2000 Ford F-150

CEASTMAN

MEMBER
2000 FORD F-150
V8
4WD
AUTOMATIC
129,866 MILES

Last year I noticed some coolant had come out of the resivoir. I thought it was the thermostant so I put in a new thermostant and coolant. Then after start up and going down the road the temp gauge comes up slowly to normal and Iwill get heat. Then the heat stops and Ijust get cold air. Then the temp gauge will jump up to the 3/4 for about 15 seconds and then dror back to normal and the heat returns and the engine runs normal for as long as you want and never overheats. The engine runs great. This only happens when the temp is 72 degrees or lower, other than that it is normal. The engine never overheats even when the temp is in the upper 90s When the temp does that jump to 3/4 som coolant overflows, but the bottle never gets totally empty. Also the coolant has turned orange.

1 Reply

CARADIODOC

MECHANIC
33,753 POSTS

Your old thermostat likely had a tiny bleed hole in it and your new one doesn't. The thermostat and the temperature sensor for the gauge are not right next to each other. Heat from the engine warms the sensor first, so the gauge goes up, then eventually the heat migrates over to the thermostat and it opens, allowing cold coolant to rush in from the radiator. The cold coolant causes the thermostat to close again and the process starts over until the system stabilizes.

I don't know why, but an aftermarket thermostat worked fine in my '88 Grand Caravan for years, then one day started doing this, only it took about a dozen cycles in five miles before it smoothed out. After living with it for a long time, I had a student pull it out and drill a 1/32" hole in the plate. That allows coolant to circulate over to the thermostat while it's warming up. Most foreign car thermostats have this bleed hole.

I suspect your coolant is expanding into the overflow reservoir until it starts to circulate. As for the orange color, that sounds like Dex-Cool, (Dex-Mud). That's more of a GM misery. Any chance it's just rust and it's time for a flush and fill?

Caradiodoc