Bleeding Radiator Express 3500 5.7

I have a 99 Chevy Express Van 3500 extended. I had a problem it started to 0verheat. It blew out the drain valve in lower driver side of radiator. I pulled the radiator and installed a new valve. I filled it with as much as it would take of Dexcool and water a 50/50 mix. I let it run as I filled it and drove about 3 miles and it overheated again. Im not sure all the air is out of the system. I put in about 2 gallons of straight Dexcool and 2 gallons water. Im sure it would take more especially with the extended version with rear heat. I even drove the front end up on the curb to allow the water to drain to the back end. Does anyone have any other ideas. There was heat coming from the vents so I dont think the thermostat is stuck closed. You do see water circulation but fluid doesnt seem to drop. The needle is pegged now. It is 100 degrees out so Ill have to wait until later.

Thanks Mike.

By the way the radiator is only 2 years old at most. It is not clogged with Dexcool.

If it built up enough pressure to blow out the drain valve, and is still overheating, there is the possibility of a bad head gasket.

gmtech1
You might be right. I just picked it up now that its cooled down its idleing real rough. No codes? Needle was pinned all the way to hot side and stuck. After it ran a second the needle unstuck and went back down but settled about 1/2 way maybe 150 degrees. Maybe sensor went bad. The bad thing is it sound like a rod knok under load. If put in neutral and revved you hear nothing except a little exhaust leak near the exhaust manifold. Maybe the donut. Could be blown head gasket. It was dark when I didnt check for water or Dexcool in the oil. Ill see tomorrow.

Checked oil no water in oil but head gasket could be blown cylinder to cylinder. Question is the noise I hear the start of a rod knock or is the head gasket making noise. Maybe ill put a little STP in the oil to see if it quiets down.

STP didnt seem to help. I would expect if it did it would be a rod knock. Maybe the head gasket. No smoke. No antifreeze in oil. Runs rough when idleing and running. Gasket between 2 cylinders? The noise I dont like.

Pulled the box inside the van. Pulled spark plu wires while running and found no difference when pulling 5 and 7 . So I guess I have my answer. Head gasket bad between 5 and 7.

Check your thermostat and flush your radiator. Remove grille and clean A/C condenser with high-pressure water like self-serv car wash and retest When the truck engine is very hot, the air blowing from the fan should be very hot, almost hot enough to burn your hand if the engine temp is around 230. Don't forget to check the coolant with an antifreeze tester.

Thanks Jeff. I had out the rediator its clean. I will have to replace the head gasket so I will replace thermostat and hoses when re-assembling. Not sure why it blew but as gmtech1 said that is why the valve in bottom of the radiator blew. I hope the noise is from the head gasket and not a rod. It could be its more knocking from the motor running on 6 cylinders.

Forgot to mention check gauges light went on no cel but when I disconnected the wire from the ect (Temperature sending unit) The cel light was on and gauge light went out.