Chevy truck w/ 5.0 V8 doesn’t warm up. Replaced thermostat which was stuck open with new 185 deg unit. Truck overheats now. Checked new thermo in boiling water, it opens. Original thermostat had hole drilled through it. Any thoughts?
I usually replace the radiator cap and the thermostat at the same time. If the radiator cap is not holding pressure the radiator will boil over.
Make sure the new one was not installed upside-down.
You have to say what year.
And you could have circulation issues with a working 'stat. Check radiator flow and water pump.