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Hey guys bought an 01 expidition 5.4 triton and after replacing the blower motor and blower motor resistor i turn on the heat and get hot air for about 15 seconds before it quickly turns cold….if i turn the heat off and give it a few minutes and turn it back on i getb15 more seconds of hot air before goin cold again..
Instantly my thoughts turned to thermostat??????
the previous owner did state the engine was replaced about 10 miles before i bought it sooooooo maybe air still trapped in the system?????
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The first thing you should do is confirm that the heater core itself is not clogged. This can be done by checking the temperatures of the hoses that run to the heater core. An infrared thermometer can be very useful and they can be found pretty cheap. Or you can use a physical thermometer to check the output temperature of the heat. With the engine at operating temperature (around 180F) the heater output temperature should be more than 120-140F. You check this at the vents with the fan on 1/2 to 3/4 speed. Here’s some reading:
http://www.ericthecarguy.com/faq/solving-automotive-hvac-problems
With this the first thing I would check is whither or not the upper radiator hose gets hot. If it does not then the impeller fins on your water pump could be about gone, which from my research is a common issue for expeditions. Also check for coolant leaks especially coming from the back side of the engine. Also you might want to check that the thermostat wasn’t put in backwards.
Also is the heat bad in both front and back? the reason why I say is that that most likely you have a rear heater core behind the rear driver wheel well, their should be a access point where you can check hose temp.
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