I've been told it sounds like a vacuum problem, but I've tested all the lines with a vacuum tester and found no leaks. When I am at idle in my truck, the engine bogs down to the point where it will sometimes even cut off when I turn on my defroster. I don't even know where to start. No blown fuses and when I run just heat or just A/C the truck doesn't do this. What could be the cause?

I've done a complete tune up to include plugs, wires, cap, & rotor.
I've replaced the idle air control valve (related to another problem)

Any suggestions?

 

When set on defrost the a/c compressor does come on, whether set to cold or hot.

 

I know that the AC compressor turns on, it dries the incoming air so it has no moisture in it.

Why is my engine bogging down?