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Intermittent heat

15K views 23 replies 7 participants last post by  05HD350 
#1 ·
I have an issue that I'm trying to get dialed in. I recently had some work done to my 2005 6.0, I upgraded my turbo and had an egr delete done on it. This was done in the summer so the issue I have hasn't come up until now. I drove my truck the other day when it was close to zero outside and I had no heat, I drove about 15 miles and stopped at a store after the initial trip I went a few more miles down the road and the heat was working. Then after driving again I had no heat. For some reason the heat seems to come and go whenever I cycle the ignition, I know all about the light switch problem on the climate control panel, it's happened to me several times during my ownership, this heat thing is new to me and I haven't seen anyone else post about it....help!
 
#2 ·
Check your coolant level. When they are low the heat can be spotty or outright not work because its high in the system.
 
#4 ·
That's possible, but the system will burp/self bleed typically. So unless you just added a bunch of coolant I wouldn't think that's your issue. Typically the system will purge the air after one or two good drives.

Check your heater control valve for operation. Its on top of the engine on the RH side back by the firewall. It was a gray vacuum hose going to it. You could pull and plug the hose and pull vacuum on the valve with a vacuum pump and see if you have heat. If you get heat the valve is good, if not the valve is bad and you've found your problem. If the valve is good and you get heat it means there's an issue somewhere else in the system because it would indicate it is not directing vacuum to the control valve when the heat is on like it should.
 
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#10 ·
I was thinking this was a valve to merely shut off coolant flow to the heater core when the A/C was on Max?
 
#5 ·
Thanks for the help so far! The temp was in the upper 30's today so I thought I'd do some investigating, the heat barely trickles out when I have my setting on 90. Not the air, the blower is working fine, I was also told to manually move the heater control by hand and tie it in place, still no real heat. I've been told to check my potentiometer whatever the heck that is, I've been told that my truck may or may not have one.....does anyone know?
 
#6 ·
A potentiometer is an analog device that increases or decreases something (current, resistance, etc.) as you turn a dial. I can't say if your truck has one or not; I have the non electronic temp controls in mine so no point of reference.
 
#7 ·
TKO, even with the manual controls, the air direction dial is VAC and the fan speed is a switch but I believe the temperature control rotary dial is a pot.

Here's the backside of the manual controls.



Here's some general info on checking Ford's controls, I haven't investigated to confirm this is exactly what is in the manual control dash of our generation of truck:

http://www.factoryairtemp.com/Uploa... Tips/FA4S 356 FORD'S BLEND DOOR ACTUATOR.PDF

Regarding the climate control, I don't know anything about it so I don't know if it has a pot to check or not. My guess would be no, that it's all solid sate run off a board, but again, I don't know.

Also, a pot issue generally results in too cold or too hot, just not changing enough or at all. Not varying random temps.
 
#9 ·
That would make sense Phil. Old systems used cables for temperature control so you could adjust the degree to which the temperature door was open, thereby controlling the mix of cold and hot air. . If there's no cable I'd have to agree that it must be done with a pot, because I know of no easy way to use just some vacuum. The doors controlled by vacuum are either open or closed. Honestly, I've never had a climate control issue on the heating side in anything built after about 1990. AC systems sure, but the heat always seems to work so long as the core is good.
 
#11 ·
It shuts off flow to the heater core whenever the heat or defrost is not selected on the function dial.
 
#13 ·
Take it out an put in a straight piece of tubing, but it will be on max all the time. The best solution would be to put in a manual ball valve and that will give you some degree of control, even though you have to stop and raise the hood to control it. Don't forget to plug the vacuum hose.
 
#14 ·
If it is the heater control valve I have always had some heat felt on the hose up to the valve. It is conceivable that the lever to vac diaphragm and the shaft are no longer locked together for some reason and the heater control valve cycling leaves it off at times and works as it should others.

The valve is normal open, vacuum to close. When I have checked the valve is only supplied vacuum on ATC systems when the a/c is on and temp diff cab to set temp is a big delta. when the temp in cab gets to set and ambient is not over something about 100F then it shouts off the vacuum and allows coolant to flow into core in case it needs to moderate vent discharge temp by mixing some warm air with cold.
Does it do the same thing if you adjust the set temp to the highest number? If cabin temp sensor is reporting the wrong temp (bad sensor) then setting to highest number is an override and you will get heat anyway if it is that problem.
 
#15 ·
**Update on heat**

I changed out the heater control valve and still no heat to speak of, even if I have my heater control on 90 it barely warms up. I'm at a loss as to what I should look at next! If anyone has any ideas please let me know....my wife is looking at this as an excuse to sell my truck and go back to a gas rig. This is the first real hiccup that this truck has given me so I don't want to get rid of it.
 
#17 ·
Have you checked to see if the heater core is plugged?
 
#19 ·
The junk usually floating around a 6.0 cooling system. Even if you've flushed the system the heater core can still be contaminated because unless you go through the backflushing procedure it only gets flushed so well (assuming the heat was on when you did the flush, which I'll admit I've forgotten more than once).
 
#20 ·
The good news is should you have to replace it, they aren't stupid expensive (Just over 100 for OEM if I recall) and it's not that hard to do other than I imagine unlocking the 2 coolant lines at the firewall might be a challenge with the engine in the way. I changed mine while the engine was out just because of the miles on it and the fact that the connections were easy to get at with the engine gone. Regarding pulling the core on the inside...not bad at all.
 
#21 ·
A preemptive change on a heater core when its accessible is always a good idea. I did the one in my '89 Mustang GT project because I was already taking the dash out. Nothing wrong with it, but they are a royal PITA to get to with everything in place. That's good advice there Phil.
 
#22 ·
I had another solution on a Dodge Dakota since the dash had to come out on it to get at it....I left the core in the box and gave it to the new owner.... :)
 
#23 ·
I seem to remember buying an F150 years ago that had a fuel tank sending unit on the bench seat when I bought it. I noticed the fuel gauge never moved and realized the previous owner never got around to changing it :winking:
 
#24 ·
So the real question is what or was this issue resolved..?

I'm currently having this issue.. However, my heat is intermittent.. If i rev the motor let off rev again. Drive or stay still it comes and goes.. For the most part I have heat but randomly if idling for long periods of time or even short times at times... I won't have heat.. Coolant level is fine (although egr cooler and most likely head gaskets are bad, "coolant/pressure blows out cap but I monitor cooling level" not much highway driving and don't pull weight)

I'm wondering if its just the coolant boiling over so much from the HGs and EGR cooler that's preventing coolant to flow through the heater core at times or if its possible my water pump is starting to go.. Is it popssible to have an intermittent water pump failure, isn't the shaft connected directly to the impeller?
 
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