Heater blows cold while driving, hot at idle.

[smythcounty]

This has been driving me crazy for the last few months. While idling the heater blows hot. After driving for a few minutes it starts to blow cool air. Change the heater control valve back in the summer because it was leaking. Burped the radiator several times, check coolant. Not sure what else to do. Could I have reverse some lines on the heater control valve? It has four. Two going to the heater core and two going to the motor. The valve is working I can see it open and close when I switch the heater on. Any ideas? Thanks.

99 Ranger, 4.0L

 

[rusty ol ranger]

Might be a clogged heater core. Or partially clogged. Might even be worth it to change the Tstat

 

[fastpakr]

Thermostat is easy to check if you've got an OBD2 adapter that gives you live data. Open up Torque or your app of choice and you can look at the coolant temperature in real time.

 

[smythcounty]

Okay. I’ll check the temp this evening. This morning no heat at idle or while moving. I’m wondering if the heater control valve is messed up or if I got the hoses backwards when I installed it. The top heater core hose is hot but the bottom one is not.

 

[fastpakr]

To confirm - are you referring to where the two hoses connect to the firewall? If one is hot and the other is not, that sounds like a blocked heater core.

 

[rusty ol ranger]

Okay. I’ll check the temp this evening. This morning no heat at idle or while moving. I’m wondering if the heater control valve is messed up or if I got the hoses backwards when I installed it. The top heater core hose is hot but the bottom one is not.

If the heater valve was bad or wrong it wouldnt matter about engine speed, just where the selector was.

 

[vigness]

I will add my vote to a partially clogged heater core.

It will depend on your hose set up, but if possible, swap them. I kow it sounds so simple, but it does work. I had an old 65 Econoline with the dog house. The heater had been having problems, and on a REALLY cold morning, I had to grab my little propane torch and run it back and forth across the inside of the window to be able to see to drive the five miles to work.

Went out at noon on my lunch break and the motor was cool, loosened the clamps and swaped the hoses, crossing my fingers as I went back inside. At first I wasn't sure I had accomplished anything, then about two minutes into the drive, I had HEAT like I never had before.

 

[smythcounty]

To confirm - are you referring to where the two hoses connect to the firewall? If one is hot and the other is not, that sounds like a blocked heater core.

Yes the two hoses going through the firewall. The valve was stuck this morning and wouldn’t move when I turned the heater switch off and on so I manually pushed it in. But it still blows cold air.

 

[smythcounty]

I will add my vote to a partially clogged heater core.

It will depend on your hose set up, but if possible, swap them. I kow it sounds so simple, but it does work. I had an old 65 Econoline with the dog house. The heater had been having problems, and on a REALLY cold morning, I had to grab my little propane torch and run it back and forth across the inside of the window to be able to see to drive the five miles to work.

Went out at noon on my lunch break and the motor was cool, loosened the clamps and swaped the hoses, crossing my fingers as I went back inside. At first I wasn't sure I had accomplished anything, then about two minutes into the drive, I had HEAT like I never had before.

Okay. I’ll give that a try. Of course I wait until it’s 10 degrees to work on it.

 

[rusty ol ranger]

Yes the two hoses going through the firewall. The valve was stuck this morning and wouldn’t move when I turned the heater switch off and on so I manually pushed it in. But it still blows cold air.

Because the core is clogging/clogged.

 

[fastpakr]

You should be able to connect a garden hose to one side of the heater core at a time and flush water through it until it comes out easily in both directions.

That said, at ten degrees I'd want to be careful about making a nasty, icy mess.

 

[James Morse]

Heater control valve doesn't control the temperature, it's either on or off. Ron D said:
"It has Vacuum in OFF and MAX AC settings
So by-pass valve has vacuum and is closed in OFF and MAX AC"
The temp is controlled by the blend door. I believe the blend door has a motor to control its position because you can hear it move if the engine is off and you change the temp selector hot/cold.

Mine has the heater control valve in-line to only ONE hose (which would be your in-hose to the core, obviously).

Here's what I'd do: 1. check that valve is always open if not on OFF or MAX AC. 2. make sure heater core is flowing freely by flushing/cleaning with hot vinegar (reverse hoses when re-connecting, core is non-directional) 3. make sure blend door is not somehow moving to the cold position (blend door closed, air not going thru heater core).

If all that stuff is working, seems like no way you couldn't have heat.

The in/out hoses on the core should have a difference in temp because the core is sucking heat out of it, but the out hose shouldn't be "cold", just less hot.

If your coolant thermostat is staying open, then the coolant never will warm up enough, but you'd know that because your temp gauge wouldn't come up to basically the mid-range where it should be. When I got mine it had -no- thermostat in it, I put one in (190-195 is what you want), but I still had to flush the core to get decent heat. Get the gasket for the t-stat too if you do that.

Heater control valve should be marked with arrows for direction of flow.

 

[PetroleumJunkie412]

Just use one of these. Will be fine.

 

[vigness]

Oh don't remind me, I had blend doors go out in two different 98's several years ago. It's a stand on your head for the after market fix, or tear the the whole dash apart for the factory fix. I went with the after market in both cases. It sits in the blower housing behind the glove box. The actuator motor sits on top near the left edge of the glove box opening. Pop the glove box all the way down and switch the temp hot to cold. You will ( should ) hear the motor move.

The quick and dirty fix it to drill a small hole in the front of the blower box, I used a 3/4" wood bit so I could see and shine my mini mag light in, and hook the edge of the blend door with a piece of mechanics wire and hold it open that way until you get some warmer temps or a warm place to work. I did that first just because you can't actually see the blend door to verify it is or is not moving, and it can be covered / closed with a good piece of duct tape. On the second one I drilled a 1/4" hole to pop just the bulb through and a 1/2" hole for me to peek through because all I had were twist drill bits handy. Either way works.

The after market is take a 'triangle' out of the bottom of the blower housing using a dremel or similar. I tried to do the first one without pulling the passenger seat and got quite the crick in my back, made it a hundred times better pulling the seat for the second one where I could lie on my back and work up on the bottom of the blower housing. Cut out the piece, break out what's left of the old door if it doesn't fall out. My replacement had a spring loaded bottom pin so I slipped it up and in place, then taped the triangle back in place to close things up.

First one was several hours, second was less than one. Paper instructions for the first, YouTube videos for the second.

 

[James Morse]

Yeah, so far I haven't had to deal with that, I know it's a pain. Actually if you have the engine off and listen real carefully you should be able to hear it without tearing anything apart - at least I could. Also of course test is, does it change the temp (if all else is good) or not.