Engine Idle

[Bimmer-Bob]

Appreciate some insight from the more technically-minded out there.

On a cold start, my engine starts up quickly but sounds a little less composed than normal. The rpms will usually be running around 1000-1100. I'll let it run for a minute or two, by which time the rpms have dropped down to the 600-700 range and the motor is purring nicely. I understand that the engine is simply warming up, but what exactly is going on here? Why does the crank run faster on a cold engine? And why does my wife's bulletproof Toyota engine seem not so fussy on a cold start?

If it makes a difference, my car is still breaking in (~250 miles).

[Sylon]

My Subaru did this every day. I believe its warming up the oil and water to a safe and optimum temperature. I rarely began driving my car after just starting up without letting it get warm. Sure, it could do the same thing at lower revs, but it may take longer.

[mephiska]

I think it also has to do with emissions, idling at a higher RPM on cold start more quickly gets the catalytic converters up to operating temperature and makes the car more enviro friendly.

Oh and my Subaru also did the same thing until I put it in gear, that dropped it down.

3rd gen RX-7's were scary. On cold start they jump right up to 3k rpms to heat up that rotary fast. Freaked out a lot of owners if they weren't used to it.

[LJKelley]

My Mercedes does this as well, starts at about 12 and drops to 8 when it is warm. If you put it in gear it drops to 8 right away though obviously the engine is not warmed up.

[Sylon]

Originally Posted by mephiska

I think it also has to do with emissions, idling at a higher RPM on cold start more quickly gets the catalytic converters up to operating temperature and makes the car more enviro friendly.

Oh and my Subaru also did the same thing until I put it in gear, that dropped it down.

3rd gen RX-7's were scary. On cold start they jump right up to 3k rpms to heat up that rotary fast. Freaked out a lot of owners if they weren't used to it.

Yeah, until you push in the clutch and boom, it shoots right back up again. Slightly embarrassing when you are trying to back out of a driveway and the revs shoot up like that, lol.

[perral1]

Originally Posted by Sylon

Yeah, until you push in the clutch and boom, it shoots right back up again. Slightly embarrassing when you are trying to back out of a driveway and the revs shoot up like that, lol.

The way mephiska said that, I'm guessing there's no clutch involved.

I have this problem with my civic si. Revs hold at 1900 when I start up, which, with the exhaust I have on it (deep and loud in that range), is very obnoxious in the morning. It takes quite a while to drop down all the way to the normal 800-900ish idle (4-5 minutes at least w/ light driving).

[Simsims]

engine runs VERY rich during those high revs in order to keep combustion going.

[Caleb]

Originally Posted by mephiska

I think it also has to do with emissions, idling at a higher RPM on cold start more quickly gets the catalytic converters up to operating temperature and makes the car more enviro friendly.

Yep, its the ECU trying to heat up the cats as fast as possible to reduce emissions. See this from the N54 docs:

In order to bring the catalytic converters up to operating temperature as quickly as
possible, the N54 engine has a catalyst-heating mode for when the engine is started from
cold. In this mode, combustion heat is intentionally introduced into the exhaust train and
not used first and foremost to develop power output.
The point of ignition is moved to 30° (crankshaft degrees) after TDC. The main quantity
of the required fuel is injected before TDC and mixed with the boost air. The piston is
situated after TDC in its downward travel such that the air/fuel mixture is already expanding
again, which reduces the ignitability of the mixture.
In order to ignite the mixture reliably, a small residual quantity of fuel is injected 25° after
TDC and this guarantees an ignitable mixture at the spark plug. This small fuel quantity
therefore provides for ignition of the residual charge in the combustion chamber.
This operating mode is set by the engine-management system after a maximum period
of 60 seconds from engine starting but is terminated if the catalytic-converter response
temperature is reached earlier

So, it looks like the wastegates are fully open, timing is changed, and fuel injection is in a sort of catalytic warm-up mode, which is probably only avaliable due to the availability of direct injection on the N54. AFAIK, all modern cars do something simmilar to heat up the cats to reduce emissions.