Heat Not Always Cold

[Silver04XLT]

Hi guys! I have been searching through posts on here for almost 3 hours and cannot seem to find the answer that I am looking for. First it started that my heat was blowing out of only the defrost vents but I got that fixed by finding a wire that had come unplugged behind the dash. Now my problem is the heat only comes out hot when I am driving. It gets cold when I stop or when I let it idle for a few minutes. I am not sure if it is the heater core which was talked about in a few of the threads or if it could be air in the lines. I also noticed in one of the threads that is said something about hearing what sounds like rushing water which meant that there was air in the lines and I noticed that sound a few weeks ago but never thought anything of it til now when it is starting to get cold out. I have been on many different forums over the years so please don't say SEARCH because I have already done that and all the problems were with older models that are different from the 04 that I own. Thanks and I hope someone out there can help me.

 

[Silver04XLT]

Whoops I just realized that I put heat not always cold in the topic. The problem is the heat is not always hot.

 

[briwayjones]

How high does the engine temp gauge get to when driving/idle? It sounds like your thermostat might be stuck open. While driving it gets the coolant hot enough to produce heat but when you stop the coolant gets cooled back down and the heater than blows cold.

 

[Silver04XLT]

the engine temp gets up to just under the half way dash mark where it usually is. I was told to add more coolant but dont think thats the problem. If the thermostat is stuck open do I just need to replace it?

 

[XtremeRanger]

If it is a v6 its as simple as draining the coolant and taking off the 3-8mm bolts on top of the thermostat housing and taking a look at it.........do a flush while ur at it...

 

[briwayjones]

If the thermostat is stuck open do I just need to replace it?

Yup

 

[Silver04XLT]

Nope I have the V-8. Is it necessary to drain the coolant to replace the T-stat or can it just be done as is?

 

[briwayjones]

I've always done it as is without loosing much coolant.

 

[trucku]

Just tossing this in, maybe you have a blend door issue.
Here is a link I found on this forum for a fix.

 

[crstanton2]

Before you mess with the blend door issue, pop your hood. On the top right hand side of the engine just along side the valve cover is a water valve. It is vacuum actuated and can get stuck if not used for a while, such as over the summer when you never use heat. Try disconnecting the vacuum port and cycling it by hand to loosen it up. Then reattachthe vacuum harness. Now have someone swith between cold and heat on the control inside the vehicle while you watch from outside, with the engine running. If the valve moves you'll see the arm move on the vaccum motor. It only takes a moment to check this out so I'd check this first. When my blend door acted up, I had a noticable cycling noise inside the vehicle, first a loud clunk followed by a repeated thump when in heat, nothing in cool.

 

[briwayjones]

I had a noticable cycling noise inside the vehicle, first a loud clunk followed by a repeated thump when in heat, nothing in cool.

Ah, wait, what was wrong when this happened? This is the first time I've seen someone mention a similar problem to one I'm having.

On my 2000, both the heat and the air work fine, no problems. But while in the heat modes it will sometimes (getting worse) make a repeated thumping noise. Any idea what this is?

 

[easnoddy]

Did the new thermostat fix the issue?

I have a V6 Mounty and makes that running water sound, so that means there is air in WHICH hoses?

Today on my way to work i noticed the heat wasn't working. On my way home the engine started overheating. I'm guessing a new thermostat will fix that issue. The thermostat seems like an easy fix, its front-center in the engine compartment with nothing blocking it. I'm sure that will fix the overheating and possibly the no-heat issue.

What about the running water sound inside the dash though?

 

[Silver04XLT]

I ended up opening the overflow coolant tank only to find out that it was bone dry...I dont know why I never thought to check that. Once I added the coolant the heat kicked out almost instantly. Now my problem is that the coolant leaks out down where the line from the overflow tank goes into the radiator. It is not really leaking. It is more of a seap coming from the line. I just ordered a new hose and got new clamps. From the reading I have been doing that is a common failure on these trucks.

 

[easnoddy]

I noticed my overflow was VERY low also, and there was definitely nothing in the top hose leading to the thermostat. Will try adding some coolant and see if I can find where its leaking. I've had to replace 2 radiators in the past 35K miles on this thing. At least the current one should be under warranty...

 

[gsbarry]

Silver04XLT, did u replace the t-stat, and did this resolve the issue? I am experiencing something similar and would appreciate the diagnostic help.

 

[Silver04XLT]

No I did not change the thermostat. It ended up being a loose wire behind the dash and very low coolant.