dasams

As I'm browsing the manual of my 06 CS, I notice the following on p73:

"Do not warm up the engine when stationary. Drive off immediately." Can anyone explain the reasoning behind this? I can only conclude that idling the car with cold oil must somehow restrict the lubrication.

Comments?

mudman2

Its emissions based I think cats don't work well when cold

Kooser

I read the same thing, however I took it to mean "do not rev the car in order to warm it up,"

I let mine idle for a few minutes prior to driving off. After a minute or so I can see and hear the rpm drop to its "warmed up" level and after another minute or so the temp guage starts to rise. Then I drive off. Granted this is primarily in the winter months but this could be incorrect, so if anyone knows for sure please respond.

worf928

Modern engine management systems have a 'cold-start' feature that keeps the idle high and the fuel mixture rich until the coolant temperature in the engine block reaches a minimum temperature. (Older systems used a 'timer'.) At this point the idle and fuel mixture will drop to near normal and the oxygen sensor loop is engaged to begin driving the air-fuel mixture stoichiometric for emissions. The engine isn't really at operating temperature until the thermostat opens. Actually, it isn't 'warm' until the engine oil is warm. If you have an oil pressure gauge you will know that the engine is warm once you see the oil pressure drop at idle. Typical pressure is 4 BAR+ when cold and 2 to 3 BAR when warm.

Engine wear is greatest at startup due to oiling - or lack thereof. Wear is next greatest during warm up because the various parts of the engine all have different heat expansion characteristics - gaps between parts are smaller or larger than they are when warm. The various parts are designed to have the correct size relationships when warm. For example an aluminium engine block expands more per degree of temperature rise than steel pistons in the cylinder bores. Thus, when cold the pistons are 'too big.' (Obviously that example discounts piston rings etc. but the general gist is the same.)

An engine under load will warm up far more quickly than an engine idling. Thus, the recommendation to avoid cold idling: engine wear. There is usually an additional recommendation: use engine 'break-in' driving limits until the engine oil is warm. Once the engine oil is near operating temperature (say 170 degrees F) the engine parts are at their 'designed size.' Minimize engine wear by not cold idling the motor and driving moderately until the oil is warm.

EDIT: Yes. Cats like to be warm. The highest emissions occur during cold running when the engine management system must keep the idle high and the mixture rich to keep the motor from stalling.

Shamus

Just turn her over and drive, just keep the revs down.

Not having a load on the engine, just sitting at idle puts a lot more stress on the engine and takes 2x to 3x longer to warm up than if you just drove it carfully and almost immediately

skl

When it's really cold here I usually upshift to second manually as it really lingers in first when cold and I don't like reving it that high when still cold. Probably overkill, but what the...

dasams

Mudman and Worf make some good points. The longer it takes for the cats to warm up, the higher the emissions. Hence the desire to heat up the cats ASAP. At the same time, engines are built with different metals and their design must include the effects of differential expansion. Consequently, the tolerances are designed for full operating temps which means big gaps when cold As a result, there's a need to heat up the oil ASAP.

I'm not convinced that I have this right and I'm like to learn more. Anyone else want to chime in?

ammo_collector

There are other components that require a warm-up besides the engine, including the whole drivetrain, which at idle remains "cold".

Snapper

The emphasis, certainly here in Europe, is to get the cat converter temperature high as quickly as possible in order to reduce emissions. This is why the CS holds onto 1st gear for the first minute or so after a cold start.

Brent 89-GT

Fortunately the Cayenne uses an aluminum block, heads and pistons. A more likely scenario is an iron block and alum pistons since nobody uses steel pistons in cars.

The Cayenne has an oil temp gauge which is a very good indicator of when you can start giving it the spurs. 200* is normal and you will notice it takes quite a bit longer to get the oil temp up than the coolant temp. Driving it right away is the right thing to do as long as you don't run it hard until the oil temp is up.

worf928

True, but the point is the same - alloys with different coefficients of expansion and not just the cylinders and pistons and not just in the engine.