The C4 UrS has a moisture trap on the boost line between the intake manifold and the ECU (computer). The moisture trap is like an old style VW bug fuel filter, except without the filter element and it only deals with air flow. The idea is is provides somewhere for moisture to drop out of the boost signal hose before it gets to MAP in the ECU.

For the UrS4, this moisture is located above the connector rack on the firewall, as shown in the photo below. (The UrS6 moisture trap is down by the ECU, in the right side kick panel).


At some point the OE clamps on the boost hose at the UrS4 moisture trap get lazy and allow boost signal leaks or, worse, allow the hose to come off the moisture trap. If this happens, your ECU never sees a boost signal so it doesn't tell the N75 wastegate refrequency valve to do anything. All you get for boost is that provided by the wastegate spring (e.g. 6 psi stock). So essentially, you now have a normally aspriated engine making less than 170 hp, not the 227 hp (plus) of an AAN.

The solution is to remove the old OE clamps and for about $1 or $1.50 each, replace the OE clamps with the smallest gear clamps you can find (e.g. NAPA, Pepsi Boys, CTC, etc.). For $3, the ECU gets the boost pressure signal at the MAP and can now control the N75 WGFV to provide more boost that just the wastegate spring. As a result, you gain at least 57 hp (depending on what you were making before the hoses came loose on the moisture trap). Not bad bang for the buck.

PS: When buying an UrS4, this can be used in negotiations if you find a car with blown clamps. Check it out but don't tell the owner. Use the info to your advantage.

Related Info:
Link to G71 MAP Sensor Info

N75 Waste Gate Frequency Valve Info

Naturally, IMHHEO™,
Dave F. aka the Librarian

93 S4, 1.8t coil pack conversion, CF (OE), S6 steering wheel, RS2'd, Samcos, MRC, TDS-1, Apikol FMIC, Eibach/Bilstein, BIRA Sys 1, relayed FP
In the basement: V8 torsen rear diff, 92 rear sway bar and cross member and too much more.
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