I have '98 Chevy PU, 4.3L V6, auto trans. Several months ago I noticed the water pump was leaking so I replaced it and the thermostat. Since then it is very
slow to warm up. It takes half an hour of driving to bring it up to normal temp. I have replaced the thermostat, the fan clutch and flushed the heater core
to no avail. I even tried blocking half the radiator with a piece of cardboard, no change. The heater works fine, hot or cold air according to the temp
dial on the control panel. Any ideas on what else I might try are very welcome.
Did you get an aftermarket thermostat?
Are you sure it’s rated at the correct temperature?
Are you absolutely sure you’ve got enough coolant in the radiator?
For what it’s worth, I’ve seen many many vehicles that had a full coolant reservoir, yet the radiator wasn’t even close to full
Are you sure that the system is fully bled? An air pocket around the temp sensor could cause your symptoms.
Also, in the past “several months” winter has descended on many of us. Could that be a factor in the slow heat-up?
Try this.
Drive the vehicle to where it should be at operating temperature. While the engine is idling, loosen the upper radiator hose clamp and take a small flat bladed screwdriver and insert it in between radiator hose and the the radiator hose neck.
Allow the engine to continue to idle until all the air is purged out of the cooling system or until heat comes out of the vent system.
Remove the screwdriver and retighten the hose clamp.
Tester
You have air trapped in your cooling system. @TonyCarlos and @Tester (as always) are right on the money.
Took me a while to get around to trying your suggestions, (cold weather, flu) but air trapped
in the cooling system was the problem. Another problem was that the highest point in the
cooling system was where the upper radiator hose goes over the fan shroud. But its working
better now so thanks for the help.
Thanks for the update