After fixing my coolant cap issue, I'm now having issues with my heat in the truck. I can start it and let it warm up to about 150 degrees on my dashdaq then the heat will blow pretty warm air, but if I start driving the truck, it cools off almost immediately. Then the truck gets to operating temp and it starts blowing hot again, but if I start down the interstate it cools off again. This is a reoccuring theme and has been for the past week now. I was only down a 1/4in from the full line on my degas bottle, so it doesn't sound like a low on coolant issue, I just filled to proper level before going to work and it still has heater problems. The only two things I can think off are the thermostats need changed, or the blend door actuator needs replaced. Any ideas?

 

Before you do anything with your blend door see if you have a vacum leak (is the vacum pump running all the time or is your heater always defaulting to defrost. These symtoms might indicate a broken vacum line or seal leak in either the four wheel hub system or your heater controls. I found this out when I discovered my front hubs weren't sealing

 

Does the engine coolant temperature cool off too, or only the heater blow cold? If the engine itself is cooling off, thermostats.

 

The blend door in my truck is electronic, so no vacuum going to the dash from what I can see. I can switch between the different blower modes no problems at all, every single one works fine.

 

I am guessing your coolant cap issue had something to do with puking out the degas right?

Might be worth pressure checking the system. Heating problems are a telltale sign of head gasket issues. It could also be as simple as an air pocket or a vacuum leak. I would still hook a gauge up and go for a drive to test those head gaskets if you have had issues with puking in the past. Even if it "seems" like the degas cap fixed it.

 

The the weird part about it, the system barely holds any pressure at an idle. I held it 1500rpm for about 5 minutes, then let it settle back down and pulled the coolant cap, and there was literally no pressure in the system. I already had the system pressure tested and it checked out fine.

 

There should be some pressure after a drive. I want to say 3-5 psi after the truck is up to temp is normal. What we need to see is once its up to temp if you do a hard pull what does the pressure do. It should rise gradually but never get up anywhere near 16 psi. If it spikes (rises really fast) or hits 16 psi when unloaded then you have problems.


Once they get bad enough blown head gaskets will vapor lock the heater core and you just won't have heat... It sounds to me more like you have an issue with the heater control valve, that is the vacuum line we are talking about. If your vacuum system isn't sealed (your pump will run at all times) you can run into heater problems.

 

The system never went near 16 psi, was at about 12 with a really hard pull on my spartan 300 tune. And I do have heat once the truck is ran for about 45min to an hour of driving, it's just when i first start it I have heater issues. I checked my coolant level just now and it's right where I filled it last night, except an 1/8 of an in low since its pressurized now. I don't know if this info will help but I do have the original radiator and the single o ring hoses going to the block. Where's this heater control valve that you guys mentioned?

 

I am actually thinking of the blend door actuator... This sounds like its not able to fully actuate.

With the blower motor turned off switch the temp control **** slowly from hot to cold then cold to hot again (do it a few times) and listen around the glove box for the motor. You may want to remove the glove box.

FordTechMakeuloco has put up a very good video on testing and replacing it on youtube. Its a bit of work but doable.

 

Alright I'll try that this morning before I leave work, what kind of sound should I listen for while testing the actuator motor? Will it be noticable if it's not working correctly? I'm also gonna replace the tstats for higher temp ones and the coolant hoses with the mishimoto silicone hose kit since I don't have double o ring hoses.

 

Also a new thing I just noticed while leaving work tonight, it's like 9 ****ing degrees so it's pissing me off, but my ect temp jumped to 205 on the freeway then dropped down to 175-180. What the he'll does that mean?? I'm honestly starting to think my thermostats are bad, the blend door works fine, and the heater core still has heated coolant running through it No problem. I'm just gonna change my hoses to the mishimoto ones with the mishimoto high temp tstats and see what happens.

 

The 6.4 has a very large cooling system, like 8 gallons, so when it's really cold out it's hard to get to temp and when your not working it it quickly will loose temp, well mine does and once I start getting heat it continues to deliver heat even when the ect cools down a little. A diesel truck owners best friend is an aux heat system, a winter cover and heated seats.
Anyways even if your truck cools to 175 degrees you should have pretty generous heat. I don't recall seeing a heater valve in any of the heater core hoses, but if I get a chance to day I'll take a quick look to see if there is one.

It's 3 degrees this morning and the wife's little POS EQUINOX is going to the grocery store this morning, even if I plug in the truck for a couple of hours it's just not going to even defrost the windows by the time I go the two miles to the grocery store, another Diesel owners best friend, a cold weather substitute, LOL.

 

Once it's gets to temp and stops Its Bull**** I rarely lose the heat, although if I have heat and give her a good pull on my 300 tune, the heat cools off and takes a bit to return to normal temp.

 

I had a derp moment there, the 6.0 has a heater control valve. The 6.4 has a blend door system that is electronically controlled. Its a very common failure in the hvac system on these trucks.
My truck doesn't build heat until about 150* but after that the air temp holds steady. Any air will screw with things a lot though. Those single oring rad hoses could be letting air into the system and this can have a similar effect as blown head gaskets on the heater core. It will also cause massive amounts of cavitation.

 

If the O ring was letting air in it would also be letting antifreeze out, IDK, I think that's a bit of a stretch myself.