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A little research is all it takes to avoid buying one of the worst used diesel trucks on the market. Some engines aren’t built the same and should be passed over in favor of more reliable options. Diesel trucks are known for their low-end torque and hard-working power, but some were barely as powerful as the average sedan. A shortlist of engines should be avoided at all costs, and every used diesel truck shopper should familiarize themselves with it. These trucks aren’t built for work.

An aged red GMC pickup truck tailgate with a dangling logo
Dangling GMC logo | Robert Alexander via Getty Images

Some 1994-2001 Chevy & GMC 6.5L Pickup are the worst for reliability

This Detroit 6.5 liter offers slightly more power than its smaller 6.2L predecessor. It put out 180hp and 360 lb-ft of torque. Even turbocharged, this engine could not compete with its Powerstroke or Cummins rivals. This truck won’t impress on the job site or off. Rife with frequent mechanical failures, the Detroit 6.5L can cause a lot of headaches. Problems from stalling to refusal to start are common for owners of this underperforming truck.

The Ford Super Duty 6.4L is a disappointing comeback

A row of Ford trucks for sale at a dealership
Ford Trucks | Joe Raedle via Getty Images

After the failures of the 6.0L Powerstroke, Ford had high hopes of repairing its reputation. Its 6.4L successor would dash those hopes. This engine has several major flaws, including oil dilution. Oil dilution can cause a lack of engine lubrication and lead to seizing or failure. To compound frustration, this engine carries the same critical fuel system issues as the 6.0L before it.

Why did the 1982-1993 Chevy & GMC 6.2L Pickup put out so little power?

This engine was a step in a new direction for Chevy and GMC. In the early ’80s, the automaker had a string of unsuccessful diesel engine builds. In an attempt to pivot and increase the quality of its diesel trucks, engine maker Detroit was hired. The resulting engine was a massive disappointment.

The 6.2L is a colossal engine that puts out very little power. It manages 130 hp, just 40 more than a Smart Car. This is not the indomitable workhorse that diesel truck buyers expect. This engine also guzzles fuel and is prone to oil leaks from its rear main seal. This is a very costly repair that involved removing the transmission. Buying this used truck will leave most buyers disappointed.

The 2003-2007 Ford Super Duty 6.0L may be the worst diesel truck of all time

The 6.0L Powerstroke is infamous. The engine is so bad that Ford and Powerstroke parent company Navistar were engaged in a serious legal battle. Ford alleged Navistar built a defective engine. Evidence came to light that Ford had ignored poor test results for the 6.0L Powerstroke, which could have prevented post-production failures.

Expensive engine replacement warranty claims poured in as the Powerstroke suffered catastrophic failures. Most of these repairs meant the cab of the truck had to be removed. Many owners lost confidence in the brand over this engine. Consumer Reports lists a series of recalls that affect this famously unreliable truck.

Owners can’t keep 1978-1981 Chevy & GMC 5.7L Pickups on the road

An illuminated Chevrolet logo
Chevrolet Logo | Mandel Ngan via Getty Images

Are Diesel Engines More Expensive to Maintain?

This engine began its life as a gas-powered 5.7L. Converting the gasoline-powered version to diesel did this engine no favors. GM made several design flaws when building the diesel version. The same number of head bolts were used in the diesel version as the gas. The bolts could not stand up to the higher-compression environment of a diesel engine and were prone to stretch, which led to blown head gaskets.

Without a water separator in the fuel system, the engine was prone to corrosion. This engined burns through oil. Its cylinder heads lifted off the block and could cause catastrophic failure. This truck was an underpowered disappointment that could cost owners a fortune in oil seal repairs. The 5.7L engine powered several sedans.

Each of these trucks serves as a reminder that not all diesel trucks are powerful and reliable. As with any class of vehicle, there are specific bad apples to watch out for. For used diesel truck shoppers, knowing what to avoid can save a lot of headaches later on.