I know this is a done-to-death topic, and there's a million write-ups out there, but nothing I've found seems to break down exactly what needs to be done, and all of the write ups have conflicting information. Hoping to sort this out with the help of everyone here.

I have an 88 S4 NA. Trying to adjust the TPS by measuring resistance. I understand the car needs to be at operating temp, and shut off. From what I understand, you need to measure the resistance on the actual TPS as you sweep from closed throttle to WOT. However, the conflicting part comes in there.

Some say you need to manually open the throttle plates and let them shut on their own, then rinse and repeat with the resistance checking. Some also say once it's set, you need to jump a connector in the engine bay to reset it, and do something with the BAC. Also seen something about adjustment the timing as well by turning the CAS.

Can anyone clear this up? Any nudges in the right direction would be much appreciated

 

there are two ways to adjust the sensor, you can use the two lights, or go by resistance. either is ok. it is best to open and close the throttle a few times too, just to make sure that its consistent. old TPS sensors can be picky. also the car needs to be warm enough that the thermowax is off, the fast idle is achieved by pushing the throttle open.

if you do have a turbo the two lights, and long screw driver are easier than pulling the intercooler...

 

So I assumd resetting the TPS is going to be the same for both a T2 and an NA (mines an NA).

So once the resistance is set properly, I've heard you need to jump a connector to essentially tell the ECU "hey, the TPS has been reset". Is that correct? Anything with the BAC or CAS that needs to be done? Does the key need to be in the on position when testing resistance?

 

You don't need to use the jumper for the TPS in particular, but it won't hurt. The jumper tells the ECU to lock the base timing and not try to change the idle with the BAC valve. This is more for setting the timing, the idle speed with the bypass screw, mixture, etc. Anything that would be affected by the ECU trying to intervene. This jumper goes on the two pin connector by the leading coils, by the drivers side retractor headlight.

The TPS is measuring something physical: the throttle linkage. The jumper doesn't affect this. Make sure the car is all the way warmed up or else the thermowax could skew the reading. Then set it with the procedure j9fd3s provided. Nothing to do with the BAC or CAS, although you can always jumper the connector just to verify your idle speed and timing. Again, it won't hurt.

By the way, I like the light method. All methods work but the light method is best IMO because it gives you direct feedback from the ECU without having to worry about resistance in the wiring harness.

For light or voltage method: Car all the way warmed up. Car off. Turn key to on position. Either use the lights at the test port (under the airbox, the FSM shows you where and I think there's a diagram on FC3Spro somewhere) aiming to have one light on but not both, or measure voltage between the wires at the TPS. Black and green IIRC, with the connector still plugged in. Try to get it to 1V.

Then push the linkage a few times. Check again. Reset if necessary. Repeat until it's always coming back to 1V / 1 lamp on.


EDIT: The bottom page of the FSM that j9fd3s provided shows the resistance method. This also works, so there are three methods total. For that one its key in the off position of course and sensor unplugged, but everything else is unchanged.

 

Used the voltage method, because I had a DMM handy.
Got it to read 1.05V with closed throttle and reads 6.5V on WOT. Connecting to the other pin on the TPS gives a constant 5.5V which, from what I can tell, is to be expected. The thing that worries me is the top end being up at 6.5. I know it ideally shouldn't be that high, should be closer to 5V from what I could tell. Not sure if that's an issue.

Took it for a test drive after adjustment and noticed a few things. Could be unrelated, could be tied to the TPS:

1. Car idles pretty high. Even with it being hot, it idles at a constant 14-1500 RPM. But, when starting it idles at around 8-900. Onlu jumps and stays at 1500 once you drive it.

2. When down shifting, the pedal feels almost unresponsive. Tried going from 4th to 3rd and tapped the gas like I usually do, and it barely revved.

3. When revved at idle, it'll come down to about 7-800 RPM, stutter a bit, then come back up to idling around 1500.

 

1. Normally the car should have a 17 second long 3000rpm accelerated warmup idle, then 1500-2000rpm for about five minutes until it reaches operating temp. Often people unplug the sensor for the 17s 3000rpm idle, but the thermowax controls the second high idle physically. I would look into the thermowax, although the behaviour is a bit strange since I'm not sure how it would start out with low idle and end up high. Maybe the thermowax is set incorrectly and not working, then when your car warms up a vacuum leak becomes apparent? Sometimes there is no vacuum leak when cold but a leak appears once everything expands from the heat.

2. This sounds electronics related. Since our cars have a physical throttle cable, you pressing the pedal means the throttle blades are opening. I would guess the TPS could be related since it was just adjusted. Does your TPS voltage sweep smoothly up from 1V to the 6V reading?

3 is harder to determine because there are other things going on. I will take a look at my TPS voltage readings next time I drive my car (probably later today) and see what it tells me. It's set with the light method but the voltage should be the same. TPS can go bad, but usually it appears as a bunch of dead spots in the sensor that cause stuttering.

 

1. The AWS only holds it at 3k for about 2-3 seconds on cold start, then comes down to about 1200. Maybe an issue with the AWS?

