Is prolonged engine idling bad?

[Old Techo]

One of the most annoying things for me in a van park are drivers who start their cold engine and leave it running for 5, 10 or 15 minutes before driving away.

Here are some Quora tech comments about excessive idling...


David Roth Auto Engineer RothAutomotiveScience, engine specialist

Is idling bad for your engine?

Idling is bad for your engine for the following reasons:

When an engine is running, every power-stroke of every cylinder lets a little bit of combustion gasses get past the piston rings. These blowby gasses, which are made up of mostly CO2 and water vapor enter the crankcase. When the engine is cold, the water vapor condenses into liquid water on all of the internal surfaces of the engine until they get warm. Along with CO2 and water, the “blowby” gas contains Hydrocarbons, Nitrogen Oxides (NOx), and some raw fuel, mostly during cold-cranking when the mixture is rich and some liquid fuel gets on the cylinder walls.

A typical cold-start probably adds a couple of tablespoons of water to the crankcase. You don’t start driving off the liquid water (and dissolved fuel) through the crankcase breather system until the oil gets up to about 150F. Most idling engines will never get close to that temp. Coolant gets hot but not the oil in the sump. So by idling to warm-up the engine, you prolong the water accumulation period and delay the time when you start driving off water.

At idle, combustion quality is at its worst due to relatively low temperatures and low turbulence in the combustion chamber, therefore the % of fuel that actually gets burned is lower than at normal operating conditions. This results in extra raw fuel getting into your oil.

At one of the OEM’s we did a torture test where we idled engines around the clock for weeks. Every couple of days we had to remove oil from the crankcase because the oil level rose due to the accumulation of fuel and water.

The above reasons are why your owner’s manual considers excessive idling as severe-service and you should change your oil more frequently

[Frank and Brenda]

When I was at work the boss told everyone, idle the bus for a maximum of 5 minutes before driving off, which was about the time to do the pre drive off check and paperwork, but that was pulling 12 to 14 ton empty, when stored to pick up a group, idle for 10 minutes maximum, of we didn't we'd get a call from the mechanic to say turn it off, it stoped some drivers leaving the bus running so they could run the air con, one driver had left the engine idle 4 hrs while waiting for a group she'd dropped off

With modern engines, there is no need to idle a car before starting off on a regular day, I start and go easy till it's warm
When I'm towing the van I'll start the car to move it into place to hook up and turn it off, once hooked up, I'll start it, move the van and it has enough time to idle while Brenda does an emu around the van spot (in case we've left anything behind) then drive off gently and take it easy till the engine gets up to operating temperature