my 95 t100 with 3.4 has started pushing water out overflow tank. I put a new water pump, thermostat, radiator cap and flushed the system. the oil looks fine and the water looks fine could it be a head gasket or cracked? or did I get bad parts?

 

If you got decent quality parts it's unlikely they are bad. Were you having this problem before installing these parts? Did you check the thermostat in a pan of hot water with a thermometer to be sure it opens at the proper temp?

 

Ya the the only 2 reseons I know for water coming out of the overflow is:

1. More pressure in the system then the radiator cap is rated for. (which opens up more possibilities, overheating and such)

2. Leaky or bad radiator cap.

 

3. Failed head gasket over pressurizing cooling system.

 

That 3. is just a reseon for 1 to occur.

 

when I fill it with water and start it the water starts pushing out

 

If its doing it when cold then id say its reseon 1. caused by reseon 3.

 

Did you make sure you burped the coolant system properly so there aren't any air pockets after your re-filled the coolant? Do a quick search, but I believe you need to have the front of the truck pointed uphill and massage the upper radiator hose. There may be another way but it something to look into.