Tonedef wrote
The combustion process has a number of byproducts, the most relevant to this discussion is water, carbon dioxide, nitrous oxide and sulfur dioxide (particularly relevant for diesel engines), and carbon soot.
When the engine is cold, combustion is incomplete, and not all of the fuel is burnt. This unburnt fuel condenses on the cylinder bores and valve stems, and it washes away the boundary oil film. That's factor number one. To make matters even worse, when the engine is cold and the management system/auto- or manual choke makes the mixture richer, resulting in more condensation.
The water byproduct also condenses when the engine is cold, and the carbon dioxide, nitrous oxide and sulfur dioxide gasses dissolve in the water droplets to form carbonic, nitric and sulfuric acid, respectively.
Now you have all these acids percolating on your combustion surfaces, and no boundary oil layer to guard against it because the unburnt fuel washed it away. THAT is where cold-running engine wear comes from.
Modern multi-grade oils have made cold-running mechanical wear on the crank and small end pretty much a thing of the past, they still give excellent protection when cold, provided there is enough oil pressure.
What all this means is that you need to get the engine up to operating temperature as quickly as possible in the fewest number of combustion cycles. If you simply idle an engine, you have low combustion pressure (because there is little compressive load on the piston) and the heat generated by the combustion process disappears out the exhaust manifold. However, if there is load on the pistons (by dint of the engine having to transfer torque to the wheels), your combustion chamber pressure under ignition is greatly increased and the temperatures generated are higher per combustion cycle, and the engine heats up faster. It's simple physics. Boyle's law.
In addition, idling makes the carbon soot build up in the ring grooves, this makes them seat improperly, and you have premature wear.
+1 letting your car idle to warm up is a bit of a myth and not a good thing.
