hissing coming from heater core

[xplorernewb]

so, as the title says, there is a hissing sound coming from the heater core. i have a stethescope that has helped me confirm this.

what could be causing it to hiss? i just burped the system a couple of times. did no good.

started on my way back from around 700 mile trip and has gotten worse. the temp gauge never went passed the N in NORMAL and now its gotten to the O but drops back down when im not moving, or if im driving at a steady speed.

im going to replace the thermostat first thing tomorrow. what are some other things i should check?

 

[xplorernewb]

btw this is a 91 4.0 sport model explorer with AC, 5 speed (if that matters). thanks

 

[Maniak]

There is a vacuum motor down there (near the heater core). My guess is either the hose got knocked off or the motor is now bad.

Basically, vacuum is applied from the heater control head and that vacuum is used to pull a lever that moves a door in the airbox (I believe it is the blend door).

I think you can get your head under there to look/feel without pulling the glove box but pulling the glove box is only 2 screws IIRC.

~Mark

 

[xplorernewb]

well the hissing sound went away today, apparently. i hit the gas pretty hard on the way to work this morning and made a bubbling sound and hasnt came back. but my temp gauge now fluctuates more than it ever has before..

 

[Maniak]

That sounds like air in the cooling system then which I've never heard described as hissing but that what it sounds like (symptom wise) now..

Air in the cooling system isn't good.. You need to burp the system to get the air out or you can get things too hot which is bad, especially since you have a 1991 which means you have the 90TM heads which is the set that is prone to crack (93TM is less prone)..

There are threads on here about burping the cooling system.. if I come up with one I'll post it..

Also, if you live in a hot climate you may want to looking into adding a heater control bypass valve to help the a/c work better. It does this by stopping hot water from getting into the heater core when the A/C is set to MAX AC.

~Mark

 

[Jason94sport]

A new heater core is $30. Just replace it. Pretty easy to do. It's likely cracked somewhere.
If you really want to isolate it. Just disconnect the 2 heater hoses going into it from the engine side & couple them together. This will bypass the heater core. Then burp the system & see what happens.

 

[xplorernewb]

ok thats what i kinda figured

so when i go to burp the system, im supposed to let it run without the cap off (cold) and squeeze the top hose, right? when i do that, water overflows the radiator where the cap goes....

 

[xplorernewb]

ok, so ive decided that the cap needed to be replaced, so i replaced it. NOW the coolant goes straight into the overflow when pressure is built... ugh im stumped

 

[smegun]

check the vacuum hose connected near the brain when accesing your brain there is a small vacuum line to a can just above to the right i had a hissing too untill i found that when i was in there i dis lodged it

 

[Arne13]

Start it up cold with radiator cap off. Let it run, are you getting air bubbles in the radiator? Going thru this with my Ranger. Didn't have the hissing sound, but bad head gasket, had the heads magnafluxed and one is good, the other bad. Good luck with yours, Arne.

 

[xplorernewb]

sorry i never got back to this, i ended up solving the problem and moved and had no way to continue on this forum. it turned out that the compressor was bad, causing it to kick on and off and caused drag, causing the temp to rise, making the thermostat open and close constantly. so i unplugged the ac (it is now deleted completely), and the hissing stopped. my temp fluctuating problem went away after i found a hose that wasnt tightened completely. tightened it and problem is gone.