What might cause a 1994 Suburban to blow only warm air from the rear air while front air is cool air front better but not cold either?

Answer

Two potential problems, either the AC isn't working at all or not working well, or the heater valve is stuck in the on position, and it's putting back more heat than the AC can take out.

With the engine running and the AC on, lift the hood and watch the AC compressor. See if the clutch is engaged or turning on and off or if it's just on all of the time. If it's on constantly and you're still blowing warm air, you probably have a sticking heater valve. If it's not running much of the time or at all, you may be out of refrigerant.

There is a small orifice in the real in the a/c evaporator compartment it maybe clogged that was the case with my 1994 suburban i took it lose blew it out and vacuumed the system, restored refrigerant, and no problems for the past 3 1/2 yrs.