Help with Infuriating Heater Issue (1 Viewer)

[amgraham]

Ok, since the temps have finally dipped here I need to sort out a problem that my 95 has always had with the heater (yes I searched).

The problem is engine is at operating temp and I slide the temp slider over to hot and the heat will work for about 10 seconds then go cool. I can slide it back to cold for a few seconds and after it starts blowing cold slide it back to hot and it will blow hot again for about 10 seconds then go cool. When it is hot it is as hot as it should be it just doesn't stay.

Things I have checked: Cooling system in good shape and bled (operating temp stays 186-195 depending on traffic, rear heat deleted, blue fan clutch with 15K fluid), pulled the radio and watched the blend door mechanism and it stays in place and doesn't drift back, same with the heater valve, swapped the hvac control panel and it it didn't make a difference.

I'm at a loss. Any ideas? It's getting frustrating sliding the temp control back and forth all the time just to get some heat in the truck. Thanks!

 

[76FJ40]

Sounds to me like your coolant flow through the heater core is restricted. The core fills with hot coolant, you turn the fan on and the coolant in the core cools faster than it can be replenished by hot coolant.

 

[amgraham]

Sounds to me like your coolant flow through the heater core is restricted. The core fills with hot coolant, you turn the fan on and the coolant in the core cools faster than it can be replenished by hot coolant.

Well I've got 170 degrees going in right now and 150 degrees coming out of the heater core (I fully flushed it less than a year ago too and got good flow through it). Temp at the vents, heat on full blast, is 78 degrees. If I slide the temp control back and forth I can get about 110 degrees at the vent before it starts to cool down again.

 

[inkpot]

I would pull and check the heater control valve and the adjacent hoses to make sure they are not blocked or collapsing. Good time to replace heater valve as the all seem to be dying from old age.

 

[amgraham]

I would pull and check the heater control valve and the adjacent hoses to make sure they are not blocked or collapsing. Good time to replace heater valve as the all seem to be dying from old age.

Already been replaced and temp is the same on both sides of the valve so it's flowing fine. I wish it was that simple. All the hoses are in good shape and not crimped or anything.

 

[BILT4ME]

Is your AC on at the same time?

AC on can be used to help dehumidify the air, but lowers your heat temp. Just fishin here.....

 

[amgraham]

Is your AC on at the same time?

AC on can be used to help dehumidify the air, but lowers your heat temp. Just fishin here.....

Nah, A/C is off. A/C works like a champ too. Kept it as cold as I wanted in the truck all summer. I get killer air flow through the vents which eliminates any possibility of blockages there. I checked the damper linkage in the passenger footwell too and it moves with the heat selector and stays fully over even when it starts to cool off.

 

[BILT4ME]

Do you have a scangauge or Ultragauge? Can you watch engine temps?

I was wondering if the thermostat is opening / closing causing this, but probably not because you said that it changes if you move the slider left/right and then it comes back.

That would indicate blend door to me, even though you've already checked it.

IDK. Sorry.

 

[amgraham]

Do you have a scangauge or Ultragauge? Can you watch engine temps?

I was wondering if the thermostat is opening / closing causing this, but probably not because you said that it changes if you move the slider left/right and then it comes back.

That would indicate blend door to me, even though you've already checked it.

IDK. Sorry.

Ultragauge. This is all at operating temp of 186*. Only thing I can figure is something is loose inside the blend mechanism unless I'm missing something. The thing that's weird is it's very consistent.. It does the same thing every time.

 

[morganism]

In the 60s, the heat exchanger gets super dirty from the crap falling in from the windshield vents. Can you drop your blower fan easily, and get a vac nozzle in there? Helps to unscrew the tip, and just use hose end...

Could also be the cable sheath holdown on the heater diversion valve. When you slide it, sheath could be sliding back away from it's holdown...

 

[trailer_trash]

I had the same problem with the HVAC.

After troubleshooting the HVAC control unit using the FSM, it turned out to be a stuck Blend Door Actuator.

To access and remove the actuator for testing, remove the trim piece, radio, HVAC control, and glove box door to reach the right most retaining screw of the actuator.

There are 3 screws holding securing the actuator in place.

It required a small ratchet-Phillips screw driver (Neko) to remove the screws.

Two fasteners can be reached from the radio side; the right most screw can be seen from the radio hole, but must be accessed through the
glove box area.

On that screw I used a hemostat to retrieve once it was loose.

When the actuator was removed, I used 2 jumper wires on the connector to the battery to cycle the motor.

Then apply Lithium grease on the Blend Door mechanism.

I also applied the lithium grease to both ends of the mix valve cable and manually cycled the mix valve.

Finally, re-install the actuator in reverse order, using the hemostat to hold the screw while positioning it into place.

The HVAC temp. control cycles the actuator and moves the mix valve and the blend door.

(I have an MP4 video of the actuator opening and closing the blend door, but the system would not upload the file)

 

[amgraham]

I had the same problem with the HVAC.

