Noise on Cold Startups

[jonathanA4]

I was looking through some threads elsewhere and noticed a lot of people mention that after their cars have been sitting for awhile (overnight) on startup their cars will go up to about 1200 rpms and be a bit louder than normal (like something else has turned on in the engine). Slowly the rpms drop down to about 850-900 (regular idle) after about 30 seconds and the noise stops (or whatever is making the noise, shuts off). I heard it's something to do with air being pumped through to the cat converter for emission control purposes or something?
Can anyone explain what exactly the car is doing when it does this at startup? I'm curious.

 

[Mikebabs1]

My 2001 B6 model A4 does the same. I'm no mechanic, but I assumed this was just the automatic choke helping out when the engines cold.

The noise is just the engine running a little faster until it warms up. I guess this saves you having to rev the engine hard when cold starting. I'm sure some of you experts out there will correct me, but either way I wasn't at all worried by this.

My first car was an old Polo that had a manual choke. If pulled right out the engine would idle much quicker. If you left it pulled out once warm, it would almost act like cruise control!! You didn't need to touch a thing and the car would tootle along nicely! /ubbthreads/images/graemlins/smile.gif

Mike

 

[Broken Byzan]

Yeah you are right, its an auxilary airpump.It pumps air thru to the cat to prevent the cat being damaged by the richer coldstart mixture.

The higher idle is just a part of the cold start procedure.

HTH
/ubbthreads/images/graemlins/crazy.gif