2. TPS voltage sweeps smoothly from 1k to 6.5k. No dead spots that I can see.

I don't believe the TPS is bad, but I have a spare sensor I can throw on there to see if that'll fix it. As for the thermowax, is there a way to tell if that's bad before replacing it?

 

The AWS is unnecessary for idling, so I wouldn't worry about it. It's there to light off the catalytic converters sooner for emissions reasons.

You can try the other TPS since it's a five minute swap and fairly easy to set once you know the process.

To test the thermowax remove the throttle body and put it on the workbench. You'll see the thermowax on the back side. At cold temperatures, the piston is holding the linkage open and this props the throttle blades open slightly. As the coolant flows through the thermowax, it heats up the little wax pellet which expands and pushes the piston outwards. This causes the small roller to roll slowly towards the end of the triangular piece. Once it falls of the end of the triangular piece the thermowax is entirely disengaged and you have your normal 750rpm idle. You can use a heat gun (gently) to simulate this and observe how it works. When starting the roller should be between the two marks on the triangular piece. When fully heated the roller should be completely free of the triangular piece. Make sure water flows properly through the nipples.

If it isn't working properly you can actually take the thermowax off and clean the passage underneath (you'll need to cut a new gasket or purchase one since it also seals the double throttle blades). Blow out the coolant nipples with brake cleaner and make sure it comes out clear. If you have any obvious signs of leakage, you can pop the little wax piston out by removing the small bracket with the two screws and pulling it out. It seals with an o-ring. If there's no leakage then don't take it apart.

When you reinstall the thermowax, you'll have to hold the little spring loaded linkage away as you tighten it down. Then you get to the point where you can set it. There are two screws to set the thermowax: one on the little linkage by the piston and one on the backside of the big round linkage where the throttle cable connects. IIRC the one on the piston controls the temperature at which the thermowax works and the one at the back controls the actual amount of air it lets through.

You will see a triangular sort of part with a little round roller sitting on it. There are two marks on the triangular piece. Adjust the screw until the roller is sitting between these marks at room temperature.

For adjusting the rear screw, I usually do a test fit and see where it idles at cold start. Then I remove the throttle body again and adjust it one turn at a time before trying again. Very tedious, but it works.

There is a procedure for this in the FSM, but it should be basically what I described.

EDIT: I would look for a vacuum leak before you go to the effort of doing this though. 99% of idle problems on this car are a vacuum leak from one of the hundred little vacuum lines on the vacuum spider.

 

Gotcha. That honestly sounds like a PITA, and I don't feel like going through that. I would honestly air on the side of a vaccum leak as well. Might asl well just replace all the vaccum lines while I'm at it. Is it that labor intensive?

 

The actual replacement of the vacuum lines isn't terribly labour intensive (just do them one at a time so you don't forget the order), but you have to remove the upper intake manifold to do it. Which means throttle body, secondary injectors and rail, etc. Then once you have it apart you're most of the way to the lower intake manifold and you've bought yourself a whole intake rebuild just trying to actually reach the stubborn vacuum lines. On the plus side this gives you an opportunity to replace the fuel lines, replace / delete the FPD, check your EGR valve, inspect the oil metering lines, etc.

You can take a look at a write-up I did awhile back in the 2nd Gen section stickied threads. That should show you the whole process.

 

Even that seems like a lot of work for a vaccum leak. I'll take a video next time I drive it and show the idle behavior. May be me being stupid for all I know

 

i've seen a bunch of cars with either air or crud in the thermowax hoses, so if you play with the thermowax, blow out the hoses with something (air, brake cleaner, pipe cleaner, etc) and top off the coolant, and see if its happier.

its warm enough in the engine bay what with plugged hoses it can still kind of work, but idle can be intermittently high

 

Seems like the most likely culprit is the aforementioned thermowax or a vaccum leak. My TPS was in fact bad. As i would accelerate, I would get spots where it woulf hang and not accelerate, and it would buck at low speeds. Threw on the spare TPS I had, and adjusted it. Now I sit at 1.08V closed throttle, and 4.95V at WOT. Drives much better, but my idle still sits at 1300.

The other night, I went out to a meet. As I was about to leave, the car would not start. Luckily I had a buddy of mine there who also owns an FC. After tinkering around for a bit, we found the plug wires were bad on the leading coil. Luckily he had a spare set of NGK wires on him and we hooked those up and she started. But, we only replaced the leading wires. If the trailing plugs were bad, and I wasn't getting spark to those plugs, could that also be causing what Im experiencing?

 

doubtful, but you should check the wires. if needed put a timing light on each one and make sure they all flash

when i get a car, i usually end up checking all of this stuff, and setting the TPS and the idle mixture and such.

 

Understood.
Speaking of idle mixture, is there a specific way to adjust that? I'm thinking that may be it. I took apart everything yesterday and the thermowax was good. No obvious vaccum leaks I could find either. So I'm thinking maybe my idle just isn't adjusted properly?

 

once the timing is set, idle speed is set, TPS is set, then idle mixture just gets turned to where it runs the best. the factory will set it where it runs the best, and then a tad leaner. it should end up near the middle, if its all the way to one side, then something else is wrong