RPetty2

Dads race 928 idles low and the rpms move with the sound of exhaust, it will go up a little then down then up and so on and just low idles like that, now I know that's a racecar but can I make my auto s4 do that? If so is it the idle thing under the airbox I have to play with? Thanks guys

IcemanG17

No.....you dads car is CIS so you can adjust the idle by a screw.......S4's are electronically set.....and tend to idle very stable once warm......

daveo90s4

Most of us spend time trying to get them idling properly. You could just install a stuffed O2 sensor if you don't want it to idle properly. Don't play with the thingy (MAF) under the airbox.

RPetty2

Iceman, what is cis?

Ethre

http://en.wikipedia.org/wiki/Jetronic

Wikipedia has a good general introduction and comparison of the systems.

danglerb

Moving idle usually means the fuel air ratio isn't tracking right, typically air is getting in via some path other than the air meter, as in vacuum leak.

The good way to get a lumpy idle is with a radical cam, high lift, overlap (intake opens before the exhaust fully closes so some weird flow can happen).

OTOH you won't pass smog if the idle drops out of a narrow rpm band in the test.

RPetty2

In other words don't bother? If its alot of trouble for a little thing that's innessasary then forget it yea?

Alan

Yes quite unnecessary... the rest of us are busy trying to avoid an uneven idle.

Always good to ask yourself:

...Why would this really be a good thing? - before making any changes from stock behaviour

Alan

RPetty2

Thanks Alan I shall always remember to ask myself that thanks !

whatudrivin

Just because it sounds 'cool' doesn't mean that the car is running properly. Wish some people around where I live would figure this out.

And the sound you are describing is called 1 of 2 things usually.

1. Loping - hotter cams and such

2. Idle Surge - idle goes up and down on its own. Caused by numerous things amiss with your motor.

Lizard928

Even with my big cams my car idles smoothly.

RPetty2

Yea true, in a way people may think its breaking down lol

danglerb

If the idle moves in a pattern, surging up and down, bleh, something is wrong.

If the idle moves in a random, almost dies but doesn't lope, sweet, something is right.

But too much lope, or can't smooth it out even raising the idle speed as much as possible, can be a PITA because from time to time the motor will die and it won't pass smog.

You might be able to sharktune with a set of chips that allow switching between several sets, and make one with a low lopey idle.

BTW I am almost never going to say don't do it, just pass along what you should be prepared for if you do do it.

bradartigue

Low idle, below around 800 RPM, isn't a good thing. If your alternator turns too slow you draw from the battery and stop charging it. Not a good thing. It is also incredibly hard on a motor to drag itself around at low idle speeds; best to keep it around where the factory said to - 850 +/- 50.

Unwanted idle variations in fuel injected cars are typically caused by air leaks. These manifest themselves as wavering idle, uncontrolled high idle, and inability to get idle speed down.

Lizard928

S4 autos idle @ 675 iirc and gt @ 775