PDP_S4

I have one of those annoying problems that doesn't appear 100% of the time, so I thought I ask the experts! I've got a 93 S4, 71K, fresh top end rebuild (long story) and a Stromung, other than that 100% stock <this week>. I just replaced all five coil packs (the problem stills exists after this fix however) and a test done a few weeks ago didn't come up with any boost leaks.

Basically under full boost and under a load (hill, etc.), the car segments or surges and almost feels like the waste gate opens prematurely. I will start to build boost, get little to any boost, but then all the sudden at 4K rpm instead of 3K, the boost will start to come back again! I readily blow off the moisture trap vacuum hose under full boost (even with a very tight clamp), but each time the car has presented this problem, the hose has been firmly attached.

I'm sure the 100 degree heat hasn't been helping the situation, but it's getting old! This sounds like a fuel issue to me??? Knock sensor / ping? Bad moisture trap unit itself? I am getting about 270 miles to a tank of gas (91 oct) as well, regardless of how I drive the car (new O2 sensor / fuel filter).

TIA, I would really appreciate anyone comments or suggestions!

Bill the Cat

STFA for the boost leak testing procedure.

Sloooo4esT

o2 sensor? t-stat?

PDP_S4

Have done this, no leaks found.

The Librarian

Surging on a long hill can happen. Try the same hill in a lower gear (higher RPM).