1999 F150 5.4l NO HEAT HELP
 

I have a 1999 F150 5.4l. The heater stopped blowing cold air. I verified the blend door is operating correctly. I flushed the heater core....had a very small amount of brownish water before it went completely clear. I flushed it from both sides. I flushed the motor and have clear good flow throughout. I verified the thermostat is operating correctly. I noticed after the motor warms up the inlet side hose to the heater core only warms up half way. Near the motor it is warm....near the heater core inlet it is cold. The return hose is completely cold. Has anyone experienced a water pump failure that would cause this???? Help I'm at a loss!!!

The impeller blades could be gone or it internally came apart.

I am going to pull the pump tomorrow....hoping that's the issue!

You mean it stopped blowing warm air, right?

I think there is a restrictor in the hose assembly before it gets to the core. May want to check on that.

How much of the hoses did you also flush?

All of them....I actually replaced both of them

Good.

Then you've either got an air bubble in the heater circuit or the pump ain't pumping.

I burped the system before and after...same issue.....looks like I'm changing a water pump this weekend lol.

I'm surprised that it's not overheating if the water pump is shot...............

Welcome to the wonderful world of Ford truck heaters. You have the same deal as I do but like mine that tows in 100 degree heat just fine, it likely is not the water pump.

A couple summers ago my truck starting to have the temp hand start raising slowly if I was in a drive through or in traffic for a few minutes, you could actually rev it up a little and the hand would start coming back down. Replaced water pump and only one impeller was left and it was about gone. Pump was original with over 230,xxx miles on it.

If you use tap water in the cooling system, it could plug up or partially plug up the heater if the water is hard in your area. After plugging up a heater core a long time ago and had the joy of replacing it, I started using distilled water to mix with the anti-freeze.

x2 on Roadie's suggestion. I always use bottled distilled water on coolant flushes OR premixed 50/50 setups.

I still have a feeling it is the impeller on the water pump. Just dealt with one not too long ago.

Turns out the problem was indeed the water pump. Truck never overheated....even in 98deg Florida heat. No idea how it didnt.
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Congratulations on the repair of your truck! And thanks for letting us know!