Subscribe to the Free

Top Tech Questions - You've Got Questions? We've Got Answers!

Jason Sands
Jul 1, 2009
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. So we're introducing our Top Tech section, 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 and we'll do our best to answer it.
Q: Want To Drive A 6.5LI'm a die-hard Chevy fan and I'm looking into buying a 6.5L turbodiesel. My question is: are there any certain years I should try to get my hands on? What about performance upgrades? My intent is to put some money in it right off the bat to upgrade the cooling system and then pursue additional power. I know the truck will never really be able to compete with the modern-day diesels, but I'm willing to live with that.
Donnie Bullock
Clinton, Missouri
Photo 2/4   |   GMC and Chevrolet 6.5L turbodiesels can be had for pennies on the dollar compared to newer offerings. While they'll never be powerhouses, they can be good workhorses with simple cooling upgrades and a few bolt-ons.
A: The 6.5L engine is by no means a powerhouse, and its injection system is limited as far as performance potential goes, so it's time to seek outside help. Back before aftermarket parts were readily available, diesels had to rely on injecting propane, a water-methanol mix, or nitrous oxide to make power. Since nitrous is more of a competition-only modification, we'd suggest propane or water-methanol as a power adder. A lot of people bash propane, but we've ridden in a few trucks that are on the 'pane, and we can tell you it makes one heck of a difference. Count on adding about 40 to 80 hp depending on how much you inject. Going the water-methanol route not only means more power, but a lot lower EGTs when towing with your truck. If you don't like the idea of having to carry around a bottle to make power, Heath Diesel (phone: [877] 894-6266 or visit offers a line of performance packages for 6.5L-equipped rigs. Prices range anywhere from a few hundred bucks to a few thousand, so there will be a kit within your price range no matter what your budget is. Another good resource is Kennedy Diesel (phone: [715] 255-9433 or visit, a 6.5L guru that sells lift pumps, tuned ECMs, and more. As for a year to buy, we'd shoot for a '97-and-up model. They have oil-cooled pistons, a revised, higher-capacity cooling system, a better turbocharger, and more performance potential. We'd expect to pay about $3,000 to $6,000 for a low-mileage truck in decent shape. Depending on the area you live in, you may be able to find one cheaper. Even with a few modifications, don't expect any more than about 200 hp at the wheels. If you're willing to live with that, then go nuts. There are many 6.5L trucks out there just doing their daily diesel duties, and fuel mileage is often in the 20-plus-mpg range with trucks equipped with 3.42:1 axle gears.
Towing Troubles
Part One

Q: All of our Duramax trucks have this disease called "limp mode." We just never know when an outbreak will happen. Does anyone have a cure for this virus that GM implanted in our vehicles to protect themselves from any kind of warranty repair? Without any programmers, I can pull a 20,000-pound trailer up a hill and floor it with no problems. It's nice to have the extra power and mileage I'm getting from a programmer, but I'm afraid it's going to break something next time it stumbles into limp mode. Does anyone have a cure?
Aaron Wiggins
Newport News, Virginia
Photo 3/4   |   If you're looking at adding more than 100 hp to an Allison transmission, or you'll be towing all the time, it's a good idea to upgrade the transmission with beefier clutches, a performance valvebody, and an aftermarket torque converter.
A: The Allison transmission found in '01 to '09 6.6L Duramax-powered trucks is one of the more complex transmissions on the market today, which also means it has its share of troubles. When the Allison transmission senses excess slippage, even for a couple of seconds, it will kick down into Third gear to save itself from burning up the clutch packs and will also unlock the torque converter in case the converter clutch is slipping. Towing is extremely hard on transmissions even in stock form, and when you add more power to the mix, you're asking for trouble. Remember that the transmission was designed from the factory to handle a 300 to 350hp engine, so modifying the engine's power output without modifying the transmission means you're exceeding the transmission's torque-handling capacity. Although it varies depending on usage, adding 60 to 80 hp is about the safe maximum for a stock Allison transmission to perform reliably. If you tow with that 60 to 80 hp, know that you're on the very edge of what the transmission can handle before going into limp mode. Over time, the transmission will start to slip and throw codes. If you're willing to drop down the power to stock levels, this will often keep the transmission alive for a few more years, but the reality is that 150,000 to 200,000 miles is not an uncommon time for a rebuild if you're using an aftermarket programmer.
Ready for the cure? It's not cheap or easy. If the truck keeps going into limp mode, that means it's transmission rebuild time. Expect at the very least to upgrade the torque converter, clutches, and valvebody. This will run anywhere from $3,000 to $5,000, depending on your upgrades and what your local shop charges for labor. While it's true that some Allison transmissions have lasted at high horsepower levels and haven't broken or limped, the vast majority will need help. The good news is, once your transmission is upgraded, it will be able to safely handle much more than 60 hp over stock, especially if it's not hooked up to a trailer. It seems like a mean trick to create a diesel engine that can make 200 hp more than stock with just a tune, and then hook it up to a transmission that can't take that type of power, but the simple fact is that in order to handle heavy towing and higher power levels, the Allison transmission needs some help.
Towing Troubles
Part Two

