Heater Core causing OVERheating?

Discussion in '1994 - 1995 Specific Tech' started by maximos, Jan 13, 2009.


  1. maximos

    maximos Member

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    What are the major signs of a bad heater core? I've suspected for quite some time that mine is going south because it started to smell like exhaust whenever I turned on the heat. Lately it's been exceptionally cold in Columbus, OH and my car has been running very hot almost to the point of overheating. The only way I can get the temperature to go back down is to turn the heat on and off until warm air comes out and takes heat out of the engine bay. Has anybody else experienced anything similar to what I've said?

    It's important to mention that I installed a Mishimoto radiator and a new OEM fan about three months ago. I can't imagine that the radiator or the fan has gone bad already. I have a small coolant leak on the driver's side in the front as well. I suspect that it's just a cracked hose though. I haven't been able to investigate it due to weather.

    If anyone has experienced this or knows a solution, please let me know.
    #1
  2. Zero_chance

    Zero_chance Founding Member

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    A bad heater core will cause some or all of the following conditions. Coolant leak onto the passenger floor. Fog of death coming out of the vents, especially noticeable when on defrost and it coats the windshield. A maple syrup kind of smell usually is quite noticeable.

    Heater cores can get plugged but theyll usually either spring a leak or give off little to no heat. Id suspect a problem with the fan or radiator, or possibly the thermostat. Any coolant leak anywhere is not a good sign. Mishimoto radiators are know to leak, regardless of how few miles are on them. The overheating being brought down by turning the heat on is because youre taking hot air away from the coolant and bringing it into the passenger compartment which makes the coolant circulating back through the engine a little cooler. A heater core is essentially a mini radiator inside of a heater box. It gives off radiant heat which then heats the passenger compartment. Id verify that your fan is actually turning on when the coolant gets up to temp, regardless of how old it is. Turning the AC on (I know its winter) should make the fan kick on automatically. Id check the fan, make sure the front of the radiator isnt blocked with snow or ice, see if there are any leaks and you may want to consider replacing the thermostat if its never been done. If its stuck closed, coolant isnt going to the radiator to be cooled and the only way it can drop heat is when you turn on the heat and move air across the heater core, into the passenger compartment.
    #2
  3. davis3

    davis3 Active Member

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    maybe there is some air trapped it the cooling system.
    i'm pretty sure its not the heater core,as Zero_chance points out.
    also check the electrical plug at the fan.be sure its plugged in all the way.try running the car til it reaches operating temp and watch to see if the fan kicks on.(be careful)

    as for the exhaust smell,could you have a small exhaust leak at the headers?
    the air for the heater is drawn from the cowl area,just under the windsheild.
    just a thought.
    #3
  4. 94-302-vert

    94-302-vert Active Member

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    +1...

    Also for the coolant leak, check around the fill for the radiator... that is where mine leaked before the damn thing actually cracked... It looked great in the box but didn't live up to expectation...
    #4
  5. maximos

    maximos Member

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    I've looked over the headers myself in addition to having it in at multiple shops...No exhaust leak of any kind coming from the headers. I've been trying to figure out the source for about a year now.

    As for the radiator and fan...Mishimoto recommended their fan when I purchased the radiator...BIG MISTAKE the fan wasn't nearly strong enough to keep the car running cool. I found this out after I had already cut off the factory wiring harness to wire it up. I found an OEM one at a local parts store and installed it. I had to re-attach the wiring harness and tape it up. I know I taped it VERY excessively to keep moisture out of there, but if moisture did manage to get in, do you think it would cause the fan to stop working? I remember noticing that the fan was not turning on at one point. I just attributed it to the abnormally cold weather.
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  6. maximos

    maximos Member

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    I just drove around for 30 minutes with the temp gauge needle consistently staying at the "L" in NORMAL and ZERO heat was coming from the vents despite being turned on high. I'm convinced that the heater core is playing a role in all of this somehow.
    #6
  7. desertcox05

    desertcox05 New Member

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    - another quick way to determine if the fans are operational is to try to pull codes. as soon as you turn the key the rad fan will turn on low then high for a couple seconds each. you'll be able to hear it from inside the car without the engine running.
    - i also don't think your heatercore is causing your overheating. to prove that disconnect both heater hoses fron inside the engine bay and connect them together with a piece of tubing. the engine knows no difference. now drive it and watch the temp.while the hoses are disconnected flush the heater core with the garden hose first one side then the other till it flows free and clear.
    - another thought is possibly a bad temp sending unit.
    #7
  8. maximos

