Heater Barely Heating and Temperature Gauge Going Up and Down | Ford Explorer Forums - Serious Explorations

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Heater Barely Heating and Temperature Gauge Going Up and Down


July 5, 2008
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Year, Model & Trim Level
1999 Eddie Bauer V8
Not sure if this is a Heater problem or an engine cooling system problem, but I think there is a connection and I’ve been searching the forum for a while and can’t find an answer. My heater blows only warm air, not hot like it should. I can turn the temperature to cool and the A/C works just fine and blows very cold air. Turn it back to warm and still only blows warm, not Hot air like it used to.
So here’s my truck and the symptoms. I have a 96 Explorer with about 317000 miles. Last summer while at the beach I noticed the temperature gauge starting to move up and down. Normally it would sit around the “O” on “NORMAL”. It would slowly move from there up to between the “A” and “L” and then quickly drop back down to the “O”. It would stay there for a few minutes and then slowly move back up again. It would repeat this over and over, no matter how fast or slow I drove, and no matter if I was in traffic or not. I assumed it was a cooling system related problem, so when I got home, I flushed the system and changed the thermostat. Still the same thing. I took the radiator out and hosed it out good, put in another thermostat and changed the radiator hoses and serpentine belt. Still the same thing. I took the thermostat out completely. The temperature gauge stayed in the lower range all the time. No more moving up and down. So I left it like that until this past fall when it started getting chilly and we needed some heat. I put another new thermostat back in and the temp gauge started doing the same thing again. But now the heater doesn’t blow very hot air. He A/C works just fine. The blend door actuator sounds to be working OK, but I have not opened the enclosure up to check the door. I also noticed a few days ago that as the temperature gauge goes up, the air from the vents gets warmer, then as the gauge drops back down, the air becomes less warm. I have used only Stant thermostat and was careful to get the bleed-by hole positioned correctly.

Any thoughts?

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Sounds like low flow from the waterpump. Has it ever been replaced or is it original?

With that many miles I guessing "head gasket". The compression leaking into the cooling system will block water flow allowing the radiator to get cooler then when the water does flow the temp gage will drop out. When it's up to operating temp. look inside the plastic coolant jug for 3 minutes while it's ideling. Do you see air bubbles? Bubbles not good.

Thanks guys.
I can see where any of these 3 could cause one or the other of my problems, but do you think the problems are related, or just coincidence.
And I think I replaced the water pump about 100000 miles ago.