Truck Overheating What Could it Be? 98 Chevy

[JonBoy]

Thanks for any help. I have a 98 Chevy with the 305. It does not always do it but lately it will reach about 250deg check gage light comes on and with in seconds it drops to a normal range. I have replaced the Thermostat and it still does it. I kinda believe that I just got a bad one and need another. It was from advanced auto about $7 so it was a cheap one. If not what else could it be? It does it the most if not always on first start up once it goes up and comes down it acts normal until its restarted. Also when its cold and does it heater blows cold until temp drops.

 

[Ron]

you have an air pocket in your system.
When the system gets locked there is no warm water in the heater core, hence the cold air.

Several thing to check but first on my list would be a collapsed radiator hose. The mid 90s bowties were notorious for this, the upper rad hose would suck itself shut and actually starve itself of fluid. If thats it a simple $7 hose fixes the problem...

 

[Diezel]

Yeah, I think air pocket as well, Try to remove the water temp sender on the top of the intake, fill and that should push the air out,

 

[upnover]

Ain't saying it can't happen, but air pockets in my experience work themselves out after coming to temp a few times.
Also check the radiator cap.
You can also try running it from cold to running temp with the Radiator cap off. watch for circulation, do that a few times.
Does not sound like the Gage since you say no heat until it drops. You might also check the thermostat too even if it is a new one. It'll be just the cost of a gasket if it's still good.

 

[Ron]

Ain't saying it can't happen, but air pockets in my experience work themselves out after coming to temp a few times.

hence my radiator hose suggestion.
Ive seen this at least 5 times on Chebbys the hose collapses and creates an air pocket, once it gets pressurized enough it will open until next time.

 

[Bullardbs]

You can also putthe truck on steep incline and take off the radiator cap and let it run then the cap is the highest place nd it will get all the air out.

This has worked for me in the past