can you use def in old diesel cars

[McMichael]

No, you should not use Diesel Exhaust Fluid (DEF) in an old diesel car that was not designed for it. DEF is a key component of Selective Catalytic Reduction (SCR) systems, which became standard on diesel trucks and cars in the United States starting around 2010 to meet stricter EPA emissions standards (Tier 2 Bin 5 and later). If your vehicle predates this requirement, its engine lacks the necessary hardware and software to use DEF. Pouring DEF into the fuel tank or a separate tank on a non-SCR vehicle will not reduce emissions and will likely cause significant and expensive damage to the fuel system and engine components.

The primary issue is chemical incompatibility. DEF is a mixture of urea and deionized water. If introduced into the fuel system of an older diesel, the water can separate from the diesel fuel, leading to corrosion of fuel lines, injectors, and the pump. Furthermore, urea does not combust properly and can leave harmful deposits. The vehicle's engine control unit (ECU) has no programming to recognize or manage DEF injection, so it provides zero benefit.

Feature	Pre-2010 Diesel (Non-SCR)	Post-2010 Diesel (with SCR)
Key Emissions Technology	Exhaust Gas Recirculation (EGR), Diesel Particulate Filter (DPF)	EGR, DPF, + Selective Catalytic Reduction (SCR)
DEF Tank	Not present	Standard equipment, usually with a blue filler cap
Emissions Standard	Tier 2 Bin 8/9 or older	Tier 2 Bin 5 or newer (much stricter)
Risk of Adding DEF	High risk of severe fuel system damage	Required for normal operation; no damage
Effect on Performance	No positive effect; can cause misfires and power loss	Essential for optimal performance and fuel economy

If you own an older diesel, your focus should be on proper maintenance of its existing systems. Using high-quality diesel fuel and keeping up with filter changes is the best way to ensure longevity and performance. Retrofitting an SCR system is technically complex, prohibitively expensive, and generally not practical for a single vehicle. The technology was a milestone for cleaning up diesel emissions, but it's only for engines built to use it.

[StReese]

Don't even think about it. My old '96 F-250 would choke on that stuff. DEF is for the new trucks, the ones with all the computers and sensors. If you pour it into the tank of a classic diesel, you're asking for a world of hurt—clogged injectors, rust in the lines, a huge repair bill. Those old engines are simple. Just run good fuel and keep up with the oil changes. They don't need, and can't handle, the modern emissions fluids.