Dear Tom and Ray: We own a 1995 6.5-liter diesel Suburban. I absolutely adore it, but my husband tells me I'm killing it by driving it short distances. Three days a week I drive it three-quarters of a mile to work, then drive it home two hours later. We live in Nebraska, so it never warms up on these trips. During the rest of the week, I generally drive longer distances. Am I driving my beloved diesel Suburban into the ground?
Ray: Your husband has a point, Kathy. From the point of view of the car, this is probably the worst kind of driving you can do. For two reasons:
Tom: One is water. When you run the engine, the combustion process creates water. But if the engine and exhaust system never get hot enough to evaporate that water, it just sits there and turns everything to rust.
Ray: The other problem, which is actually a bigger concern for diesels, is oil dilution. When the engine is cold, a lot of the fuel that pours into the cylinders never gets combusted. And some of the uncombusted fuel sneaks down past the rings and gets into your oil.
Tom: That dilutes your oil and makes it less slippery (because diesel fuel is not as good a lubricant as oil). And when you try to lubricate your engine with a mixture of oil and diesel fuel, the end result is that your engine won't last as long.
Ray: And since the vast majority of your driving is done when your engine is cold, you'll be diluting your oil much more than the average diesel-engine owner. (By the way, if you had a gasoline-engine Suburban and drove it like this, you'd dilute your oil, too. But gasoline engines have a way of purging excess fuel from the oil when they get good and hot. Diesels really don't.)
Tom: So what to do? I'd start by getting a block heater for the Suburban. That's an engine heater you plug in at night. At least that would give the engine a head start warming up in the morning.
Ray: And I'd also change the oil much more frequently than your owner's manual recommends - to get that diesel fuel out of there. And I'd base my oil-change interval on time - say, once a month - instead of mileage. If you wait until you've driven 2,500 miles before changing the oil, you might be driving around with severely diluted oil for a year and a half.