Hello ,I'm new to this forum and hope to get some usefull info as well as giving any help I can.I have an 99 malibu (v6-3.1l)and it ran great till a few weeks ago when it started to bog after hitting the 20-30 mph the it would downshift and accelerate hard up to 40-50 mph then bog again.Lately it just won't get past 20.I've changed the plugs,wires,fuel filter and one of the coils wich the obd signaled as a missfire in cylinder 6.Still have the same problem.Any ideas as where to check next?The obd light hasn't come on again.Thanks for any help.

 

Check for a plugged catalytic converter.

 

Had similar problem last year with my daughters 2001. Would go 30-40 miles then bog down. First dealership could find nothing wrong. Drove ok when we got it back, then started doing the same thing next day. Had to tow it 40 miles to a second dealership. They actually put two mech's in the car , one driving and one reading the OBD, and had them drive it until it bogged down. No codes.
Turned out the insides of the converter was completely gone. Nothing but crumbled ceramic that was pluging up the exhaust and the engine was choking on its on waste.