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20 Valve Loss of HP and MPG


I'm not sure if this message went through the first time so here it is

Car: 1991 90Q 20V

Problem: About 6 months ago I observed my car had noticeable less HP and
Mileage had dropped by about 15%. All my driving habits have remained the
same and I cannot figure out what the problems is.

The car runs well and does not feel rough. Idle is normal (750-800RPM). No
warning lights are lit.

This is what I have done to try and correct the issue:
1. Replace the O2 sensor
2. Replace the air filter

Other Maintenance:
1. Changed all drive fluids to RedLine (not necessarily related)
2. Changed all belts including timing and H2O pump.
3. Oil changes and fuel additives on regular interval
4. Emissions are very clean (DMV Test)

Possible causes:
1. The engine does not kill or sputter when removing the oil cap (is this
2. the crank case breather hose is squishy (is this bad?).
3. Knock sensor or engine management system problems
4. Plugged Cat Converter

Any help would be appreciated.

Michael Benno
(612) 593-4555