I have a 95 f150 with a 351. It recently developed a rougher idle with some light popping coming from the exhaust. It seems to drive fine and when its cold it doesn’t seem to be a problem. It usually starts after about 15 minutes of running. I have changed all the basics including plugs, wires, cap, rotor, fuel filter and coil. Have also confirmed timing is correct. Today i realized something odd, if the heater switch is on a/c, max a/c or defrost the popping stops and the truck seems to smooth right out. If its on off, vent or floor the truck will start running rough again. I am now wondering how the heat control could be relevant to my problem. I checked some of the vacum lines to the heater control and didnt notice anything obviously leaking. Has anyone seen this before? Any help is appreciated. Thanks, Eric.

 

My money would be on a vacuum leak.
Check all the rubber connectors and make sure they aren't deteriorated and leaking like my truck was.
May even clean your IAC to see if that helps as well

 

The HVAC system uses manifold vacuum to power vacuum motors to move louvers directing air flow. It is common for the under hood vacuum lines to crack or even disintegrate. The hoses in the cab do much better. That is where I would start. My advice is to replace all your under hood vacuum hoses, and move forward from there.

Given your stated symptoms you most likely have a cracked or broken vacuum line near the passenger side firewall that feeds the blend door vacuum motor. This is a notorious failure.

You can use a hand operated vacuum pump to check the vacuum circuits, but you will need accurate understanding of what you are testing as some of the circuits have "controlled" leaks by design.

 

this will help 93 F150 351w vacuum line replacement - Ford Truck Enthusiasts Forums

pic of the white disintegrating vacuum line