Because of the high-compression ratios and resulting high cylinder pressure in diesel engines, they must be built to withstand more punishment than gas engines. Beefed-up parts include a thicker block and cylinder heads and stronger connecting rods, pistons, crankshaft, and valves. These parts can be costly. As an example, if you want to upgrade an '02 F-250 from the standard 5.4L V-8 to the 7.3L turbodiesel V-8, you're going to spend around $4800. However, to go from the 5.4L to the 6.8L V-10 gasoline engine, the price is a more manageable $600. Another diesel disadvantage that comes as a byproduct of needing heavy-duty components is increased weight. A diesel engine can weigh several hundred pounds more than a comparable gas model.
Despite huge improvements in noise isolation and engine-noise technology in pickup trucks in the past 10 years, diesels are still louder and shake more than their gasoline brothers. However, a recent back-to-back drive in two Ford trucks, one equipped with a 5.4L gas V-8 and the other fitted with a 7.3L diesel V-8, demonstrated that diesels aren't that far behind. At idle, the clatter and shake of the diesel are clearly noticeable, while it's tough to tell if the gas engine is even running. Under low-speed acceleration, the diesel still makes more noise. But once you're up to speed, there's little difference between the two even when accelerating on the highway.
Anyone who's tried to start a diesel engine on a cold winter morning knows the winner in this category. Diesels don't have spark plugs like gasoline engines do. The fuel is ignited spontaneously once it's injected into the cylinder that's already under pressure. When it's cold (below 30), the air isn't hot enough to ignite the diesel fuel. To help counter this, today's diesel pickup-truck engines use a computer that senses cylinder temperature and injects the fuel later in the engine rotation. By injecting the fuel when the piston is closer to top dead center, the cylinder is under more pressure and the air inside is hotter, which aids combustion. As an added measure, most modern diesels come equipped with a 110-volt heating element designed to keep the engine block warm when the truck is parked.
Despite petroleum companies best efforts at producing diesel fuel with lower sulfur levels, burned diesel fuel still smells much worse than burned gasoline. Beyond the smelly tailpipe, diesel lags behind gasoline in the areas of oxides of nitrogen (NOx) and particulate matter emissions. It's the particulate matter that causes the black soot seen emanating from diesel-vehicle tailpipes, while NOx is one of the components of smog. The next generation of diesel fuel is generally thought to improve on this situation.
Regular maintenance on a diesel is more costly, thanks to several items including the larger volume of oil in the engine and the fact that fuel filters and water separators must be serviced more often. Modern gas engines have an even bigger advantage thanks to extended service intervals on spark plugs, engine oil, and antifreeze.