can you put antifreeze in a running car

[CooperDella]

No, you should not add antifreeze to a running car. The primary reason is safety. A running engine operates at extremely high temperatures, and the cooling system is under significant pressure. Opening the radiator or coolant reservoir cap on a hot engine can cause a sudden eruption of scalding-hot coolant and steam, leading to severe burns. The correct procedure is to always turn off the engine and allow it to cool down completely before attempting to add coolant.

Understanding Cooling System Pressure When your engine runs, the coolant temperature can exceed 200°F (95°C). This heat causes the coolant to expand, pressurizing the system—typically to between 13 and 18 psi. This pressure raises the coolant's boiling point, which is essential for efficient engine cooling. The following table illustrates how pressure affects the boiling point of a 50/50 water-antifreeze mixture.

System Pressure (PSI)	Boiling Point of 50/50 Coolant (°F)	Boiling Point of 50/50 Coolant (°C)
0 (Atmospheric)	223°F	106°C
8 psi	239°F	115°C
15 psi	265°F	129°C
18 psi	276°F	136°C

The Safe Step-by-Step Process

1. Park Safely and Turn Off the Engine: Engage the parking brake and let the engine sit for at least 30-60 minutes, or until the radiator hose feels cool to the touch.
2. Locate the Coolant Reservoir: Modern cars have a translucent plastic overflow tank marked with "Full" and "Low" lines. This is the safest place to add coolant. Only open the radiator cap if you must check the level directly, and even then, only when the engine is cold.
3. Add the Correct Coolant: Check your owner's manual for the specific type required (e.g., Dex-Cool, HOAT). Slowly pour the antifreeze to the "Full" line.
4. Replace Cap and Check for Leaks: Securely tighten the cap. Start the engine and let it reach operating temperature with the heater on to circulate the new coolant, then recheck the level after the engine cools.

Adding coolant to a running engine is unnecessary and dangerous. The few minutes saved are not worth the risk of serious injury.

[LeJake]

Absolutely not. That's a great way to end up in the emergency room. My dad, a mechanic for 40 years, drilled this into me: a hot engine is a pressurized bomb. That cap isn't just a lid; it's holding back boiling liquid and steam. Wait until the engine is completely cool. No shortcut is worth a face full of scalding antifreeze. Just be patient and do it safely.