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Weird car thermostat question

Discussion in 'The Okie Corral' started by sviking, Mar 15, 2010.

  1. sviking

    sviking

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    '99 Honda CR-V EX. 120K miles. Stock radiator and thermostat.

    Car warms up and runs fine. But, whenever I turn on the heater, the coolant temp needle takes a dump all the way to the bottom. The car still runs fine, has good power and has been getting better gas mileage than ever. The heater air blows warm, but it seems slightly cooler than before, but maybe that's just my imagination. Obviously, waste heat is a byproduct of burning gasoline, so the engine MUST warm up. If I keep on driving, the needle usually SLOWLY rises up to near normal, but sometimes it doesn't.

    So, anyone know why the coolant needle would rapidly to drop to near bottom when turning the heater on with the car fully warmed up? It did this slightly before, but it's really been noticeable after the last coolant flush/fill service a few thousand miles ago. Zero other noted problems with the vehicle.
     
    Last edited: Mar 15, 2010
  2. Geeorge

    Geeorge Sarcasm Inc.

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    The heater core is a mini radiator and when you start the blower it works like the engine fan does and draws away heat
     

  3. Adams454

    Adams454

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    Sounds to me like the thermostat is sticking open. When you turn on the heat. You are sending hot coolant into the heater core. And sending the already cool coolant thats in the core back into the system. Then everything gradually warms up together.
     
  4. Eyescream

    Eyescream hates you

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    I think probably so.

    It's a handy thing to know, though, if ever the car's overheating, you can draw some heat off the engine by turning on the heat in the cabin so you cycle hot coolant through the heater core and blow it off into the cabin of the vehicle. I had an old Nissan that would get bubbles in the coolant lines and overheat something fierce, so I got good at the turn-the-heat-on trick.
     
  5. sviking

    sviking

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    I fully understand the thermodynamics of convective heat transfer and how it relates to the electic fan in the dash blowing outside (or recirc) air over the "mini radiator" and into the cabin after it is heated via convection.

    There is not enough hot coolant flowing through that "mini radiator", nor is there enough convective heat transfer to cause the coolant temp needle to dive to the bottom in mere seconds with a fully warmed up car. It's something else. Think about it...does your car do this when you turn the heater on? I've never had one do it...until now...

    I was thinking maybe the sending unit to the temp guage was bad because the car operates normally in every other aspect.
     
    Last edited: Mar 15, 2010
  6. jetmech

    jetmech

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    your low on coolant
     
  7. sviking

    sviking

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    Negative. Just had it flushed/filled and completed a 2500+ mile drive from the east coast to Arizona. Checked it before, during and after. Zero change from full status in the overflow reservior next to the radiator.
     
  8. 9mmXRAY

    9mmXRAY IDPA MW2 GSSF

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    Over cooling... I do it intentially on my Chevys because its better for them. Check these first...Bad thermostat or clogged... improper anti freeze mix...

    FYI if you remove the thermostat from most cars/trucks you will create an overcooling and the heater core might not work at all in ambient temps below 70 degrees.

    Oh forgot to add I live in a hot place so normally I run a 160deg thermostat in my Blazer its only about 20 deg cooler than the normal but if its like 60 deg outside my heater core will not work but man it sure puts out the horsepower!
     
    Last edited: Mar 15, 2010
  9. Geeorge

    Geeorge Sarcasm Inc.

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    Put a piece of black tape over the gauge and forget about it:whistling:
     
  10. Fixxer

    Fixxer Got ointment?

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    You're right- that is a weird car. Now to the question...

    Actually, it makes perfect sense that the thermostat is stuck open. The thermostat only comes into play after the heater core; ie it only controls coolant that was not cooled from the heater core.

    Assuming an open thermostat, before the car is fully warmed up, when you turn on the heat, you are cooling it (the coolant) with both the radiator (and not necessarily the electric fan) and the heater core (with assistance from the blower). Any air through the radiator (from driving) is going to cool it more than is necessary, and the needle will plummet.

    As a quick test, get a hose pliers (two parallel smooth jaws used to temporarily squeeze a hose) from Sears, and lightly clamp the upper radiator hose (don't pinch it completely shut- it's not good for the hose, and is unnecessary for the test anyway). Alternatively, you could cover both sides of the radiator with heavy cardboard, but that would be less effective as a test, unless you could block all air in and around the radiator.