Q: I own an '06 Dodge Ram 2500 Quad Cab that has 47,000 miles on it. I've performed some modifications to the truck, like a K&N cold-air intake, 5-inch turbo-back exhaust, CFM Plus intake elbow, and a Bully Dog programmer with a boost fooler. I tow a 21-foot toy hauler with three quads--all together the trailer probably weighs around 10,000 pounds loaded. When I go to Flagstaff, Arizona, to ride, there is a hill that is an 18-mile climb. When going up the hill, my EGTs will get as high as 1,250 degrees. Some people say it is OK, and others say it is too hot. What's the real answer?
Bill Shaw
Henderson, Nevada
Photo 4/4   |   If you're one of our readers who tows regularly, you may be interested to know that Chevy, Ford, and Dodge all offer less powerful engines in their medium-duty trucks. The lesson here is if you want your diesel to tow reliably forever, you need less horsepower, not more.
A: This is another common question we get here at the magazine. A lot of our readers tow, and while there isn't really one right answer, there are guidelines. Think of exhaust gas temperatures as sticking your hand in a hot oven. If you put your hand in just for a split second (say, to put some pizza in there) you're none the worse for wear. But if you leave it in (your hand, or the pizza) for a longer period of time, it will get burned. A similar effect occurs in diesel engines. While seeing 1,500 degrees on the pyrometer for a few seconds is usually OK, having it pegged at 1,500 for five minutes is sure to cause damage. While 1,200 degrees is often considered the limit for prolonged towing, time is also a factor in how an engine will respond to high EGTs. Engine design also comes into play, as the newer common-rail engines are built to handle higher temperatures from the factory.
Common-rail Cummins diesel engines are among the best heat handlers on the market today, so we'd say your 1,250 degrees is still on the safe side, but just barely. Anything above 1,300 should be kept to a few minutes, and anything above 1,400 should be limited to just a few seconds. Keep in mind these are also conservative maximums--we know of a few Dodge Cummins owners who have towed at 1,400 degrees for five minutes and lived to tell about it, but we'd never recommend someone actually go out and try it. Also, all of these temperature recommendations are pre-turbo--if your pyrometer sensor is mounted in the downpipe instead of the exhaust manifold, we'd try to stay below 1,000 degrees while towing.
If you're still worried about your exhaust gas temperatures, installing an aftermarket intercooler or water injection kit is a sure way to lower your towing EGTs. Expect a 100- to 200-degree drop with either one of these add-ons, which will bring your truck well below the safe towing threshold. If it were us, we'd probably just keep our eye on the gauge, and try to keep it below 1,300 degrees.


Subscribe Today and Save up to 83%!

Subscribe Truck Trend Magazine

Subscribe to:

Truck Trend

Subscribe Diesel Power Magazine

Subscribe to:

Diesel Power

Subscribe Truckin Magazine

Subscribe to: