The Boss engine is generally regarded as reliable, as it was developed from Ford's 4.6-liter modular V8, which powered countless police-cruiser Crown Vics. It's not uncommon for them to exceed 300,000 miles or more with proper care. Ford gave it an iron block for added durability, knowing that it would be used in a truck. Unsurprisingly, there are few catastrophic 6.2L V8 problems. Still, there are common problems to keep an eye out for. Valve-spring failure leads to misfires, power loss, rattling sounds, and rough running. Replacing valve springs is not a tough job, though it is time-consuming because you have to replace a total of 16 on this engine. The springs themselves are so cheap, you can just as well replace the lot to be sure, while you're at it.
The second thing to keep an eye on is oil leaks. This isn't necessarily a common problem with the Boss engine, but rather a general wear-and-tear issue that all cars tend to develop. The weak spots on this engine include the rubber valve-cover gasket, oil-pan gaskets, main seals, and the baffle on the right-hand side valve cover. Keeping an eye on the oil level is crucial, and a small amount of leakage isn't a major problem. You might want to take a closer look once you smell oil burning, as it indicates a larger leak. Thankfully, oil gaskets are cheap, and if you know your way around an engine you should have no trouble replacing them. The Boss engine uses 16 spark plugs, so the ignition system is rather complicated and anything that goes wrong with it can affect the engine's running. Changing sparkplugs is routine maintenance, and worth doing if you experience any sort of misfits, shudder, or rough idling. Spark plugs lead shorter lives in hardworking engines used for hauling and towing, so they have to be replaced timeously to avoid running problems.
Oil consumption can also become an issue on the Boss as it ages. It's more often a faulty positive crankcase ventilation (PCV) system causing it rather than the engine itself and can be avoided by servicing and cleaning the PCV system regularly and avoiding periods of prolonged idling. Meticulous maintenance and frequent oil changes can defer PCV problems for a long time.
Mileage: Oil leaks from around 100,000 miles. Faulty valve springs can fail at any time. Spark-plug life varies according to usage patterns, so keep an eye on the engine's running. PCV valves can get stuck due to usage patterns and lax maintenance at any time.
Cost: Valve springs only cost around $6 each but the labor to replace them all can be as much as $700. Oil gaskets are cheap, but depending on which ones must be replaced, the labor can add up to over $500.
How to spot: Valve-spring failure will trigger fault codes, illuminate the Check Engine light, and cause misfiring, rough running, rattling sounds, and power loss. Oil leaks should be visible and may cause the oil level to drop if they're big, leading to other problems if left unchecked. Tired spark plugs will cause stuttering, misfires, hesitation, rough running, and power loss. Increased oil consumption will show in a dropping oil level and perhaps even some smoke from the exhaust.