Bill S

No, both reasons are valid. Idling increases engine wear AND emissions.

BigDog

and on idle, what causes higher wear when the engine is cold? The only thing that wears an engine is frcition, and in a cold engine metals are contracted over operating conditions so if the oild flows roughly the same as at operating conditions, there should not be increased friction causing wear. Older OHV engines used to need engine temps higher to get the oil flowing up into the heads better, so idling was good before you started driving. Now, oiling systmes are better, engines smaller, and heads much more efficient since overhead cams need that oil fast. Why does an engine wear more at idle? I would honestly like to know.

WClark

Once again ancient history is creeping into modern engine discussions.

Oil is the key here.

In the old days of heavy weight dino oils life for a cold start engine was tough. When you combine a very thick (cold) 30W oil with an engine idling (low oil pump pressure), you had all sorts of places where in-sufficient oil flow would occur and wear would result.

Today, with 0W synthetic oils...well first of all synthetic oil cling to metal much longer and even if the pump didnt run there would be protection for a while...but mainly low viscocity synthetics flow VERY well when cold (and I mean REAL COLD) so even a slow turning oil pump can deliver plenty of oil almost immediately to every reach of the engine.

The increased pollution when cold, is a real problem. So much so that in places where population densities and cold weather combine, the EPA has basically forced use of "reformulated" fuels to reduce the pollution from all the cars cold idling for longer periods than in warm weather.

WYSIWYG

that drip off parts dry overnight

G. Hale

I think there's more to it than just the metal to metal wear, which is where the oil part comes into play. The worst part of a cold engine is that the fuel does not burn completely and the engine has much more carbon deposits built up from running cold. That's why they usually say it's good every now and then to run the car hard to burn out the carbon deposits. I'm guessing that is probably only one of the drawbacks to a cold engine. Like mentioned in an earlier post, the engine is designed to run at normal operating temperature. Too hot or too cold is both bad.

BigDog

cars, but with modern cars the "cold engine" problems are almost non-existent. It is very easy for the computer to change timing and A/F mixture to compensate.

Bill S

I can't explain the engineering principles. But unless demonstrated otherwise, the advice in the owner's manual is authoritative to me. And it makes sense to me that the engine operates with least wear and most efficiency at the design operating temp, not colder or hotter.

My suspicion is that idling at start now and again is not a big problem. But doing this constantly, as with a remote start device, may well contribute significantly to engine wear and higher maintenance and repair costs later.

The Red October

Think about it like this... How long will it take the oil temp to reach 175 while idiling? While driving? The engine is designed to run within a certain temperature range, it will have the least amount of wear in that range. The quicker you get to that range the better.

Also the engine expands slightly when it heats up, right? That means the oil flow will be improved in those tight areas. The higher rpms heat the car quicker, and allow more oil to circulate.

Ever see big trucks cover their grills in the winter?