I have a 97 f150 with a 4.2l in it. I use the truck to pull a trailer for a business. The truck runs good with no load. If I turn on the Ac I notice sometimes it seems like there is a big loss of power below the 2k range roughly. It feels like a elephant sits on the back of the truck and all the sudden jumps off because the power comes back instantly once the motor get up in the rpms more. The problem is even worse with the trailer behind the truck. For the first few miles its fine, but once I get a few good miles at 55 or better the problem shows its head when I try to take off from a stop of a slow turn. Sometimes when the trailer is behind it the truck will idle very rough and low too when sitting at a lite, but it has to be up to temp and driven a while for that to happen. The truck has had alot done in the last year. I have a new radiator, clutch, plugs (bosch platinum), wires, coil, and catalytic converters ( stock ones fell apart and clogged up). I also had to change a rubber elbow on the intake that goes to the back of the passanger side valve, because it was cracked and caused a lean engine code. The only other thing I notice is that the truck has had a smell of hot metal ever since the new catalytic converters were added. They told me it would go away its the oil burning off, but it hasn't in 2500 miles. So maybe my exhaust is clogged beyond the "Y" pipe? I have no other idea as to what my problem could be please help! I am tired of my local mechanic's throwing my money at a problem with no end result.

 

The power loss because of the AC is due to the draw it adds to the engine, and it kicks in at about 3k because the torque curve is steep and that's where it kicks in.

Its like someone kicked you in the *** when you get to about 3500rpm's in my truck

Its super bad to run your truck hard not warmed up, as for any engine, the loss of power is your now pulling in hotter air into the engine rather than cold, so your not getting as good of a burn.

Hope this helps.