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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.

Many major engine problems can display themselves during startup. In particular, listen for engine misfires, or erratic engine speed when initially started. This may indicate a worn injector pump or drive system. The drive system would include the cam chain, or the geardrive, if so equipped. As these parts wear out, the injector pump becomes less accurate as to when it injects the fuel. This can produce a substantial amount of extra smoke and noise. Pay attention to any other initial knocks and rattles, but remember, these mechanically injected engines can be noisy, so don't be alarmed by the typical diesel clatter.

Smoke can be another telltale sign of troubles. Here is the truth: Diesels smoke, especially when they are cold. It is normal to see a fair-sized puff of black, white, or gray smoke come out of the pipe on initial start-up. However, when it settles down to an idle, there should be very little visible smoke coming out of the pipe. Even under moderate acceleration, there should not be smoke that you can see while driving. You may want a friend to drive along behind the test-drive to have a look.

Excessive black smoke is another sign of worn-out injector-related items. Remember that having the injector pump replaced can be pretty expensive, so make sure that you are ready for that cost if you decide to purchase an old oiler.

Black smoke can also occur when the compression of the engine is low. This could indicate worn-out cylinders, pistons and piston rings, or cylinder heads. When this starts happening, it is probably time for a complete engine and injector pump rebuild. There is always a chance that it merely needs some adjustments or new injector nozzles, but it is never wise to count on being lucky when your wallet is involved.

Continuous white smoke can indicate a problem with the glow plug system staying on or an engine that never warms up due to a stuck thermostat. These are dangerous conditions and can damage the engine, so it would need to be dealt with right away. Replacing the glow plug controller or thermostat is not an expensive proposition as long as it's done before engine damage occurs.

Due to the larger than normal amounts of transmission failures that occurred with the automatic transmissions, it would be wise to make sure that during your road test you verify that the transmission shifts firmly into all gears, and does not hunt for the next gear or slip for any reason. Also pay close attention to how long it takes for the transmission to engage when a gear is selected.

Obviously, it is imperative that you verify all other aspects of the truck as you would with any other used vehicle. Check the oil, brakes, shocks, and frame sag, just like you would on any heavy duty truck, keeping in mind that diesel trucks have probably seen more use and abuse than their gasoline brothers.

Diesel trucks were generally purchased by someone who wanted to get the most life out of a work truck. This could mean your next purchase may have traveled hundreds of thousands of miles with lots of towing and load hauling. So, look for other signs of wear and tear, if the odometer reads only five digits, making sure that the claimed mileage agrees with the general look and feel of the truck. It can also help if the owner kept his service records from various mechanics, which almost always include a date and mileage.

Why Used Diesels Are Better The advantages of buying a used diesel, especially an older mechanically injected model are many. It can be a very economical proposition. By their very design, diesel trucks left the factory with strongest components that a manufacturer had to offer. Checking the diesel option typically got you the strongest axles, beefiest transmissions, biggest radiators, and even dual batteries. So, even if down the road you need to spring for something expensive, like a transmission, an injector pump, or a complete engine, you have a truck that is worth investing money into.

What these trucks lack in power they make up for with simplicity and reliability. Owning an older vehicle guarantees that you will have a usable truck that is easy to service and inexpensive to maintain. Plus, you'll never be afraid to use your diesel truck-as a truck!

If you live in a state that has a smog check law, you can smile every time you pay your registration and do not have to pay for a smog test. It is also valuable to know that diesels can run on a variety of oils and fuels. As crude oil costs continue to rise, you can count on advances being made in alternate fuels, such as biodiesel or even straight vegetable oil (from a secondary fuel system). Biodiesel is being sold as a blend (with petroleum based diesel) in some states and is sometimes available even in pure form by some private distributors. Having a diesel vehicle will enable you to take advantage of these less expensive fuels as they become more widely available (see related article on biodiesel elsewhere in this issue).

These general pointers on buying a used diesel truck should get you off to good start. For more detailed answers, you may want to first track down a qualified diesel service center. The manager there will likely be happy to speak with you, since you're a potential customer, and he may even know of some good deals out there as well.

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