2004 Impala, Lack of Power

Hi Alll
I'm new to this forum, posted this problem at a different site, thought I would give this one a try.

I have a problem with a friend's car that I can't figure out. The 2004 Impala has lack of power on the high way IE: Your in over drive going down the high way, push the gas pedal to the floor, the transmission doesn't shift down to passing gear. The engine speed doesn't increase, nor does it rev really high as though a clutch is slipping, just lack of power. My friend brought the car to a dealership, they couldn't find the problem, no stored codes and the check engine light doesn't come on, and the car idles fine.

I checked the air filter, it's dirty, but not clogged, blew out the filter with compressed air. Put some gas line antifreeze in the tank as it's really cold outside. Checked the spark plug wires they all seem to be fine, no loose spark plugs. None of the above fixed the problem

Loosened up the bolts Cat to exhaust manifold, just enough so that it wiggled and had some space for the exhaust to escape. The car was loud, however, it had power and acted normal when driving it down the highway.

Odd thing is that the Cat didn't glow cherry red when the car was heated up and I didn't notice a rotten egg smell that usually occurs from a plugged Cat. Maybe this is because the Cat is of a newer type, any thoughts???
Could there be a exhaust restriction futher down the exhaust system??

Duke:)

Welcome to AF, if the car ran ok with the cat partially out of the picture then there must be some exhaust restriction, have the cat back pressure tested, they are covered for 8/80 or at least 7/70 depending on the state.