My nephew's 1995 Sportage, DHOC, 5 speed, has lost a lot of power in 4th and 5th gears. We just installed a new timing belt and water pump a couple of weeks ago and every thing was fine except for a moderate exhaust leak. At the same time we installed a new timing belt on my 2001 Sportage and both vehicles ran fine.

I discovered that I had had a slight Brain-Fart and failed to perform the last step in the porcedure which is to rotate the exhaust cam to the S mark and then tighten the tensoner. We corrected that last weekend. We at that time installed new plugs in the 1995, the center electrode on the old plugs was burnt off. When re restarted the 1995 all appeared well but we later discovered that there was a serious loss of power in 4th and 5th gears. The afore mentioned exhaust leak is much worse and it is located at the flange between the header and the down-pipe which is about two inches above the O2 sensor.

Would this leak cause the O2 sensor to indicate a lean condition and make the mixture rich causing the loss of power. Fuel economy has fallen from 27mpg to about 17mpg.