A used diesel can be the best truck deal on the lot or a five-figure repair wearing a shiny paint job. The difference is usually visible in fifteen minutes if you know where to look. Diesels tell on themselves through fluids, cold starts, smoke, and a handful of platform-specific weak points. This is the checklist a tech runs before money changes hands, ordered roughly the way you'd actually inspect a truck.
Before you go: know the platform's weak points
Every diesel has a signature failure or two. Know them before you look. A 6.0 Power Stroke has EGR coolers, oil coolers, and head gaskets. A 6.7 Power Stroke and 2019-plus Cummins have CP4 and, on the Cummins, the grid-heater bolt. An LB7 Duramax has injectors. A 6.7 Cummins has EGR and emissions items. Knowing the specific truck's known issues tells you exactly what to check and how much a problem will cost. Walking in blind is how buyers overpay.
The cold start: the single most revealing test
Insist on inspecting the truck dead cold, not after the seller has warmed it up in the driveway. A cold start reveals more than any other single test.
- Cranking and fire-up. It should catch quickly. Extended cranking points to glow plugs or grid heater, injectors, fuel supply, or compression issues.
- Startup smoke. A brief puff on a cold start can be normal, but heavy white smoke that lingers suggests injectors, glow/grid heating, or worse, coolant entering a cylinder. Persistent blue smoke means oil. Heavy black smoke at idle isn't normal.
- Idle quality. Listen for a steady idle versus a miss or stumble that can indicate a dead or weak cylinder.
If the seller "already warmed it up for you," that's a reason to come back, not a convenience.
Fluids tell the truth
Pop the caps and pull the dipstick. Fluids are where hidden problems surface.
- Engine oil. Check level and look for a milky, coffee-with-cream appearance that signals coolant intrusion, an oil cooler or head-gasket tell on the wrong platform. A fuel smell or an over-full reading can indicate fuel dilution.
- Coolant. Look for oil sheen or discoloration in the coolant, the flip side of the same cross-contamination story. On some engines, a specific dye and a combustion-gas test are worth doing.
- Oil cap and dipstick tube. Heavy sludge or a mayonnaise film hints at neglect or a coolant/oil crossover.
Any evidence of oil and coolant mixing is a stop-and-investigate flag, because on some platforms it's a cheap oil-cooler job and on others it's a head-gasket teardown.
Under the truck and under the hood
- Leaks. Wet oil, coolant, or fuel around the front cover, oil cooler, water pump, and injection components. A little seep is age; active dripping is a negotiation point or a walk-away.
- Exhaust and emissions hardware. Confirm the emissions equipment is intact and functional. A deleted or tampered emissions system is a legal and resale problem and can hide why the truck was sold. Missing DPF, EGR, or a tuner in the glovebox is a red flag, not a feature.
- Turbo and intake. Check the intake tract and turbo for excessive oil and shaft play. Boots and clamps should be intact.
- Belts, hoses, and the cooling system. Cracked belts and swollen hoses are cheap but tell you about maintenance habits.
Drive it under load
A test drive that never asks anything of the engine tells you little. Get it up to speed and, safely, put it under load, an on-ramp or a grade.
- Power delivery. It should pull cleanly without hesitation, dead spots, or a sudden loss of power that could indicate limp mode.
- Warning lights and limp mode. A truck that derates under load is telling you something. Note any check-engine, water-in-fuel, or emissions lights.
- Temperature under load. Coolant temp should stay controlled climbing a grade. Creeping temperature under load points to cooling-system or head issues.
- Transmission behavior. Diesels are hard on transmissions. Feel for clean, firm shifts and no slipping.
Scan it before you sign
Bring a scan tool or bring a truck-savvy friend who has one. Pull codes, including pending and stored, not just active. Where the platform supports it, check injector balance rates and any available fuel-system data. Codes that were recently cleared, a suspiciously blank memory on an older high-mileage truck, or stored faults the seller didn't mention are all worth a hard conversation. A pre-purchase inspection at a diesel shop is cheap relative to what these repairs cost.
The deal-breakers
Walk away, or price the repair in hard, when you find:
- Oil and coolant mixing with no clear cheap cause on that platform.
- Deleted or tampered emissions equipment.
- Heavy blow-by (excessive vapor or pressure from the crankcase), which points at engine wear.
- A seller who won't allow a cold start, a load test, or an independent inspection.
None of these automatically means the truck is worthless, but each one changes the price by thousands, and you should know that before you sign.
FAQ
What's the most important thing to check on a used diesel? A genuine cold start and the fluids. The cold start reveals glow/injection and combustion health, and the oil and coolant reveal cross-contamination that hides the most expensive repairs.
Is high mileage a deal-breaker on a diesel? Not by itself. A well-maintained diesel with documented service can be sound at high mileage, while a neglected low-mile truck can be a money pit. Maintenance history and the inspection matter more than the odometer.
Should I pay for a pre-purchase inspection? Yes, especially on a platform with expensive known failures. A diesel-shop inspection costs a fraction of one head-gasket or injector job and routinely pays for itself in negotiation or avoided disaster.
Where we land
Buy the used diesel that starts clean cold, shows honest fluids, drives strong under load, and scans clean, on a platform whose weak points you understand and have priced in. Refuse the truck the seller won't let you inspect properly. Do that, and you land the deal instead of the repair bill.
Shop 6.7 Cummins diesel parts at coredieselparts.com. Free shipping over $70, ships fast.
Related guides: - Best Years for a Used Cummins - Diesel Injector Balance Rates Explained