    No, your car does not transcend the immutable laws of the universe- it is making heat, it is just being cooled in the wrong place at the wrong time.
     
  11. paynter2

    paynter2 It ain't over Millennium Member

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    This would be exaggerated if you are low on coolant to begin with. Check/re-check your coolant level - check it according to the instructions in your owner's manual.
     
  12. 220-9er

    220-9er

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    Another vote for a thermostat that's stuck open. That's why its there.
    The thermostat keeps the engine at it's ideal operating temperature in cold weather and during engine warn up by restricting the flow. By using the heater, you are adding cooling capacity by bypassing this restriction.
    You do need to make sure the heater valve was opened after the coolant was replaced. On some cars not doing this will allow a large air bubble in the system that will later result in low coolant level after the heater is opened. Check the coolant reservoir once the engine is warm to make sure its at the correct level.
     
    Last edited: Mar 15, 2010
  13. Adams454

    Adams454

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    If you already know better, why ask us the question? You should have just told us that you had a theory, and only wanted someone to agree with it. It would have been easier that way.
     
  14. hoffy

    hoffy

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    +1 on possible stuck thermostat, or even more likely an air bubble, if your coolant was just flushed/changed, some cars are harder to "burp" than others. Could also be a water pump, but I doubt it, I think that would manifest itself in other ways. I certainly hope that is not the problem. Good luck. Oh, feel the heater hoses too, one should be pretty warm.
     
  15. Glocks&Ducs

    Glocks&Ducs

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    Many vehicles have two temperature sensors. One for the computer and one for the gage. Seeing how your car is running fine, I would say you probably have a bad gage sensor.

    I had the exact same symptoms, when I performed a complete drain and flush on my Blazer when I replaced the lower intake gaskets. I think the different type of coolants probably messed with any residual build up on the sensor and caused it to go whacky for a while. Mine would intermittently drop to zero and later on it would just read erroneously. I was about to change it but it started acting normally after a few days.
     
  16. DR. HOUSE

    DR. HOUSE Everybody Lies

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    '
    The way he described his problem, it isnt a sensor. A bad sensor wont go from showing the wrong temp to showing the right temp after a while.

    It either works or it dont. like you said, it dropped to zero out of no where. GM was notorious for bad clusters and gauges for a few years. His acted up when he turned his heater on but it slowely came back up to tepm. temp sensor is clearly working. Also, the vehicle will still run "fine" with a bad thermostat.

    + 1 on the thermostat.

    When was the last time you had it replaced?

    Is that the one thats mounted horizontally?
     
    Last edited: Mar 15, 2010
  17. Glocks&Ducs

    Glocks&Ducs

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    Live well sensors can appear to fail intermittently due to contamination on the probe.
     
  18. DR. HOUSE

    DR. HOUSE Everybody Lies

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    live well? Wtf is that?

    His sensor did not fail, did you read his thread.

    It was warm untill he turned his heater on. Then his needle went to cool and im guessing it didnt just all of a sunned drop to cool, i would bet it declined.

    Also if it were a sensor, the needle wouldnt slowly come back up to temp. The needle would jump up to temp in a split second.

    First clue, air temp is warm not hot. Sensor wont cause this.
     
  19. sviking

    sviking

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    Thanks for all the help guys. With your help and some more research, I'm pretty sure now it's a stuck open thermostat causing an overcooling condition. It's a '99 with 120K+ miles...on the original OEM thermostat. Never been replaced, so I'm sure it's past due. Never had any problems with in until the last 5K miles or so... If that's the problem, which is sounds like it is, thankfully it's a relatively easy and inexpensive repair. Thanks again! The dealer wanted $170 for parts/labor. Seems a bit high, but I just moved and don't have all my tools, jack, jack stands, etc here yet and I have another long trip coming up this week so I need it done.
     
  20. Clem Eastwood

    Clem Eastwood

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    ive never seen one with a T stat stuck open, or w/o a t stat or with a restrictor warm up then take a dump in temp. they usually just take forever to warm up.

    i can see an air bubble causing an issue though.