Buying a High Mileage Diesel

Service Tips & Considerations for High Mileage Diesel Trucks

By virtue, diesel engines are generally known for their remarkable longevity comparative to a typical gas engine. The architecture of a diesel engine tends to be much stouter and most applications demand a long engine life in order to offset the higher initial purchase price. As a result, a high mileage diesel can retain significantly more value and pickups with several hundred thousand miles can be purchased as reliable means of transportation. As is the case with purchasing any used vehicle, however, it is often unknown just how strict the previous owner stuck to a maintenance regiment, and regular service is the key to entering the million mile club.

There are great deals to be had on used Dodge, Chevrolet, and Ford diesel pickups with 250,000+ miles on the odometer. But can you reliably purchase a pickup with that many miles? Of course. Our recommendation to purchasers of high mileage diesel pickups is to perform a thorough service once you take ownership. Not only will this prepare you for the long haul, but it may uncover any necessary repairs that require immediate attention. Below you'll find many notes, recommendations, and considerations for purchasing a high mileage pickup.

Documentation can greatly increase value and add peace of mind - Recent service and repairs are always advertised and rarely documented. If a seller can not produce receipts or similar documentation supporting their claims you may as well treat them as if they never occurred. If the seller has documentation, expect tighter negotiations and less bargaining power as records may increase the potential value. Since you don't know the seller's level of mechanical aptitude, receipts from a reputable shop are much more valuable than a seller's own craftsmanship.

Perform an engine service, immediately - At minimum, change the engine oil and replace the fuel filter(s). This is cheap insurance and will kick start your maintenance routine as you'll know just how many miles since these were last performed. Additionally, you'll get a chance to see the motor oil. Although engine oil turns black almost immediately in diesels, you can examine the consistency and look for filings on magnetic drain plugs, which may indicate significant problems. Now would also be a good time to check the air filter and replace as necessary.

Perform a transmission service, immediately - This should include dropping the pan, replacing the internal transmission filter, and replacing any external/auxiliary filters before topping it off with fresh fluid. Unless you have records from the previous owner, it's anyone's guess when the transmission was last serviced. Routine transmission service promotes longevity and allows you to examine for excessive wear by checking for clutch material and metal particles collected in the sump of the transmission pan.

Some oil leaks are normal, others more alarming - It is normal for an engine to develop slight oil leaks around various seals and gaskets. An example of a normal oil leak is engine oil seeping around the front and rear main seals; it's unavoidable and to be expected. A light coating of oil around a seal or gasket is not necessarily alarming. However, a continuous oil drip should raise suspicion.

The engine may purr, but what about the chassis - Just because you're buying a "million mile motor" doesn't mean that the rest of the truck is holding up as well. Shocks, bushings, and steering components will nickel and dime you if they require attention. If the steering feels loose or the truck doesn't steer straight, you may want to research repair costs - steering boxes, for example, are rather expensive for 3/4 and 1 ton pickups. Additionally, a brake job can quickly run you several hundred dollars. After purchasing a high mileage pickup, be sure to service the chassis at all grease points, thoroughly inspect for wear, and make repairs as necessary.

Begin a fuel additive regiment - Older diesels, primarily referring to those with mechanical injection systems (12v Cummins, 6.2/6.5 GM, 6.9/7.3 International), were created long before the introduction of ULSD. ULSD lacks the lubricating characteristics of straight LSD and prolonged operation without an additive can cause excessive wear and premature failure of injection pumps. There are dozens of fuel additives to choose from that supplement the lubrication properties that modern diesel fuel lacks.

However, additives are not advised solely for older diesels. Are you getting that 25 miles per gallon that the previous owner boasted of in his Craigslist ad? No, you’re probably not. But fuel additives can also provide a boost in fuel economy. We religiously use a supplemental fuel additive in anything that runs on diesel, period. To fully realize the fuel efficiency aspect of many fuel additives you'll need to replace your fuel filter(s) after using the additive for a few hundred miles.

Drivetrain service is often neglected and can lead to expensive repairs - Differential and transfer case fluids are often neglected. Gear oil is cheap, a new rear end is not. Servicing these components once again grants you the opportunity to check for excessive wear and/or damage. Additionally, the condition of universal joints can be checked by rotating the driveshaft back and forth; excessive play indicates they should be replaced. If no metal filings are found in the transfer case and/or differential(s), they won't need to be serviced again for quite some time and you'll have the peace of mind that there are no problems.

Synthetic oil is better oil - Albeit more expensive, synthetic engine and gear oils are far superior to conventional "dino" fluids, period. It's never too late to switch to synthetic, regardless of mileage. For a detailed list of benefits and additional information on synthetic motor and gear oils, see: Synthetic engine and gear oils for diesels

Inspect cooling system condition - Consider inspecting the engine coolant and cooling system. Check hoses for cracks, leaks, and deterioration. For certain engine models, such as the International 7.3L IDI, it is important to test the SCA/DCA concentration of the engine coolant and adjust as necessary. Additionally, look for mineral deposits and build up in/around the coolant overflow tank. If the condition and/or color of the coolant is questionable a coolant flush is advised.

Specific engines have specific problems - Keep in mind that certain model truck/engine combinations had their own common problems, flaws, and demons. Benefit from the findings (and possible demise) of others with the same vehicle and address these concerns early on. The old 7.3L IDI, for example, experienced problems with cavitation. Rather than take your chances, check the SCA/DCA concentration prior to purchasing. If you bought a high mileage 12 valve Cummins that has survived this long without losing a dowel pin, don’t necessarily assume that it won’t ever happen to you. If the cooling system in your Duramax is running on the verge of normal, don’t forget that it is relatively common for the water pumps to fail every 100,000 miles or so. Know what you’re buying and what you can do to get the most from your truck.