels only). Supposedly the resistor is used to compensate for
manufacturing variances in fuel injection pumps. I'm starting to think that
the fuel efficient trucks have resistors in them that are on the leaner side
of the curve. One of these days I'm going to remote mount my FSD and
experiment with different resistors.
Here is a thread from the dieselpage, it is the TDC offset that can be changed, it takes a tech 2 scan tool.
A setting of -1.50 had given the best fuel economy. You might pickup 1 to 1.5mpg. I don't remember how the power was compared to a more advanced -1.94. My truck is at -1.85 and I wouldn't change it....fuel ecomony is great.
-1.5deg for lower rpm torque - pulling a load where no downshifting is done above 45mph, and -1.94deg for increased horsepower in higher rpm ranges.
Well i just recently set my TDC offset from -1.14 to the -1.94 and back to a setting of -1.58 here is what i have learned. Yesterday i hauled 2 loads of trees back to back, 70 mi round trip, temp 84, road speed 74 to 80, scaled at 16k + each load. The -1.94 builds more heat, builds it quick, seems to overwhelm the cooling system, 230 in shot order, egt went up 50 to 75 deg. it starts better, better throttle responce, has good torq all the way thru, Makes this engine rattle a bit much for my liking at freeway pulling speed. I dont have the 97 cooling mods yet, but do have the d max fan and clutch mod. Inbetween loads i whipped the tech1 on it and reset TDC off set to -1.58 and off i went, this engine in this truck likes this setting better because it can more easily manage the heat, the timing of -1.94 seems too much for it. It pulled impressivly with both settings, i am of opinion that it pulled better in the rpm range of 24 to 2600 better set at the -1.58 .
A few years ago I did some testing with TDC settings.
-1.94 is aggressive. It builds EGT's faster on long hard pulls with no intercooler. It starts the truck quicker. You're basically using the fuel more efficiently. Cold starts do "rattle" a little more. I personally think the truck gets off the line slightly better too.
-1.50 is really close in performance. EGT's build slightly slower. Without an intercooler any setting is going to get you to the red zone. Smoke might be slightly less here as well.
Either of these settings or anything in between is much better than the factory settings. Fuel mileage is about the same with these settings. It's all about performance!
Fuel Additives
Some claim that using Stanadyne Performance formula diesel fuel additive will increase your miles per gallon. It does in fact smooth the engine down and it seems to increase the power a bit. I never got better economy with it but others swear by it.
Another Hummer owner recommended Hydrotex. Their fuel additive, Essentialube does what they claim. I notice a smoother and faster idle and increased acceleration immediately. You have to locate a Hydrotex dealer and then can set up an account to order. Hydrotex has been used by commercial truck fleets for many years, and is not generally sold to the public
