Topic: Heater core heating up without heater turned on

[jfrabat]

OK, so now, my heater core seems to be heating up even when the heater is turned off.  This would not be a big issue if I did not live in Florida, where it is still in the 80's, so as you can imagine, the cabin can get pretty hot if I dont use the AC (and even with the AC, I now have to overcome the heat of the core). 

I have not checked deeply into this (it started the day beofre yesterday), but I first felt some heat from under the dash, and then I turned on the defroster with the temp set to the coolest setting, and hot air is coming out (less than when I turn the heater on, but not by that much).

Any ideas on what could be causing this?

[Jeffy]

The passage for the heater core is vacuum actuated.  If you look above/behind the ash tray there is a white 3 port manifold with two green hoses going to it.  That controls the heater.  I forget if you want vacuum or not.  I know if you move the outer tube to one side it will stay closed and run normally.   If you put it on the other side it will stay open and you'll feel the heat.  Also make sure the heat you're feeling isn't from the transmission tunnel.  That area gets pretty warm as well as the floor in front of the passenger.  You'll want to insulate the floor if that's the case.

[jfrabat]

The passage for the heater core is vacuum actuated.  If you look above/behind the ash tray there is a white 3 port manifold with two green hoses going to it.  That controls the heater.  I forget if you want vacuum or not.  I know if you move the outer tube to one side it will stay closed and run normally.   If you put it on the other side it will stay open and you'll feel the heat.  Also make sure the heat you're feeling isn't from the transmission tunnel.  That area gets pretty warm as well as the floor in front of the passenger.  You'll want to insulate the floor if that's the case.

Thanks, Jeffy; I'll check the actuator this weekend.  I touched the floorboard (pulled the carpet and touched next to the thifter), and it does not seem to be coming from the transmission tunnel, it seems to be coming from the heater outlet under the dash, so I am guessing it is the core.  Still, I'll take a closer look at it this weekend.

Felipe

[95yjman]

Would a vaccum problem also cause the heater to barely run on the low but work fine on high?  I mean say I'm sitting idle with the heater on low and its not really working but then when I get up to speed it starts blowing warm air.  I know that probbely doesn't make sense...

[Jeffy]

Would a vaccum problem also cause the heater to barely run on the low but work fine on high?  I mean say I'm sitting idle with the heater on low and its not really working but then when I get up to speed it starts blowing warm air.  I know that probbely doesn't make sense...

You mean the fan is still running at the same speed but when you drive slow, there is colder air?

[Got Wood?]

If you don't need to use your heat anytime soon, I think I would bypass the heatere core. This way there isn't any heat even getting into the cab at all. Unless, of course, that would cause the heater core to dry out.

[jfrabat]

I dont want to permanently shut off the heater because we do get a cold wave every once in a blue moon here, and I do run the heater then (that happens maybe 4 or 5 times a year).  Also, when I have the top off, it is nice to be able to turn the heater on at night when driving on the highway, as the wind makes it cold.  So I guess I'll just try to fix it by following Jeffy's advice.  If I cant fix it, I may just have to permanently shut it down, though.

[95yjman]

You mean the fan is still running at the same speed but when you drive slow, there is colder air?

No, I'll have the fan switch on low and when I'm sitting at idle I get no air coming from the vents.  Then say I'm cruising at say 40 or more I get warm air but barely.  I also can't get the controller to say in the off positon.  I don't know if thats part of the problem.  I'm told that is a rod problem under neath the dase though

[Jeffy]

No, I'll have the fan switch on low and when I'm sitting at idle I get no air coming from the vents.  Then say I'm cruising at say 40 or more I get warm air but barely.  I also can't get the controller to say in the off positon.  I don't know if thats part of the problem.  I'm told that is a rod problem under neath the dase though

So the fan isn't turning on or blowing?  Even with no heat the fan should be blowing air.  Your controller is probably screwy and needs replacing.  The controller has the manifold mentioned previously on it.

[jfrabat]

The passage for the heater core is vacuum actuated.  If you look above/behind the ash tray there is a white 3 port manifold with two green hoses going to it.  That controls the heater.  I forget if you want vacuum or not.  I know if you move the outer tube to one side it will stay closed and run normally.   If you put it on the other side it will stay open and you'll feel the heat.  Also make sure the heat you're feeling isn't from the transmission tunnel.  That area gets pretty warm as well as the floor in front of the passenger.  You'll want to insulate the floor if that's the case.

Jeffy,

Just to be sure, so if I am driving with the windows open and the vent open, there should be no reason for hot air to be coming out from under the dash (where the air comes out when the heat is turned on), right?  This I would fix by just checking the manifold?  I'll be getting in there this weekend, and I want to get that fix so I dont need to drive with the AC on all the time (hardtop windows are SMALL!).

Felipe

[Jeffy]

I don't believe so.  I get heat from the exhaust and trans tunnel though.

[jfrabat]

I don't believe so.  I get heat from the exhaust and trans tunnel though.

This one is coming DOWN from behind the dashboard, and it stops if I close the outside vent (which is why I end up with AC).  Before, with the vent open, only cool air would come through (most on the passanger side floor, but some under the dash as well).  Now, cool air comes from the passanger side floor vent, but hot air comes out under the dash (and it is 80+ degree here, which means it get VERY hot with this thing heating the cab!).  My Jeep has the original carpet, which helps insulate from the tranny tunnel and exhaust (you can feel the difference when I take the carpet out). 

Anyway, I will check the manifold and the vacuum lines...

[FourbangerYJ]

  My Jeep has the original carpet, which helps insulate from the tranny tunnel and exhaust (you can feel the difference when I take the carpet out). 

I took the carpet out and have rubber floor mats. The passanger floor gets real hot! My daughter dropped a crayon on the rubber mat. I did not notice it until later. It was totally melted. Just a pool of crayon and a empty crayon paper. If you don't wear shoes your feet would be toast. At least in my Jeep.

[jfrabat]

Just to let everyone know that the problem was a simple one to solve; it seems that me, being the smart and experienced mecahnic that I am, did not change the thermostat housing gasket when I changed the thermostat.  Of course, the gasket was totally ripped to shreds, and it was allowing the engine coolant to filter into the heater core even when the heater was turned off.  I took a test drive today to see if this was the problem (when I saw the gasket, I figured this could be the source of my issues), and it was not sending hot air under the dash, so problem solved (and it cost less that $2 to fix!).

[Jeffy]

Just to let everyone know that the problem was a simple one to solve; it seems that me, being the smart and experienced mecahnic that I am, did not change the thermostat housing gasket when I changed the thermostat.  Of course, the gasket was totally ripped to shreds, and it was allowing the engine coolant to filter into the heater core even when the heater was turned off.  I took a test drive today to see if this was the problem (when I saw the gasket, I figured this could be the source of my issues), and it was not sending hot air under the dash, so problem solved (and it cost less that $2 to fix!).

There's nothing in the thermostat housing control flow to the heater core so I suspect it still will happen.