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.