Welcome to top tech questions. oftentimes, readers contact us with questions about articles, or to praise us on what a good job we are doing. But our favorite form of reader communication is tech questions. Our Top Tech section is a place where you ask what’s on your mind, and we answer it. Got a trouble code? Wondering how to get your engine to make more power? Send us an email at jason.sands@sorc.com and we’ll do our best to answer it.
Duramaximizing Mileage Question: I have an ’09 GMC Sierra 2500HD with a Duramax engine and Allison six-speed automatic. I have 50,000 miles on it now and am very pleased with the performance. I am not looking to hot-rod the truck. It pulls my 12,000-pound, 30-foot, fifth-wheel pretty well. I do have a fuel mileage question, though. My fuel mileage empty, with straight and level freeway driving, is averaging 21.5 to 22.8 mpg. In town, I get 17 to 19 mpg. Pulling the trailer, I drop to about 8.5 or 9 mpg on the freeway. Any thoughts?
Tim Baughman Bosque Farms, New Mexico
| Aerodynamics play a huge part in towing. Even though
it weighs more than 30,000 pounds, the Airflow Bullet
Truck is capable of more than 13 mpg—nearly twice
that of a regular semi—mainly because of its radical
aerodynamic enhancements.
Answer: Although we answer fuel economy questions all the time, this one had us a little perplexed because of the drastic difference between towing and non-towing fuel economy. For freeway and around-town mileage, 17 to 23 mpg is excellent and is about what we have seen with our own Duramax projects. When moving on to towing, however, 8 to 9 mpg isn’t all that great—especially if it is flat ground. Our recent experience with towing a trailer at ludicrous speeds (see “141.998 mph With a Trailer” Feb ’13) has taught us just how important aerodynamics are. It’s our guess that the speed at which you are towing combined with a pretty un-aerodynamic trailer is what is leading to the poor fuel economy while towing. Remember those big wings you used to see on the top of a truck’s cab in the ’80s and ’90s? Well, it turns out those things actually worked, by directing air up and over the camper trailer at speeds. Unfortunately, there’s not much to do, aerodynamically speaking, if your fifth-wheel extends over the cab of your truck (like many current designs do). The only way you’ll drastically improve your fuel economy is by slowing down.
Knock Knock Question...
Question: I got a line on a ’91 W350 Dodge with a service bed, automatic transmission, and what looks like dualie axles. Two years ago, somebody tried to run used oil (I assume vegetable oil, but really, who knows what) through it, and according to the guy I’m emailing “…it seized up.” He said, “It runs but knocks like crazy.” Any thoughts? Honestly, I’m hoping I can just flush the fuel system and do a little tweaking with the pump or injectors and have a running 12-valve to cart junk around with.
Christian Hazel
Escondido, California
| Many diesels from the ’80s and ’90s have been pretty used and abused
by now, so be careful when purchasing one and look for signs of smoke,
noises, rust, and slipping clutches or transmissions.
Answer: We hate to break it to you, but any “knocking” in a diesel is usually a very bad sign. In gasoline-powered engines, a loud knock usually indicates a rod is about to go. In diesel engines, while this can also be the case, knocking is usually the result of an injector firing at the wrong time, or dumping too much fuel in. A slightly worn injector will not do this. More often than not, the nozzle has to be completely cracked in order for the truck to rattle like crazy. When an injector goes, there is usually a good amount of smoke that comes along with the territory— most likely white. If it were us, we would buy the truck if you can use some parts off of it, but we wouldn’t count on actually getting it running right any time soon. When an injector is that bad, internal engine parts will fail, such as pistons or rings, often necessitating a complete rebuild.
More-Power Mercedes Question: I don’t need a truck, and the VW TDI options are too small for my 6-foot 4-inch frame if I intend to put the kids in their car seats behind me. So, I’ve decided my first diesel will be a Mercedes E Class CDI. I’ve narrowed the options down to two: the ’03 to ’06 with the I-6 (OM648) or the ’07 and up with the V-6 (OM642). All my research is leading me toward picking the car with the inline motor. I would also like to be able to wrench on it and extract some more power—likely a larger non-VGT turbo, some water-methanol injection, and possibly a shot of nitrous. Based on those goals, do you have any suggestions or guidance?
Ryan Geyer
Santa Barbara, California
| For those looking to bridge the gap between a Volkswagen Tdi and a diesel truck, the mercedes E320 Cdi can be a
very good choice and is quite reasonably priced as well.
Answer: While the Mercedes inline engines are known for their reliability, you’re probably going to be blazing your own trails as far as performance stuff goes. You could always try Extrude-Honing the injector nozzles or tuning for more fuel, but how it would actually affect the performance of the vehicle is unknown. Keep in mind that the newer models also have a DPF and will have to go into regeneration, which will hamper fuel economy numbers. If you’re not married to the idea of having a Mercedes, we’d suggest the BMW 335D, which is very underrated power-wise and is capable of more than 35 mpg and 0-to-60 times in the 5-second range in factory form. There’s also programming available for them, which is capable of increasing horsepower to more than 300 hp at the wheels which, for a pretty light car, is a whole lot. Even if you do decide to go with the Mercedes (which, by the way, can be found for less than $20,000, used) you’ll be treated to 210 hp and 369 lb-ft of torque, which still makes for a pretty rapid ride.
