I"m having some problems with my 1990 N/A. The car idles all wonky. When I rev it up with no load it drops down below idle like it going to die, just barely sputters back to life and goes back to erratically ideling. If I leave it idling for a long time (a few minutes) it eventually evens itself out, then as soon as I touch the pedal it's bac k to it's old self.

On acelleration it's choppy (not smooth) but the power is still there. Stopping at stop signs without easing the revs back down risks killing the car. I have no idea where to start troubleshooting. Any help would be greatly appreciated.

 

I'm thinking TPS, though returning to idle can also be the dashpot.

Refer to the S5 FSM for both of those, though you should also make sure the resistance changes smoothly on the TPS in both narrow and wide ranges. There may be a cross-over point with your digital multi meter that will look like resistance roughness.

I'll be updating the TPS Adjustment page with the S5 FSM info in a few minutes. Also, have you tapped into your diagnostics plug? It can help you quite a bit, especially with your S5.

 

Ok, I updated the TPS Adjustment to cover S4 and S5s, with diagrams.

 

UPDATE: I adjusted my TPS. That took away most of the problems I was having and the rx was running much better. I was still having some hesitation issues above idle and the car wouldn't come back to idle smoothly. It comes down smooth to about 1500 then hitches and eases it's was down to 800.

Unfortunately, as I drove the car things began reverting back to the old condiction. It seems that every time I drive the car the TPS goes back slightly out of adjustment. I have no idea why this is happening.

I tapped into the diagnostics plug and I came up with an indication to check the Air Flow Meter. I may have a bad AFM, so I'm replacing it. If anybody knows how I can test it before I buy the new part I'd like to do that. I have a multimeter.

Now, the wierdest part: I realized that I was setting my idle using the screw on top of the throttle body only. I realized while searching around that there was a variable resistor which affects the air fuel mixture, which I would also need to adjust to get things just right. The problem? MY VARIABLE RESISTOR IS MISSING. That's right it's not there. From reading I deduced that the resistor only affects idle and I'm getting hesitation at higher RPMs, but it's disconcerting that I'm missing somthing from my wiring harness. Could this be causing the problem? How do I fix this.

So, I'm down to 2 causes: AFM or Missing resistor.

What do you guys think? Am I going in the right direction?

BTW, Steve, THANKS SO MUCH FOR YOUR HELP!

 

No problem! But I have good news and bad news - the good news is S5's don't have variable resistors. The bad news is you probably looked for one because of my idle AFR adjustment reference! I'll fix that right now, sorry about that.

 

Check out page F1-82 in the S5 FSM (fuel and emissions section) for an AFM test. It's basically checking to make sure the AFM moves smoothly and testing some resistances.