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. An idling engine 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
Idle is worst case for cam-lobe wear because the wear tends to be proportional to the level of contact stress between the tappet and the cam-lobe. This is always highest at the nose of the cam at full valve-lift die to the combined factors of small radius of curvature at the tip of the cam and the load of the valve-spring at full lift. The load of the valve-spring is countered by the inertia-forces at high engine rpm (which is roughly zero at valve-float speed), but at idle-speed the cam lobe tip sees nearly the full valve-spring load. On engines with cam-lobe switchers, this should only apply to the low-speed cam lobes. At near redline the load on the cam flanks are higher than the valve-spring load, but the radius of curvature is nearly flat, so the contact stress is probably not higher than worst-case. The other issue with idle on some engines is that the cam lobes are splash-lubricated, and there is much less splash at idle than at higher speeds. Some engines do not have the drip-rails to aid in low-rpm lobe oiling, so the drip-rails are important for these engines. Note that at hot-idle when oil pressure is lowest, there is a lot less oil coming out of the drip rails and the cam lobes don’t splash it around that much. It is interesting to look at the cam oiling at idle with a transparent cam-cover.
