So here's a new one...

[mjlcobra]

Long story short... My heat only works when the vehicle is stopped. It's very toasty. When cruising I only have "warm" air.

It's a '99 XLT 4.6 Windsor
I just did a coolant flush this weekend, distilled water ONLY.
New thermostat and reservioir cap.
Coolant level correct, air has been bled.
New water pump and heater core 10 months ago.
Yes both heater hoses are hot. Engine usually runs between 198-212 degrees, as high as 218-220 on the freeway.
Usually people have the opposite complaint don't they?

Any ideas?

 

[01yellerCobra]

Maybe something with the blend door? Or a seal that controls how much fresh air gets in?

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[mjlcobra]

Thanks for the reply. I can leave my temp settings untouched. When I stop at a light it will blow HOT. When I take off it loses it's heat. My blend door actuator was also replaced last year when I originally chased this issue. The door was verified to be in working order when I replaced the core. By the way, the water pump was the last thing replaced last year, which afforded my the luxury of heat for 6 months. Now I've just got this weird issue... I have no problems getting cool air out of the panel setting? Do you think the door is opening when the engine is under load and closing itself back up when it's not?

 

[mjlcobra]

The fact that it happens under load has me wondering if electrolysis has anything to do with it. I could not find any info to support this idea, but my mind is wandering....

 

[Stoned06]

Could be vaccuum related. Under load the vaccuum will increase and maybe there is a loose line that goes to the blend door or heater controls that is not allowing the correct flow of vaccuum to that control and it closes.

 

[bedrck46]

this is for the manual system if you have etac let me know

Item Part Number Description
1 18A318 Vacuum Control motor — panel/defrost door
2 18A478 Panel/Defrost door (full vacuum position)
3 — Defrost airflow
4 — Side window demister airflow
5 18B545 Temperature blend door (full heat position)
6 19860 A/C evaporator core
7 19A813 Air inlet duct door (full vacuum position)
8 — Outside air inlet
9 18A318 Vacuum control motor — air inlet duct door
10 — Recirculated air inlet
11 19805 Blower motor
12 — Vacuum to emission control
13 — Vacuum from the engine intake manifold
14 19A563 A/C vacuum check valve
15 19A566 A/C vacuum reservoir tank and bracket
16 19B888 A/C-heater function selector switch
17 18476 Heater core
18 18A559 Floor/panel door (full vacuum position)
19 — Floor airflow
20 18A318 Vacuum control motor — floor/panel door
21 — Panel vent airflow

Port Hose Color Function
1 White Air inlet duct door
2 Yellow Floor/panel door
3 Black Vacuum source
4 — Not used
5 Blue Floor/panel door
6 Red Panel/defrost door

VACUUM APPLICATION CHART—MANUAL A/C Switch Port Color Function Function Selector Switch Position
MAX A/C A/C PNL/ VENT OFF FLR/ PNL FLOOR FLR/ DEF DEF
1 White Recirc/ fresh V NV NV V NV NV NV NV
2 Yellow Floor/ panel NV NV NV V NV V NV NV
3 Black Vacuum source V V V V V V V V
5 Blue Full floor NV NV NV V V V V NV
6 Red Panel/ defrost V V V NV V NV NV NV

V = Vacuum

NV= No Vacuum

 

[mjlcobra]

Wow, thanks for the help Stoned and Bedrck. I will check for a vacuum leak.

 

[mjlcobra]

Ok.. So an update....
Could not find any vacuum leaks. On a hunch that air is trapped, I parked on my semi-steep driveway. I jacked up the driver front wheel and let it run for an hour and a half with reservoir cap off and heat on high to get very hot air from the vents. I lowered the truck, continued to let her run. After 10 minutes I confirmed that I still had great heat. Shut her down for about 7 hours. No heat. For what it's worth, the rear HEAT has never worked, but last year I had hot hot heat up front for the winter, despite the rear not getting hot. Could this issue be something bigger? Or would a Lisle funnel, overfilled change the "top" of my cooling system during bleeding to force the air out for good?

 

[mjlcobra]

Ok, final update...
My vacuum holds very well. Occasionally I'll leave my hvac set on something other than "off" for the 12+ hours that I'm at work. I'll notice and turn it to off before cranking the engine. I'll actually hear the vacuum release pressure (all before the key is in the ignition).

Now for the goods! I burped the system again on the driveway, cap open, driver side extra elevated. This time I made sure to put the CAP ON before lowering the truck and also took it out for a drive for about 20 mins before shutting it off. Let it cool and took it out. I still have heat. I suppose the lesson here is that I must've allowed air back in when lowering the truck. I'm one of the few people who is excited to have heat in the middle of May. Thanks to those that offered help.