4.6l new: heater core, water pump, thermo, all new hoses, flushed system and rad many times.

Below freezing. Start truck and start driving. Engine slowly warmes up as normal. Heat is blowing warm. Then temp guage bottoms out fast. Heat blows cold almost as fast. Then engine more slowly warms up again. Heat blows very hot as expected. It never over heats or fluctuates after that.

Anyone else have this happen with their vehicle?

Thanks
Joe

 

Was it doing this *before* you changed the parts and flushed it?

 

Sounds like an air void.

 

Or a bad thermostat.

 

Agree with Roadie. Sounds to me like the thermostat is going to full open and staying there, instead of opening gradually like it should.

- Jack

 

You can check it on the stove with a thermometer, and pot of water. Or just replace it. A new one is $7. If you want oem, $12

Don't forget the rubber o ring as well.

 

I had a '97 4.6 that did the same thing. On really cold days (sub-freezing) the gauge would go up near normal, heat would begin to blow, then the gauge would bottom out, air would turn cold. About two minutes or less of running and the gauge would return to the normal position and the heat would be fine after that "blip". I just assumed it was the thermostat opening fully and letting cold water flow into the heat/block system again. I probably should have replaced the thermostat, but never did. Sold the truck at just over 80K with that same factory thermostat in it.

 

If it is a air void wouldn't the air pocket bleed itself out once temp starts to come back up to normal operating range? I also hv this issue with my 97 4.6

 

Ot if it cavitates somewhere. That is why proper bleeding of the system needs to be done....