The diesel maintenance that prevents the big repairs

Plenty of big diesel repair bills start small: a fuel filter left in too long, coolant that wore out, transmission fluid cooked by a summer of towing.

Your owner's manual is the authority on intervals for your exact truck. This is what each service protects, so you know what you're paying for.

Fuel filters and the water separator

Filters and the water separator keep water and dirt away from the high-pressure pump and the injectors, which are two of the most expensive parts on the truck. Drain the separator on schedule. If the fuel ever looks cloudy or milky, it's contaminated. After a filter change the system has to be primed fully, and stalling or rough running afterward usually means air got in.

Skip it long enough and you're buying a pump and a set of injectors.

Coolant

Coolant wears out, and on some engines the damage runs in a chain. On a 6.0 Power Stroke, old coolant helps plug the oil cooler, which leads to the EGR cooler and then head gaskets. On the 6.4, it clogs the EGR coolers. Early Duramax engines tend to lose head gaskets after overheating.

Use the coolant type your manual calls for. Mixing types is how some of this starts.

Transmission fluid

Heat is what wears out a diesel transmission, and towing makes heat. Overheated fluid can't control friction the way it should, so clutches slip and wear. If you tow, follow the severe-duty schedule in your manual. Dark fluid or a burnt smell means it's overdue.

Oil, and an oil level that goes up

Change it on schedule and pull the dipstick in between. A level that rises means fuel is getting into the oil and thinning it. That's a known problem on several engines, and on an LB7 Duramax it's a classic injector sign.

Leaks and reseals

Gaskets and seals wear out too. An oil or coolant leak that gets ignored can cost belts, hoses and, with coolant, a head gasket. More on that in when a reseal makes sense.

Some published intervals

These examples come from published service schedules. Check them against your owner's manual, because intervals change by model year and by how the truck is used.