If you’re in the market for a used diesel truck, you already know they can be great investments. They tow hard, run forever, and don’t mind putting in long hours. But here’s the catch — if you grab the wrong one, you could be stuck with repair bills bigger than your monthly payment. The trick is knowing what to check before you hand over the cash.

We’ve worked on a lot of these trucks here at Brandell Diesel, and we see the same problems show up again and again. Below are some things worth paying attention to when you’re looking at a truck on the lot, in someone’s driveway, or even at an auction.

Maintenance Records Tell the Story

Diesels need regular care. Oil changes, filter swaps, injector service — if the owner skipped this stuff, the engine will eventually show it. Ask for paperwork. A stack of receipts is a good sign. If the seller shrugs and says, “I always kept up with it” but has no proof, that’s not very reassuring.

Mileage vs. Engine Hours

A lot of folks only look at the odometer, but that doesn’t tell the whole story. Some trucks rack up “easy” highway miles, others sit idling all day on job sites. A truck with 200,000 well-driven miles might be healthier than one with 100,000 miles and thousands of idle hours. Don’t just chase low mileage — look at how it’s been used.

Oil Leaks, Blow-By, and Smoke

Pop the hood. Do you see oil around the valve covers, turbo, or oil pan? Leaks usually show up there first. Take off the oil cap with the engine running. If it’s puffing smoke (blow-by), the motor could be tired.
And check the exhaust:

Black smoke = too much fuel.
White smoke = coolant issue.
Blue smoke = burning oil.

Any of those can mean expensive repairs down the line.

Turbo and Fuel System

Most diesels live and die by the turbo. If it’s whining, lagging, or not making power, plan on spending money. The same goes for the fuel system — injectors and pumps aren’t cheap. If you notice rough running, hard starts, or poor fuel economy, have a shop check it out.

Transmission and Drivetrain

Engines get all the attention, but don’t ignore the transmission. On the test drive, the shifts should feel smooth. Grinding, slipping, or jerky changes? Walk away. Also check u-joints, axles, and the transfer case if it’s 4×4. Those repairs add up fast.

Rust and Suspension

Crawl underneath if you can. Light surface rust is normal, but deep rust on the frame or patched-up welds is a deal breaker. Look at the suspension too — shocks, bushings, springs. Worn parts here affect how the truck drives and how safe it is.

Scan It Before You Buy

Plug in a scanner (OBD-II). Codes don’t lie. Even if the seller cleared the dash lights, hidden problems can still show up. At Brandell Diesel, we run full diagnostic scans during pre-purchase inspections so nothing gets missed. We also check if emissions systems have been deleted or tuned — something that can cause more trouble than it’s worth.

How It Was Used (and Modified)

A truck that spent life towing campers is different from one that pulled heavy equipment. Ask what it was used for. Also, look at the mods. A good tow package? Fine. But cheap lift kits, huge tires, or a “hot tune” can shorten the truck’s life. Make sure upgrades were installed properly.

What to Look for When Buying a Used Diesel Truck