bac22

Okay, so battling an idle problem when the car is cold, idles great when warm. BAC seems okay, but the TPS seems out. Been trying to adjust it, but it's a pain to get it right.

I'm suspecting the TPS is just outright bad, was wondering if a TPS going bad was common or not?

ace626

iTrader: (2)

From what I've read it is fairly common and I've had issues that I thought were TPS related. Use a multimeter to check the voltage on the green-red wire from the engine harness side of the TPS connector. First turn the key to on (car doesn't have to be running). At idle it should be 1 volt at full extension it should be about 5 volts.

HAILERS2

Try this: set your TPS whatever is your favorite way/method.

The end result will be a output of the TPS of approx 1vdc at idle, no matter which method you used (LED lights, ohm value at idle, reading the output with a meter and setting it to one volt dc).

So now you have set the TPS with a fully hot engine. Now idle the engine while monitoring the output of the green/red wire at the TPS connector or at the pin shown in the attached jpg.

IF the reading is a steady one volt and your engine is idling crummy...........don't blame the TPS.

Remember the TPS output will be much higher when the engnine is cold vs hot (like in the 2-3vdc range when cold) so don't bother monitoring the TPS output when the engine is cold).

Cold engines might very well idle smoother when cold because the ECU enrichens the mixture when it sees a cold value being output from the water thermo sensor. When the water gets hotter the mixture gets leaner at idle.

IF you had a series four you could probably just turn the variable resistor to the full rich and see if there is a difference, proving the mixture is what is causing a funny idle or proving that there is a air leak causing the mixture to be too lean at idle (variable resistor being turned to full rich making up for the air leak).

The TPS is used by the ECU to determine Zones of operation ...not to apply a bit more fuel as you turn the screw a little bit one way or the other.

Meant to add.............APPROX is just that. A TPS reading at idle of 0.95vdc or 1.05vdc is what I call APPROX and frankly I can't tell the difference using those settings when driving vs a solid one voltdc driving/idling.

bac22

Thanks for the info, see what I can dig up. Problem is it completely stalls out when cold, but idles fine when warm. Vacuum lines have been replace and the BAC seems to work correctly, so now I'm thinking maybe the TPS is the culprit.

BTW, it's an S5 NA.

HAILERS2

Stalls out when cold..............could very well be the water thermowax isn't rigged right. See page F1-36 of the online non turbo series five manual. OR someone might have removed the water thermowax assy altogether. Previous owner type thing.

bac22

LOL...I'm the original owner, bought brand new in 1989...yeah I'm a geezer now...hahaha.

Thought I had checked the thermowax, but maybe I didn't. Started working on this last problem last September, but put away for winter. With the recent warm weather, want to get her out on the road soon.

bac22

BINGO!!! Took off the thermowax, did the hot water test, it passed. So I put it back on then started looking at the way the vacuum lines were running and something wasn't right!

Sure enough the vacuum lines were not only backwards, but the the damn check value was backwards too...LOL! Fixed vacuum lines and check value and now no cold idle problems !

My own damn fault though, I must of screwed that up at some point since I'm the only one who works on the car!

Anyway, thanks for putting me on the right track!