Hey Guys! Chevy 350 Tech Question - Don't shoot!

[70stangcoupe]

I was thinking about finding a Chevy board and asking this, but I know you guys are the smartest entity of car enthusiasts on the internet, so I'm hoping you'll help me out even though it is a bowtie.

My dad and I recently rebuilt the motor in his 92 Chevy Pickup (350v8) and put it all back to stock except a mild towing cam, since we use this truck to pull. Well, we've got a problem.

It heats all the way up to 250° before the Thermostat opens and then it rockets down to 180° and runs forever just like a champ. Even when the gauge reads 230°, the top radiator hose has no fluid in it and isn't hot. Then all of a sudden when the gauge shoots down the hose becomes hot and fills up.

This is the third thermostat we've tried, and they've all done the same thing. Different brands, everything. I dont think the thermostat is at fault. I've ran the thing for about 100 miles with the heater wide open trying to work any air pockets out of the cooling system, but it doesn't help. What else could it be?

Thanks guys!

 

[pabear89]

Air in system.
With large rad's and hi temp thermostats you need to do a slow fill with
coolant.
fill rad and leave cap off,heater temp control on. run engine till temp is approx themo's temp and refill rad SLOWLYwith engine running.
when the colder coolant gets to the thermostat it will close it and trap air pockets inside. thats why your upper hose is cold then a sudden flash of heat.

It take a little longer but seems to work.

PB

 

[rbohm]

i agree with pabear. what i usually do is drill the tstat(1/8" hole) so it will bleed air as i fill the system. that will minimize the time it takes the system to purge itself of air pockets.

 

[HistoricMustang]

Yes, I think the old air bubble is in the system.

Jack the front of the truck up when putting water in the system. I do this on both my '65 Mustang and Z-28 daily driver.

HistoricMustang
www.historicmustang.com

 

[TT670]

yup, its air..sometimes when theyre real tough to purge it helps to park on a steep incline with the front facing up. It helps make the radiator the highest point in the system to the air can work its way out. Usually just running it for a while with the cap off is all it takes.