Re: 2000 Expedition Heater Issue
If the coolant level is fine and within proper range, then there are a couple possible causes.
First, the coolant pump is on its way out. Not always will a failing coolant pump leak through the weep hole. Some pumps use internal steel components. If the coolant isn't changed/flushed regularly the impeller fins inside the coolant pump can slowly corrode causing less, and less flow over time. Once you increase engine RPM (increase in speed), would explain why you get heat.
Second thing, is if you have a rear heater unit, this unit is run by a booster, or aux. pump. If you keep the rear heat off this shouldn't be an issue for the front heat, but to rule out a coolant pump issue you can try running the rear heat if your vehicle is equipped.
The blend door effectively closes/opens too allow heat to enter the cabin. The 'cool' setting on the dash control knob over rides the heater core. If your coolant level is down (nothing in the reservoir) this can mean that there is one or more air pockets in the system. Now, this can also be the cause of lack of coolant reaching the pump to effectively circulate the coolant through the heater core. Should there be an air pocket, refill the coolant reservoir, drive the vehicle and check and refill as necessary to work out all the air.
The heater core may also be partially blocked inside with rust/debris. A back flush of the core generally helps alleviate any future problems. There are garden hose kits for this, but be cautious as engine coolant will come out when you remove the hoses from the core that enter in through the firewall.
If uncertain contact a local shop and they can perform a coolant pressure test, which can help diagnose a bad/faulty pump, or a block in the system.
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