my 2005 chevy venture (3400 SFI) suffers from chronic coolant back pressure and overheating. What might be the causes of this?

 

Hi!.I would check the radiator cap, or replace it if you have not done it in the last year or two. I would also check to make sure the radiator fans are running when you switch on the AC (they are supposed to), at least at lower speeds. The same thing should happen when you switch to the heater mode that uses defrost. This is supposed to turn on the compressor and any time the compressor comes on, the radiator fans should be on too.

 

if the pressure develops in the surge tank right away; it could be a blown head gasket. if the surge tank is overflowing with coolant after the engine gets hot; it could be as described above.

 

sounds like it's not circulating ! it could be a clogged radiator or the thermostat stuck closed ! check those things out ! it might just need a good flushing out !