This problem actually goes back a few months. The engine used to run perfectly: solid idle @ 750, etc, etc. Then I pulled everything off the top of the motor to do some cleaning and when I put it back together I had a new problem.

With the tps connected, the engine will rev fine up to 3k. Past 3k, it starts surging. If I try to drive with the tps connected, at and past 3k power all but vanishes and the exhaust does an excellent immitation of a machine gun (bra-a-a-a-a-a-a-a-a-p, not bang-bang-bang.)

With the tps disconnected, she revs smoothly and is quite driveable, aside from a smidge of instability when holding the throttle steady. But (not surprisingly) my mileage is down by 20% and she won't hold idle when she's warm.

I've checked the resistances on both tps', and one of them is out by a little. I think it's the full-range one. It's hard to picture that as being the cause, but then maybe that's what the symptoms setting in at the 3k mark is telling me...

Linked below is a short video of the tach, shot with my phone. The tps is connected.

Revving:
http://postdownload.filefront.com/13...7de47ad44af5e7


I'm still hopeful that this is an easy fix.


[edit] Yup, I checked for vacuum leaks. Found the thermoswitch was leaking, changed it out and haven't found any others...

 

Readjust your tps, or see if it's still in spec.

 

I always use the sensor voltage to adjust the tps. It's kind of nice to have that feature on my SAFC. Just turn the screw until it says 1.000 volts, rev a few times, adjust, rev, adjust, rev, repeat until the sensor voltage stays at 1.000 volts.

Whenever I get idle issues, I always do it in this order:
1- warm it up
2- initial set coupler
3- idle
4- tps
5- timing


For some reason, I never do one without doing any of them, mostly because if one's out of spec, all 3 adjustments usually need tweaking.

 

The tps is set. I used an S-AFC and a pocket torch on the thermowax to make sure that no throttle means no throttle. Then I backstabbed the full range pin at the cpu and set the tps for 0.8v, as per the fsm (F1-80.) No change. The surging still starts precisely at 3,000 rpm.

I yanked the ecu and had a look and found that two... thingies had fried. I don't know if they were already toast when the car was stored or if it happened later. I took the ecu to shop that replaced the fried bits with bits from a spare s4 ecu. The guy said that while the replacement bits weren't identical, they were close. Put the ecu back in the car and the problem hasn't changed. The picture below was taken after the 'repair.'



My best guess right now is that something is switching on at 3k rpm that is fouling the mixture. Any ideas of what that might be?

 

*bump*

 

*bump*

Yes, I saw the blown resistor in the pic and replaced it. I traced it back to the pins and found it ran to the port air solenoid valve. I tried tracing the circled bits; one side went to ground, the other went through two resistors and went to a chip. Didn't trace the middle.

 

ba-dump

 

Checked the port air solenoid valve. When the car has passed 20k kms, it's only supposed to put out 2.5v. It was putting out 12v-13v. A replacement is on the way. Still haven't solved the original problem...