Problem believed to be with Aluminum Flywheel and Crank sensor(sort of long).....
Well after battling with annoying "limp mode" issues for almost a year, Germantoy mentioned that he had the same problems due to his Aluminum flywheel from Tmtek. I also have a Tmtek unit.
I just pulled the engine and took some measurements of the size/location of the CPS pin on the flywheel and sure enough the dimensions are way out of spec. Tmtek machined the flywheel incorrectly.
For anyone who is having similar problems (search my previous posts for the exact symptoms) here are some of the dimensions. Keep in mind i took these dimensions and they are not exact. All are in millimeters.
Pin length (stock S4) = 10.12
Pin length (tmtek) = 5.0 (WTF!!)
(Note: The S4 flywheel is offset forward an additional 0.79mm so the difference in lengths is really more like 5.91mm)I could have been more accurate with a godd@mn hacksaw.
Audi spec: sensor to pin clearance= 0.45-1.25mm
S4 actual sensor to pin clearance = 0.90mm
Tmtek sensor to pin clearance = 2.08mm(again WTF!)
Just by looking at the sensor in relation to the tmtek pin, the end of the pin barely reaches the outside diameter of the sensor, when it should actually at least reach the center of the magnetic sensor. On top of that, the pin is too far away from the sensor. Here are pics of the two pins. I believe Germantoy had the bracket machined to bring the sensor both closer to the pin and also closer to the flywheel to make up for the short pin. Since i have the engine out i think i am going to try to get a longer pin with a full diameter (tmtek's has a flat on it??) and make up the difference by milling alittle of the bracket to bring the sensor into spec.
Sorry for the long post but this has been kicking my @ss for the last year and as a mechanical engineer this horrible example of manufacturing has totally pissed me off, so hopefully this info will save someone else alot of time.
1988 90Q (20vt 3b conversion):
Lehmann/Hoppen software, Nice Paint Job Delete, custom 3" exhaust w/Magnaflow muffler, No S2 parts, Coupe steering damper, Custom rear swaybar(used 1" Front bar w/front control arms from 90quattro), Eibach springs, Custom FMIC, K&N intake, TMTek Aluminum flywheel w/Sachs 4-puck metallic race clutch, B4 hood & headlights, 17" Forged Centerline's, summit battery reloc., B4 fenders, coupe control arms, glass RS2 front bumper, custom cold air intake.
MODS: 200 Coinholder, One 15" wheel from 100CS, walmart mats, 710N, New Front DS fender(courtesy of woman in office parking lot), boost gage, custom aluminum anti-carjacking device. H&R's/Bilstein's, B6 S4 wheels.