Weak heat in cabin

[JAWIT]

Having an odd problem with the 2008 4.0L Mountaineer that has been going on for a few months now. Vehicle has the Auto Temp control (EATC).

Situation is that even when the EATC is set in the 80-90 range, the heat coming out of any selected mode (Auto, floor, defrost, vents) is barely warm. Ive tried enabling/disabling the dual button but that has no effect. Even when letting the vehicle warm up for 15 minutes and taking it for a spin around the block for an extra 10 minutes, and really getting on the gas, the air is still barely warm.

The upper radiator hose is hot like it should be, but the lower hose is only slightly warm after a spirited drive. The same goes for the heater hoses through the firewall to the heater core....the one closest to the passenger side is hot while the one immediately to its right is pretty cold. I believe the heater hose that is noticeably colder routes to the water pump. The heater control valve was replaced with a new Motorcraft part 2 years ago and that seem to be working correctly as the hoses on each side are hot to the touch. Only the lower radiator hose and the heater hose coming from the water pump are cold.

The radiator fluid has been replaced twice with G05 fluid in the last 4 years and there are no leaks in the system. I also replaced the thermostat housing with a metal one about 3 years ago or so, as well as a new Motorcraft thermostat at that time. Would the no/weak heat situation be caused by a failing water pump? I've never had an overheat situation and the temp needle on the dash has always been right in the middle. The radiator fluid level is always full when i remove the radiator cap and there is fluid in the overflow reservoir.

Any help would be appreciated!

JAWIT

 

[JAWIT]

Bump

 

[J_C]

It is unlikely to have a water pump failure that doesn't cause overheating. Did the heat work last winter, after you had done the last repair to the cooling system? If so then I feel we may be able to eliminate needing to bleed any air out.

My first guess would be the blend door actuator (or blend door itself), or a clogged heater core.

If the blend door is blocking airflow to the heater core, I'd "guess" that should result in the core return hose being warmer than "cold", so I'd suspect clogged heater core more than blend door.

I'd take both hoses off the heater core and see if I could flush some water through it, how much resistance there is in doing that. If only a little (but at least some) gets through, then I might try either low pressure, compressed air, or flushing through a citric acid solution and plugging the heater core pipes to let the solution sit in the core for a while before water flushing that out, and remember citric acid is harmful to your eyes. Most acids are but some people try vinegar or CLR first because they have that around the house. Pick an organic acid as the mineral acids attack metal more.

 

[JAWIT]

It is unlikely to have a water pump failure that doesn't cause overheating. Did the heat work last winter, after you had done the last repair to the cooling system? If so then I feel we may be able to eliminate needing to bleed any air out.

My first guess would be the blend door actuator (or blend door itself), or a clogged heater core.

If the blend door is blocking airflow to the heater core, I'd "guess" that should result in the core return hose being warmer than "cold", so I'd suspect clogged heater core more than blend door.

I'd take both hoses off the heater core and see if I could flush some water through it, how much resistance there is in doing that.

Thanks J_C.

I think I recall the heat working at least better last winter than this one, but the coolant was replaced since last winter, which made me think perhaps air is trapped in the core somewhere. I'll have my mechanic take the heater hoses to and from the core off to see if there seems to be a blockage there or if it is trapped air. I've replaced both front blend door actuators since owning the vehicle (the drives side just a month ago), but I've heard no typical noises from either that would indicate them failing. I'll report back once they can take a look.

JAWIT

 

[J_C]

I edited and wrote a bit more in my prior post, and I am guessing your vehicle does not have a heater core valve on the hose to it, that needs open to flow coolant and isn't fully opening, since you stated the heater core intake hose was hot, but if the core doesn't seem clogged and you do have a heater core valve, I'd check that next.

 

[JAWIT]

I edited and wrote a bit more in my prior post, and I am guessing your vehicle does not have a heater core valve on the hose to it, that needs open to flow coolant and isn't fully opening, since you stated the heater core intake hose was hot, but if the core doesn't seem clogged and you do have a heater core valve, I'd check that next, just turn the engine and heat on and leave the heater hose off the valve and see if coolant has good flow rate (and makes a mess!).

Do you mean the heater control valve? If so, I mentioned that had been replaced. It's hot on both sides of the valve, so I am guessing that means it is open as it should be when heat is on? Seems to be working as far as I can tell.

 

[J_C]

Yes, sorry I overlooked that. Hot on both sides could still mean it's not fully flowing, just enough to heat the hose but with it having been replaced, seems less likely to have a problem only two years later, yet I'd still check it after the heater core itself, if the heater core doesn't seem clogged.

Note that I constantly edit my posts with afterthoughts, so if you read something right after I posted, odds are the post will change in a few minutes.

 

[JAWIT]

So just an update. The issue apparently ended up being the thermostat and/or the heater core. I had the thermostat replaced back in 2019/2020 with a new Motorcraft part, along with the coolant, and then the coolant was drained/refilled again in 2024, so I was surprised there was a potential blockage in the core. Perhaps the thermostat was just stuck open.

In any case, they replaced the thermostat and blew the heater core out this time. Heat in the car will run you out now!

 

[94Eddie]

Thanks for reporting back with the fix. It will help others in the future who have the same problem.