    maximos Member

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    I thought about that, but my car actually overheated when I left it at idle for 20 minutes (keep in mind it was 30 degrees out at the time).
    #8
  9. davis3

    davis3 Active Member

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    i forgot all about the temp sensor.
    here's some info on how to test it.
    AutoZone.com | Vehicle Selection - Year

    i dont think the heater core is your problem.it MAY be a problem.it MAY be clogged,but the only way i think it could cause overheating is if it had a massive leak and you lost all your coolant.in that case,you would notice it in two seconds.
    just to be sure,check your carpet around the tranny tunnel on both the passanger side and drivers side.if you have a leak,the carpet will be soaked.

    right now the immediate problem is the overheating.bypass the heater core like desertcox05 described.and go from there.

    you said you got a new replacement fan from the parts store?
    did they give you the right one?
    cant put it past some of those guys to not know what they are doing.
    (just some guys,i know there are a lot of guys who know what they are doing and are very helpful.i'm not taking shots at ALL the guys who work at parts stores:))
    #9
  10. Zero_chance

    Zero_chance Founding Member

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    If its overheating at idle then the fan definitely isnt working or is very inadequate. Taped up connections are not good enough to keep moisture out. A little water can cause corrosion, corrosion in the circuits creates resistance and resistance sucks up the voltage that should be going to the fan. If youve been in there, Id recheck all your connections looking for any green corrosion. Hope you find something.
    #10
  11. desertcox05

    desertcox05 New Member

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    i'm also wondering if the parts store gave you one from a fox. it has a opposite rotation. did you compare the 2 prior to install?
    #11
  12. divit250r

    divit250r Member

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    It's worth noting that if your car is overheating and you're not getting any hot air from the heater, there's a great chance there is air trapped in the system. Hot air in the cooling system will not allow the thermostat to open, only hot coolant will. If the thermostat doesn't open, the car can overheat like it did for you. Just a thought. Good luck!

    Scott
    #12
  13. maximos

    maximos Member

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    If I'm not mistaken, Fox body fans are belt driven while SN95 fans are electric. This fan was made with an electric setup and fit perfectly into the car without any modifications. I'd also like to point out that up until the extremely cold weather everything was working fine. As soon as I can gather enough courage to battle the insane wind outside I will check the connections.
    #13
  14. davis3

    davis3 Active Member

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    when you pull the fan plug out,look at the fan connector.
    there should be 3 prongs there.
    #14
  15. maximos

    maximos Member

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    There are 3 plugs. I know it's the correct fan. It definitely isn't turning on. I just had my car sit at idle with the A/C at Max and it didn't kick on. I wonder if there are just a bunch of little things happening that are causing this larger problem. If there is air in the system, why would it just now become a problem after several months? I think I might take it somewhere to have the entire coolant system flushed, I'd imagine when they pressurize it everything will be taken care of (lines, heater core, etc.) I haven't checked my wiring job because the car is drivable right now and I don't want to get into the wiring without all of my equipment in a garage.

    It's a shame the SHO died...The cobra would never need heat that way.
    #15
  16. desertcox05

    desertcox05 New Member

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    i was talking about the water pump being backwards. i thought you replaced the wp too but after rereading the thread i see that was not the case. only the rad and fan. my bad.
    with the rad cap off can you see the coolant moving after the car is up to temp.? (note: take the cap off before you start). did you replace the rad hoses when the rad was replaced?
    #16
  17. Zero_chance

    Zero_chance Founding Member

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    Doesnt the CCRM control the fan? Those are known to go bad. A way to check the fan would be to disconnect and then jump power and ground to it and see if the fan turns on. If it does, then the fan is good and its likely the CCRM.
    #17
  18. Zero_chance

    Zero_chance Founding Member

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    On the fan connector, the black is the ground and the other two are low and high speed settings. You should be able to test the actual fan op that way. If its bad then you know what to replace. If it turns on hooked directly to power and ground, then the CCRM has gone south. It controls fan speeds as well as a few other PCM operations. Theres non serviceable relays inside of the CCRM, likely solid state though Ive never actually taken one part to confirm.
    #18
  19. maximos

    maximos Member

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    What exactly do I need to do to test the fan? I can wire and re-wire with the best of them, but the actual testing of individual wires is probably my weak point.
    #19
  20. turbogt

    turbogt Member

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    a speaker wire connected to the battery would work.. easy to test
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