Hi All,

My heater's been only mildly warm, and so with winter approaching, I replaced my heater core last weekend.

While I was in there, I confirmed that everything was working okay, cleaned out the leaves, etc. The blend door opened and closed completely and all the seals were nice and tight. Blower works well.

Also, this is an '81, so everything having to do with the heater and vent is cable operated. The only two selections are Hot to Cold (which moves the blend door), and Floor or Defrost (windshield). (No A/C.)
The coolant going through the hoses in the engine bay is piping hot.


Yet, after getting it all in working order, my heater is still only mildly warm. Just enough to take the chill off, but no where near hot enough to really heat up the cab. My other Bronco (an '84) will singe your hand if you put it right in front of where the heat's blowing out down near the floor, but this one is just "warm".

It's a pretty simple setup, but I can't imagine why it isn't nice and toasty after a new heater core. Any ideas?

 

what Tstat do you have in the 81?

 

I believe it's a 195°. I replaced it about a month or two ago.

 

Are BOTH hoses to the heater core equally hot? If not, its most likely the core. If the system is not completely full, you could have an air bubble in it. Typically the heater core is the highest point in the system so if there is any lack of coolant, any air in the system will bubble up into it. This will be noticeable as an intermittent cooling of the hoses to the core while the engine is running... and not much cooler at that.

The fix requires starting dead cold and running the engine with the radiator cap OFF up through the point where the thermostat opens. Top the system off and put the cap back on. This process needs to be MONITORED CONSTANTLY. Boil-over is possible especially if there is a large enough air bubble to push coolant above it. Its tempting to squeeze hoses to get air to bubble out... don't you will have a mess.

This is procedure is not typically necessary unless for some reason the coolant level has gotten so low that the overflow tank has gone dry or there is no syphoning of the tank occurring during operation.

 

I'll have to check for an air bubble, that'd make sense. However, when I installed the heater core, I removed the hoses and pointed them upwards (so they wouldn't drain), and then filled the new heater core with coolant with a funnel before reattaching hoses. The amount of air in the system should be minor, but that doesn't mean it is.

 

You say the blend door went full swing....did you check that with the cable attached properly? Same thing with the temp. cable.....a little out of adjustment and it can make a difference.

 

I did. While it was apart, I moved the door manually to be sure it had full swing and was sealing fully.
After I got it all put back together, I had my wife slide the heater controls inside the cab and I could hear the blend door "thumping" into place as it hit the extreme limits of its swing.
Then, after she moved the controls to "hot", I manually moved the lever in the engine bay to see if I could move it any further and I couldn't.




Checked for air bubbles last night and there didn't appear to be any.


Frustrating, it was 26° this morning and it took 10 minutes after the engine was warmed up for the defroster to melt the frost on the windshield.

 

Perhaps the blower fan itself is weak and needing replaced.

 

I hadn't thought of that, it doesn't put out a high volume of air, but it's never really felt "weak".

Who knows though, maybe it's not enough.

 

A.B,

Broncos and full-size pickups from 1980-1996 all use the same heater set-up. I have changed out a core on one in less than thirty-minutes. There should be no cable under the hood; just the two hoses with no valve. Of course, someone could have added a cable-operated valve.

Both heater necks are on the same end of the core, so there is a baffle between them to direct the coolant flow through the core. I have seen new cores with bad baffles. You should get a minimum of 125 degrees from the floor vents.

HTH

 

Yeah, the heater core change is very easy, especially compared to some I've done!

The cable I'm referring to is the one that comes out of the cab and operates the blend door inside the box in the engine bay. When I move the lever in the cab, the cable moves and either swings the blend door shut or pulls it open.


How would I even know if it had a bad baffle? When I pulled it out of the box, I ran a hose through it and it had great flow, but I suppose I have no idea how much of the core was filling up when I did it.

 

Here's a picture of the cable in the engine bay I'm talking about:

 

Are you sure you have good water flow from the engine. Maybe a hose is bad or corroded fittings cutting the water flow. Also, is there a specific in and out on these?

 

I had the engine out earlier this year, so most everything's been gone over. I could check them again, but there should be good flow. I don't think any component (radiator, hose, thermostat housing, heater core, or otherwise) that's over two years old. Worth a look though.

As for an in and out, I don't think so. If you swap the hoses, it's just going to flow through the heater core in the opposite direction.

 

If you have one of those infra red thermometers, do a check on the actual temps along the hoses and the heater core.....any place you can get a good reading at. That will let you know the coolant is flowing good and the temps are good. Compare that to the actual engine temps with the infra red.

If temps are all close, then it becomes an air flow issue or control issue. I have been known to operate a heater sitting on the fender with the hoses hooked up so I could work air flow issues.