Why warming up an engine in winter is obsolete

Modern engines shouldn’t be warmed up by idling.

Idling to warm up an engine was for carbureted engines

A carburetor is an atomizer. It uses engine vacuum to draw fuel from the carburetor bowl and spray it into the intake manifold as droplets. But that fuel can’t burn well until it’s vaporized, and that’s where the warm-up requirement came into the picture. Without a warm up, driving a cold carbureted engine caused stalling and flooding.

How the carburetor warm-up process worked

The heat riser valve

Since the carburetor dumped the atomized fuel into the center of the intake manifold, engineers had to find a way to heat up the area directly under the carburetor. The answer was the “heat riser valve;” a thermostatically controlled mechanical valve that closed off a one bank of the engine’s exhaust manifold to prevent that heat from going out the tailpipe. Instead, the heat riser routed the hot exhaust gas up to the area directly under the carburetor.

Carburetor pre-heat

Plus, they installed shrouds around the exhaust manifold to capture heat to warm the underhood air before it entered the air cleaner. Heating the intake with hot exhaust gas and pre-heating the air into the air cleaner helped the fuel vaporize.

But all of that is now obsolete

Port fuel injectors do a much better job of atomizing and vaporizing the fuel. In port injection, where the fuel is sprayed onto the backside of the intake valve, vaporization is helped by the almost instant heat of combustion that heats up the intake valve.

On gasoline direct injection engines, where the fuel is injected directly into the hot combustion chamber, vaporization is instantaneous.

That’s why warm-up is obsolete

For most late model fuel-injected cars and trucks, idling to warm up your engine is not recommended by any carmaker. In most cases, 30-seconds is all  you need. In other cases, carmakers recommend no longer the two minutes of idling after a cold start. Idling longer than that is actually bad for your engine.

©, 2023 Rick Muscoplat