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Used Diesel Trucks - Old Oilers

Tips For Buying A Used Diesel Truck

Photography by , Steve Temple

Has the volatile price of gasoline got you worried? Does your gasoline-fueled vehicle get worse mileage than a '69 Chrysler Town & Country with a 440 Wedge? Or maybe that last truck of yours just doesn't seem as tough and reliable as you originally thought, and just can't hack it towing up a long grade.

These are just a few of the reasons that may have stirred your interest in a diesel truck. Or maybe you've been around diesels for a long time and are aware of the tremendous torque, ease of maintenance, reliability, and fuel economy that an oil-burner can deliver. On the other hand, you may have also noticed that a new diesel truck can cost a bundle. A stripped-down model with no real options other than the diesel powertrain can cost about $5,000 more than one with a gasoline engine. Start adding things like four-wheel drive or four doors and the price of a new diesel vehicle can start looking like the down payment on a house.

Considering that, along with the durability and longevity already mentioned, you may be more inclined to look for a good used diesel truck. Since it can still demand a premium price over a regular gasoline-burning truck, it's a good idea to know what you are looking for in a used diesel.

To help you make an informed choice when looking for used diesel truck, and to keep things simple and low cost, the type we'll be considering are of the earlier domestic makes with mechanical injection pumps, as opposed to the computer-controlled common-rail injection systems. For the most part, this means that the Chevrolet and Ford versions will be naturally aspirated, meaning no turbo. However, sometimes a turbo was installed by the dealer, or as an aftermarket piece by the owner. There are also some earlier models with turbos that had mechanical injection pumps. Of course, the Dodge with its Cummins engine had a turbo from the start.

Turbo or not, these engines and powertrains are all very reliable. Adding a turbo will add complexity and power. With more power comes more heat. This can affect the longevity of engine parts. However, the simplicity of these engines is really one of the things that makes them so durable. Let's break down the major brands of trucks and the engines that they have.

Dodge 12-Valve Cummins TrucksThe 12-valve Cummins-powered Ram was really a revolution in diesel-powered trucks. These engines were designed as industrial powerplants, and it really shows by how much power and torque they make. This was not the first passenger truck to have a diesel option, but it is probably the truck that started the manufacturers trend of offering heavy-duty diesel-powered light trucks. The main advantage of the Cummins engine is its tremendous power and reliability. The recommended rebuild interval for a Cummins six-cylinder is around 300,000 miles. This means that with the proper care, or an easy life, these engines can last even longer. Of course, its popularity makes even old worn-out hulks demand a high resale value. There is one downside to all that power. While the Chrysler-supplied automatic transmissions were nearly bulletproof in gasoline-powered engines, that's not the case when subjected to all of that low-end torque from a diesel. Some owners cite weaknesses in the torque converter as the real culprit, which in turn results in the transmission's shortened lifespan.

Whatever the root of the problem, an important factor that is often overlooked with these transmissions is that the Chrysler automatic transmission fluid is a necessity. It is a slightly different formula than the usual fluid that people are tempted to use, and it makes all the difference in transmission life.

A manual transmission is probably a better option for a used Cummins-powered truck, but due to there rarity, they can be difficult to find. If the Dodge truck you're looking at has an automatic with Overdrive, be prepared to have it rebuilt or replaced with an aftermarket unit.

Ford 6.9- And 7.3L (Non-Power Stroke) TrucksThe engines powering the '94-and-earlier Fords were designed by International/Navistar. Obviously, Navistar is another very strong brand in the diesel industry, and the naturally aspirated 6.9- and 7.3L eight-cylinder engines are quite reliable. Even when an aftermarket turbo system is added, they still can easily last more than 200,000 miles.

These trucks are probably the most plentiful of all the older diesel-powered light trucks. Their initial cost was usually less than that of the Dodge, so the Ford/Navistar trucks became very prolific. This is good for a used truck buyer, because it means that parts are widely available from multiple sources. A wide array of turbos and performance aftermarket products exist because of their popularity, so it is pretty simple and relatively inexpensive to upgrade to more power.

As with the Chrysler, the automatic transmissions in the Fords did not always live as long as you might expect, but they fare pretty well. In most cases, however, the engine will outlive the transmission on these Fords, especially if it is an E40D automatic with Overdrive. The biggest trouble will come when a turbo is added. The extra power can send the transmission downhill in a hurry if the transmission is feeling weak.

General Motors 6.2- And 6.5L TrucksGeneral Motors built its own 6.2- and 6.5L engines for these trucks through its Detroit Diesel division. Contrary to popular rumor, these engines are not "just converted gas engines." These engines were designed to be very serviceable and reliable. A testament to this fact is that the U.S. military has used the 6.2- and 6.5L diesel for many years and continues to use these engines with and without turbos. (In fact, one potential source for used diesel vehicles is military surplus auctions.) One interesting fact about the 6.2- and 6.5L diesel is that it can run at a higher rpm than other similar diesels, making maximum horsepower around 3,600 rpm and maximum torque at about 2,000 rpm. This trait can make for a broader power range than other non-turbo engines, allowing a little more versatility when climbing hills.

The bad news is that these engines make the least power out of the group. Of course, power is not the only goal in buying a diesel truck. The 6.2/6.5L engines are also comparatively quiet, and before you say that you don't care about noise pollution, you might want to think what your neighbors will say if you plan on starting your truck at 5 a.m. to go to work. Diesel-equipped GM trucks are still plentifully available and can provide years of dependable and economical service. The earlier 6.2L engines can be replaced with a new style 6.5L without much difficulty, so you don't need to be concerned about not being able to find parts. Unlike the automatic transmissions in other trucks, the GM TH400 three-speed automatic transmission does not seem to exhibit any unusual weakness or unreliability. Early 700-R4 (four-speed automatic transmissions with Overdrive) had problems behind every engine they were paired with

Inspecting A Used DieselSo what specifics should you look for when contemplating a used diesel purchase? One of the best signs of the age and abuse a diesel has endured is listening to it start up cold. Insist to the owner of the truck that you want to hear it started cold. This means no running for at least 8 hours prior to your inspection. If the owner refuses to do so, or agrees, but the engine feels warm when you open the hood, be suspicious.

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Lars Melger
Lars Melger

What would you recommend me getting for a personal hauling service? Apparently where I live, there's a high demand for hauling equipment places. I could make some decent money just moving equipment around, but I need a truck to do that. I'm considering getting one with a 6.2 L engine or should I go for a 6.9?