What am I missing? Heater still cold at idle w/fan on full.

Holy sh...
Gotta love engineers, it's illegal to kill them.

 

You would create a whole lot of other problems if you put the tubes at what top. Sometimes they do things for a reason

 

The pictures always make it look so simple too, if only an entire dashboard wasn't in the way of that core, I would replace it just because. $50 part why not. Lol Already wasted the time and money on a water pump after all.

 

Go to youtube - search on -> back flush heater core.

 

Done and done kind sir, gonna do that this weekend. Throw some CLR in there and let her soak for bit, then rinse it clean. Burp the system and hopefully thats all it needed all along. Lol. I get what seems like full flow back flushing with just a garden hose and water, but that's no to say it isnt partially blocked. After all I have heat at high RPM's like the original poster does.

 

Now THAT is hilarious. You have no clue what you're writing about.

I've replaced 100's of heater cores and ALL had the pipes exiting on the top of the heater core.

 

I take it you have a 2010 Offroad with 4.0 L. Looking at mine where the two heater hoses connect to the core just before this connection one of the two has a union in it. I would remove the upper hose from the union and fill at the radiator until coolant exits hose and or union. It looks as this upper hose is just slightly higher than radiator cap creating a point where air could get trapped. If it is determined there is no air in system it would have to be the air blend door or the door controls. Not sure if it is vacuum operated or operated by a electric motor.

 

The one problem with this is that you'll build up a lot of moisture in your cab air and your windows will fog over. The only time you should really use recirculate, is when it's hot out and you want maximum efficiency from your A/C.

 

Don't think it is the air blend doors, at least with my issue. Only one of the two hoses coming out of the heater core gets real hot, the other just warm until I rev the engine up and hold at higher RPM's. I do have that same union on mine though, so after I soak the core and rinse it I will be sure to pull that hose when I refill everything to get the air out. Thanks for the tip!

 

This is what I think it is and what I do. I don’t think the core has enough heat capacity with really cold air coming in and the fan on full.

 

Thanks for taking one for the team on the water pump replacement. I figured that’s not it. So it was good to here it didn't make a difference in your case although I'm sorry for the time and money to do it. I did cut and splice in a bleeder valve at the top of the hose near the firewall which is the highest point of the system. Also I forced coolant in the core before reattaching the hoses after my pressure flush.

I appreciate all the comments because, as I stated in my OP, I just think this is normal. I really don't think the core has the capacity to throw full heat with an idling RPM coolant volume, fan blowing full, and very cold outside air. One other unique situation I have is the when I drive to work at O'dark:30 in the winter I come off a mountain. I really don't hit the gas much until the freeway 4 miles from my house. As soon as I climb the freeway onramp it blows warm air. It's the occasional traffic light I hit that I notice it.

 

Bleeder valve is clever, great idea.

In my situation, I had really good heat when I first bought the truck no matter the outside air temp (unless it was a wind chill of -15 type of day lol). Then this whole issue started around the time the stealership flushed my coolant system. Same as yours but in my case air temp would drop to like 70 degrees at idle, then up to 130+ when driving. I finally got it figured out today though after two years of dealing with this. I decided to use the ol CLR to clean the core trick. Now I have 128 degree air at idle with fan on high and recirc off, outside temp around 36 degrees today. Worth a shot if your temp varies at idle as much as mine did. Here's what I did if you wanna give it a shot:

Disconnected the two hoses under the hood going into the heat core, connected tubing on each, return side to an air compressor hose (valve to regulate air flow), supply side into an empty gallon jug to catch the coolant. Blew out the coolant from the core with air, removed the air line and replaced with a tap water line (hot), rinsed through twice. Blew out with air to remove the water, then filled the return side with CLR until it was coming out the supply side. Soaked for about 15 minutes, then blew out with air, and rinsed with hot water several times. Blew out with air once more, then filled with new coolant until both lines were full. Reconnected everything, raised the front of the truck up (parked on ramps on my inclined driveway) and burped the crap out of the rad (about an hour) to make sure every last bubble of air was out. The no spill funnel recommended before really comes in handy as it is made out of a near clear plastic so I was able to sit in the cab and rev the engine while watching for air at the same time. Nothing major came out of the core, maybe ten small chunks of something weird looking, but must have been enough to partially block the flow.

As it stands my problem is solved for now or at least much much less of a problem, we will find out on a really cold day in the snow. Lol. I must say it is really nice to be sitting at a red light and actually have consistent hot air come out the vents while idling. It's been so long! And if the issue arises again, might be time to just replace that core.

Thank you everyone for the advice and tips, and thank you knayrb for donating post space on yours. Lol.