After troubleshooting the HVAC control unit using the FSM, it turned out to be a stuck Blend Door Actuator.

To access and remove the actuator for testing, remove the trim piece, radio, HVAC control, and glove box door to reach the right most retaining screw of the actuator.

There are 3 screws holding securing the actuator in place.

It required a small ratchet-Phillips screw driver (Neko) to remove the screws.

Two fasteners can be reached from the radio side; the right most screw can be seen from the radio hole, but must be accessed through the
glove box area.

On that screw I used a hemostat to retrieve once it was loose.

When the actuator was removed, I used 2 jumper wires on the connector to the battery to cycle the motor.

Then apply Lithium grease on the Blend Door mechanism.

I also applied the lithium grease to both ends of the mix valve cable and manually cycled the mix valve.

Finally, re-install the actuator in reverse order, using the hemostat to hold the screw while positioning it into place.

The HVAC temp. control cycles the actuator and moves the mix valve and the blend door.

(I have an MP4 video of the actuator opening and closing the blend door, but the system would not upload the file)

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Thanks man. The problem is all that stuff moves freely and as it should per the FSM. I'm really at a loss.

I changed the thermostat the other day for kicks and it didn't make a difference.

 

[BILT4ME]

Thanks man. The problem is all that stuff moves freely and as it should per the FSM. I'm really at a loss.

I changed the thermostat the other day for kicks and it didn't make a difference.

Is the arm sheared off inside the enclosure?

 

[amgraham]

Is the arm sheared off inside the enclosure?

I don't think so because when you slide it over to hot you get a burst of hot air for about 10-20 seconds as it gradually cools off. I've checked all the arms and actuators and they move freely and don't drift back or anything. Next I will probably check the water pump and make sure it is moving enough volume.. It's weird that the engine temp stays exactly where it should though.

 

[BILT4ME]

Have you back-flushed the heater core?

Take off both hoses at the firewall and flush both directions with a garden hose. It sounds like gunk is floating, then when you open the valve, it clogs, then works its way loose again......

 

[blkprj80]

I'd try removing the cable from the valve on the firewall outside. Tie it open with some soft wire and see what happens.

Is it possible you have an automatic climate controller and don't know it?

When you pulled your rear heater, you didn't inadvertently reverse the flow through the main heater core did you?

 

[amgraham]

Have you back-flushed the heater core?

Take off both hoses at the firewall and flush both directions with a garden hose. It sounds like gunk is floating, then when you open the valve, it clogs, then works its way loose again......

Yep, I flushed it a year ago and again a couple weeks ago and let CLR sit in it for 30 mins. Got no residue at all out of it and flowed freely.

I'd try removing the cable from the valve on the firewall outside. Tie it open with some soft wire and see what happens.

Is it possible you have an automatic climate controller and don't know it?

When you pulled your rear heater, you didn't inadvertently reverse the flow through the main heater core did you?

I am going to pop the cable off the heater valve and see if it has any effect holding the valve open. I don't think it will though because the valve opens fully with the cable and stays open and there are no restrictions inside.

I do not have automatic climate and the switch is on manual on both of the control units I have.

The rear heater bypass is routed just like every other one I've seen on here.

 

[BILT4ME]

Have you "burped" your cooling system?

Set the front wheels up on ramps or point the nose way uphill.

Remove the radiator cap and start the engine. Fill with coolant and let it run until it starts to overflow, then install the cap.

Make sure the overflow tank is at the full mark.

Drive it 20 miles.

Bring it back, place back uphill, and shut it off to let it cool down to ambient again.

Repeat multiple times until you don't have any air in the radiator and you don't have to refill anything.

It took a week of this for mine, as i drove it to work in the am, then shut it off overnight and recheck in the morning.

 

[leonard_nemoy]

This sounds like the issue on my 91, I have done everything besides digging into the dash. I think it is something behind the dash because I noticed if I slam the control from cold to hot with a lot of force it will keep blowing hot, but if I just lightly push it over to hot it starts blowing cold within the minute. I am too lazy to pull the dash apart....

 

[amgraham]

I FIXED IT!! And I feel a little dumb but that's ok.

First, I replaced the water pump since all signs led to a circulation issue. It does seem like the original pump but of course it looked perfect inside. Oh well, put the new Aisin pump on and fired it up. As I was letting it warm up I started screwing with the heat.

I noticed as it was warming up the inlet and outlet on the heater core were ice cold. I checked the function on the heater valve and it was opening and closing fully with the cable. However, I noticed the heater hoses got warm after I flipped it to cold a few times. I pulled the cable off the valve and manually moved it to "cold" and now I have heat. My effing valve is backwards somehow! Also it started dripping coolant from the pivot while I was messing with it haha.

So, I humbly apologize to everyone who said heater valve. From the outside it looked like it was doing what it should. Live and learn.