Ive been having a slight problem with my car when its been startup in the cold. Basically after I start it up, if i lightly tap the gas, the engine will drop from say 1100rpm to 700rpm then jump up to 1500rpm before idling back down to 1100 or 1000rpm. It will do this a few times before responding normally. What could be causing this?

 

i think this is normal...
something to do with the electric choke...
someone may know more but i would say it is fine...

 

If its normal, then I dont think my car was normal before. Ive never noticed this, and not that I make a habit of tapping the gas when the engine is cold or idling, but still.

 

Your engine does NOT have a choke. It's programmed to start up in open loop and thus run a richer program until a set temperature. At which point it goes into closed loop and then controls the mixture.

Your bizarre idle seems to be a sticking IAC. I just replaced one in my p/up. Don't get a cheap auto parts store version. Either OEM or standard.

 

IAC huh? Hmm, ill have to check that out.

 

The idle RPM is set by the PCM based on the coolant temperature. When the coolant temps are very low (e.g. - 0*F) the idle speed on an A4 is increased to 1,200 RPM in neutral. Are you experiencing lower ambient temperatures than you are used to?

 

Not when I was noticing this problem. I know how the car idles when its cold and its idling fine, the problem is just if I lightly tap the gas, the engine speed drops rapidly down about 400rpm then jumps rapidly back up to about 1400 or so rpm then drops back down to 1100 or wherever it was idling before i lightly tapped the gas.

 

I have a similar problem... but not quite the same, if my cars been sitting over night it'll startup and idle like "normal" (1000 rpm ish).
When I'm at work and start the car up around lunch time to head out, (4 hours?) it does exactly what you describe... 1400 rpm and then gradually back down over the next few seconds and then it idles at the normal 1000 rpm. Oh it also takes a few turns if it's been sitting anywhere from 2-6 hours.

I haven't tried the tap gas thing though

 

Could be a sluggish IAC motor, as noted in the first response.

 

Would a sluggish IAC motor go away after a few taps or after its more warmed up?

 

Have you done the TB bypass? and live in a cold area that gets cold enough to form ice?

 

maybe this is why i think it is normal...

mine does the same thing...

 

Personally I doubt it would happen, the tb is quite close to the intake which picks up heat relatively quick, but if the IAC is already having trouble it could contribute.

 

Nope, havent done it yet

 

If I were to guess I would put my money on an IAC for all our cars, in fact I still have the one I replaced still kicking around so maybe I'll swap that in and see how it does