1999 engine pulsation / power loss @ speed

[scrappy]

99 4.0 ltr SOHC...just bought from used car lot. Check engine light on and engine pulsates/looses power after 45 minutes of driving. At 50 -70 MPH it gets bad. Wife driving it once and couldn't go faster than 40 MPH and "check engine" light was FLASHING (not good right??). Had code scanned @ auto store they said "downstream O2 sensor" which there are two of. Used lot said they got codes for too lean and too rich and said they found and fixed leak in upper air plenum gasket. Half way home from the lot...light came on again and problem still exists....still getting downstream O2 sensor code. I don't want to just change O2 sensor if problem is something else like more vacuum leaks?!?!? Any guidance/thoughts would be appreciated. Thanks! (my first post) Scrappy

 

[Majisto]

Flashing means mis-firing. I had that problem on my old Taurus. Check the ignition system because that can cause your car to also run poorly. I'm hoping the not going over 40 means your cat is clogged. I think it can get clogged if your truck runs extremely rich.

 

[scrappy]

Thanks Majisto! Do you think clogged Cat would also give me the code for bad O2 sensors?...if so, maybe clogged cat is main problem ?!?...causing computer to adjust..??? Scrappy