I've had my S5 TII since July and it idled OK then but now it doesnt.... I have to hold my foot on the gas to keep it from stalling. I tried adjusting the idle screw and it doesn't do any good. I checked the plugs and they seem fine. My car also bucks a lot and is really jerky when I let off and get on the gas. Does this have to do with a bad TPS?

 

test your tps with a ohm meter. it should be a smooth sweep. if its all over the place when u press the tps. then thats means its gone bad. i forgot what it should be set at but i think its around 1k at idle on a warm engine. if its shot i have a spare for sale

 

I'm pretty sure it's more like 850-900 but I could be mistaken.

 

If you've adjusted the idle screw on top of the throttle body, and seen no change, you've probably got a significant vacuum leak somewhere. Time and heat can cause the vacuum lines to become brittle and crack, Or the fitting around the nipple has gotten loose. Gaskets at the UIM and LIM can begin to leak.

As far as the TPS, with a fully warmed up engine the resistance should go from 1000 ohms at idle setting to 4000-6000 ohms at WOT, evenly increasing throughout the throttle travel. If you can't get a solid 1000 ohm setting your tps is probably bad.

Once the tps is set, ground the initial set connector and try to get idle speed at 750 by turning the screw on top of the TB. If you can't then you've got some other problem .

Summary:
Check and set the TPS
Set the idle speed
Check for vacuum leaks and fix them
Check the function of the BAC valve
Clogged cats will play hell with idle
Of course, plugs and wires need to be in good working order
With idle problems, possible causes need to be eliminated one at a time

 

plugs and wires are new-ish, cat is brand spanking new, I'll have to check the rest now

 

OK, I replaced the TPS and did a cursory check of the vacuum lines. I capped off a loose line, which I think might've been for the air pump or something. No difference. I'm thinking it has to be a vacuum leak I can't see. I watched my boost guage, and at 1000rpms it showed 10 below the zero mark, and when the idle speed dropped to about 400rpms, the guage showed 4 below the zero mark. Dunno if that means anything.

 

I take it you have a aftermarker boost gauge? you should have 20 inHG at idol. if itat 10 or so then you have a vacum leak. buy a new UIM gasket (cheap as) pull the UIM off - replace all you vacum lines adn remove you emissions equipment. its easy as then put it all back together with some hylomar for good measure and you'll be away

 

there's no emissions equipment to speak of. What is the UIM for the record? The vacuum lines have all been replaced already, as well.

 

UIM = upper inlet manifold.

hmmmmmmm well if you have replaced all the lines then maybe you need to go thu and put some gasget sealant inbetween every mating surface to make sure your not leaking anywhere.
where are you taking you boost gauge reading from ( what nipple)

 

Not a clue, I'll have to trace the line. The car came with all this work done, so it's all new to me.

 

OK, it's reading 10mm Hg at idle, and it drops to about 4mm when the idle drops to 400rpm's, So that obviously says "leak" to me. Could be at the intake manifold, I checked the lines and they seemed OK but it's hard to tell. I'm taking it to a rotart specialist this week.

 

is the tps adjusted correctly? I believe there is a screw so that you can adjust it, and there is a tool with 2 led lights that you can hook up to the tps so that you can see if it is adjusted correctly. the jerkiness sounds like the tps isn't working or isn't adjusted right. If the 7 starts and idles then shoot some carb cleaner around the intake and see if the idle changes. might be able to find a leak that way, good luck.