Simple, reliable, rugged, the 6.9L and 7.3L IDI are not the most powerful machines, but they get the job done and potential buyers should be comfortable with purchasing one. No major defects, though it's worth noting that the 7.3L IDI has larger head bolts than the 6.9L. 7.3L IDI engines were also prone to cavitation, so it's worth asking an owner if SCA/DCA has been used regularly. A factory turbocharged version of the 7.3L was optional for model years 93 & 94, though there's reason to believe that these engines are less reliable than the naturally aspirated variation. Both engines can be turbocharged with aftermarket kits to improve performance. Expect decent fuel economy, not a speed demon; these engines were built to work.
7.3L Power Stroke
The tried and true 7.3L Power Stroke is the most sought after Ford powerplant. The 7.3L Power Stroke varies greatly from the 7.3L IDI, so do not confuse the two. The 7.3L Power Stroke combines the reliability of the older IDI with a much greater performance potential. Owners can expect the engine to last anywhere from 350,000 miles and up - there are plenty of trucks in the million mile club. The automatic transmissions that backed these engines were nothing special, though they are typically reliable and there are many upgrades available for potential buyers. The engines can be tuned and modified to produce a substantial amount of horsepower/torque, though the 2001-2003 engines used powered metal connecting rods that are a concern in any engine making power in the 400 rwhp range. Expect respectable fuel economy somewhere between that of the IDI and the 6.0L Power Stroke, all depending on driving patterns.
6.0L Power Stroke
Everyone knows the horror stories of the 6.0L Power Stroke, but these have been largely bloated by the media attention they attracted. There are many reliability concerns with this engine, including EGR problems and blown head gaskets. Fortunately, most of the engines that have had problems have been repaired with updated components. Look for a truck that has been well maintained, the 6.0L is very sensitive to maintenance neglect. If an owner can provide maintenance records don't be afraid to strike a deal. Once the initial hiccups are worked out, the 6.0L Power Stroke delivers great performance and fuel economy (expect 17-21 highway under the right conditions). Ford backed the 6.0L with the 5 speed TorqShift transmission, which is an extremely reliable unit that offers quick, crisp shifts. Avoid a truck that is using coolant, oil, or that displays starting issues unless you can pick it up cheap. Be sure to take the truck on a test drive that allows it to reach normal operating temperature. The 6.0L can be a hot rod with the proper parts and precautions. Don't be afraid to buy one with 150,000-200,000 miles; if the motor has lasted that long without any major issues, it's certainly not a "lemon" and is good for 350,000+ (though it might nickel and dime you in sensor replacements). Be prepared to learn the ins and outs of the 6.0L, as this is the key to owning a reliable one. EGR valves clog often, become familiar with how to clean one.
6.4L Power Stroke
The biggest complaint about the 6.4L Power Stroke is the fuel economy, though it is only fair to note that during inception, both GM and Dodge were experiencing similar issues. This was an industry wide problem, with the root cause being the introduction of the DPF aftertreatment system. It is difficult to estimate the fuel economy you can expect considering that owners report figures all over the board. A reasonable expectation would be 12-15 mpg city and 15-18 mpg highway. Aside from that, the engines are relatively reliable, with only a few common issues. They feature an industry first twin turbocharger system that offers crisp throttle response and minimal turbo lag. The engine itself is strong (a medium duty design), and the cylinder head design eliminates head gasket failure problems associated with the 6.0L.
6.7L Power Stroke
The 6.7L Power Stroke is manufactured by Ford Motor Company, not International Navistar. The engine uses a common rail injection system and a single sequential turbocharger to generate 400 hp and 800 lb-ft with minimal turbo lag. It builds torque quickly off idle and the torque curve is relatively flat.. The trucks currently do not have any major reliability concerns, and fuel economy is respectable. The engine does require the use of DEF (urea injection) as part of its emissions control system.
1994.5 - 1997 Ford OBS
Nicknamed the "OBS" for "old body style", the 94.5 - 97 Ford F-250s and F-350s are extremely popular and hold their value well. A diesel owner's dream, the trucks are rugged, dependable, and are powered by the 7.3L Power Stroke. Additionally, they are exempt from emissions testing in states that require it, keeping the demand for these trucks high amongst the younger crowd. F-250 models were equipped with Ford's TTB (twin-traction beam) IFS, while the F-350 models were equipped with a solid front axle (the solid axle being favorable).
1999 - 2003 Super Duty (7.3L)
1999 was the first year of the Super Duty platform (with the exception of the older "F Super Duty", which is what we now know of as the F-450 model). Most 2001 - 2003 Ford Super Duty pickups were equipped with a 7.3L Power Stroke that had powdered metal rods (PMRs). These connecting rods are significantly weaker than forged rods, and are prone to failure in the 400 rwhp range. If you do not plan on extensively modifying your engine (larger injectors or other major upgrades), than this will not be an issue. The PMRs do not cause reliability concerns in stock trucks, and these engines can even withstand tuning upgrades such as chips & modules. Beyond on that, there is not much worth mentioning regarding these model years - good, solid, dependable trucks.
2003 - 2007 Super Duty
Everyone knows the horror stories of the 6.0L Power Stroke. Potential buyers should not fear purchasing a truck powered by the 6.0L so long as the price and conditions are right. The 6.0L is sensitive to maintenance neglect, so look for a seller who can provide at least some maintenance records (engine oil and fuel filters need to be changed religiously on these engines to minimize reliability issues). Most of these engines that had major problems have been in the shop enough to receive upgraded parts, so reliability concerns are lower now than when they rolled off the assembly line. The TorqShift transmissions are reliable and long lasting, and the trucks themselves are extremely rugged. They make great tow rigs, work trucks, and commuters (combined fuel economy often in the 16-18 mpg range). Would not recommend the engine if you plan on extensive modifications, though with the proper precautions taken, the stock bottom end has proven reliable through the 550 rwhp range. If you want to preemptively address the engine's fatal flaws, plan on spending $2500-$3500 in upgrades after you purchase the truck. Avoid a truck that is using coolant or is extremely difficult to start, as you may end up chasing someone's problems.
2008 - 2010 Super Duty
Pickups with the 6.4L Power Stroke suffered a few "annoyances" and minor problems, but are overall reliable. In 2008, the diesel particulate filter and active regeneration were introduced as standard emissions equipment, killing fuel economy. The truck's twin turbocharger setup gives excellent throttle response and minimal turbo lag, though the engine does not make peak torque until 2,000 rpm. 2008 also introduced a fresh new body style for the Ford Super Duty. The engine's continue to deliver power through the TorqShift 5R110W automatic transmission or a ZF 6 speed manual gearbox.
2011 - Current Super Duty
To reduce emissions and improve fuel economy, the 2011 6.7L Power Stroke introduced a variety of new design features to match the freshly redesigned body. A DualBoost, single sequential turbocharger (1 turbine that controls 2 compressors), was used through the 2014 model year, but was replaced in 2015 by a single VGT turbocharger. DEF (urea injection) was introduced as part of the SOC system, which combats NOx emissions. While the need to keep the 5 gallon tank full is a bit of a hassle, the system allows for improved fuel economy and uses DEF at a reasonable rate. Trucks from these years handle like a Cadillac, can move a skyscraper, and offer a variety of creature comforts & luxury options. The 6.7L Power Stroke has been one of the most reliable engines used in the Super Duty, with very few major flaws and reports suggest a low warranty claims rate.