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  1. #1

    Default Heater Core Replacement

    I tried searching the forums for an answer and couldn't really find one. Sorry if I'm re-posting an existing discussion - just point me in the right direction ;)

    I have a 99 silverado that takes FOREVER to pump hot air in the cab. The truck will get to normal operating temps in about 10 minutes of driving, but when I stop at a light the air will drop in temperature significantly. Also, it will take about 45 minutes of driving before I feel warm and cozy. Being it's the dead of winter and I'm in MO (around 15 degrees everyday) and I HATE being cold, I NEED my heat!

    I had to replace a quick connect (I think that's what it was called. It was a connector that hooked a hose into the firewall that carried coolant) not to long ago. I found it because my coolant light came on and it was dripping all over the place. I replaced it while it was still warm and I didn't use my heat, so I have no idea if it has had an effect on the heat (I bought the truck in July). I refilled the coolant and everything was fine.

    I haven't noticed any liquid inside my truck, or in the garage after I've parked it, but my engine compartment does smell funky after I've driven it.

    Now, I'm assuming it's a heater core, but I figured before I went out and bought one, and started to tear my dash apart I would get another opinion.

    Did I mention I HATE being cold?


  2. #2
    Jr. Apprentice
    Join Date
    Nov 2009
    Santaquin, Utah


    Have you tried just putting in a new thermostat first? Sounds like that is probably your problem. If the heater core is rusted out it will leak and if it is just plugged then you got a lot of junk in your cooling system. Try taking off the heater hoses from the engine and running water through them under pressure like your home hose. Try to back flush it and the junk should come out. I would try the thermostat first it is a lot cheaper and easier to do than the core.
    Last edited by jnbascom; 12-17-2009 at 08:38 AM.

  3. #3


    Thanks, I'll give that a shot. I was talking two some friends yesterday and they were thinking it was the thermostat as well. I also noticed my coolant level was quite low, so I added some last night. Could the coolant level affect the heater?

    Any idea where the thermostat is located?

  4. #4
    Jr. Mechanic
    Join Date
    Dec 2009
    Deep River Ontario


    I don't belive the theromstat is causing this problem as it does get to operating temperature, however it could be tested with boiling water,

    remove the T-stat, and you will notice that the 'valve' is closed, boil some water on the stove, and get it nice and hot at least 200 deg, and place the t-stat in the water, with a pair of tongs, and the valve should 'open' if it does then the t-stat is working correctly, if it doesn't move then it needs to be replaced

    and with the coolant level that will effect how your system functions,

  5. #5


    I'll give that a shot. If it's not the thermostat, then what is it?

  6. #6
    Sr. Apprentice brianj's Avatar
    Join Date
    Oct 2009
    New Hampshire


    If your coolant level is low, the first place that stops getting coolant flow is the heater core. Also, if you get air in the system, it will go to the highest point in the system- usually the heater core. I have seen water pumps that the pump vanes have corroded off of the impeller and they were not pushing enough coolant to circulate into the heater core, but that is not too likely. To me it sounds like you have no flow to the heater core at idle, and i suspect you are lower on coolant than you think- make sure the radiator and the overflow tank are full when cold. Start the truck up, and let it warm up. keep your hand on the heater hoses, and you should feel them get hot long before the truck reaches operating temp. If they do not, you have no flow. You may be air locked, need a water pump, or have a plugged heater core, especially if you used any of that radiator stop leak crap.
    Last edited by brianj; 12-17-2009 at 11:11 PM.
    1995 chevy 2500:fighting0040:
    1996 tahoe
    1946 plymouth
    1946 ford
    1931 model a
    1975 H.D. sportster
    1986 Monte Carlo SS
    Many rusted , half finished projects- want to buy one? Cheap?? Please?!!

  7. #7
    Master Mechanic
    Join Date
    Apr 2009
    Owens Cross Roads, Alabama


    id say swap out the,quick and easy..if that doesnt fix it, then at least you know you have a good me it does sound like the heater doesnt nessecarily have to rust out to be bad..could be clocked full of junk as previously stated..had a truck last winter with that same problem..heater core was full of junk and i couldnt flush it so i had to replace it.good luck!
    2004 Silverado 4x4
    1986 K5 Blazer
    8" Superlift w/39.5 super swampers
    1992 GMC Jimmy "Blue Bomber"
    4.3L Vortec

  8. #8


    I topped off the coolant (filled it all the way up) and that doesn't seem to make a noticeable difference in the heater warming up faster. I had checked the hoses on Wednesday (before I filled it with coolant) and they were flowing, I could feel the hot water moving through them, so I don't think I have a "flow" issue.

    If it was the heater core, wouldn't the truck always be cold? It does warm up, it just takes a lot longer than it should.

    Anyone have a picture of where the thermostat is on the truck? Honestly, I'm not really sure what I'm looking for. I'm going to try and get that swapped out tonight if I can find it.

    I am by no means mechanically inclined, so bare with me as I process and try to understand everything.

  9. #9
    Jr. Mechanic
    Join Date
    Dec 2009
    Deep River Ontario


    If you look at your rad there should be 2 large hose's and one smaller one, the line on the top will lead to your thermostat housing, and the lower one will lead to your water pump, the smaller of the 3 will lead to your overflow bottle, if you can find your rad cap, I would check to make sure the level in the rad is good as well, also check for any kind of gooey paste inside the rad, as that stuff can cause blockages in the coolant system

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