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  1. #1

    Default 99 Suburban Won't Heat Up

    My Mom's 99 Suburban runs normally at 155* and when your hard on the throttle it will get to 170* max. This is a problem because winter is coming and the heater blows cool to very lukewarm air. I've replaced the thermostat twice with a dealership 195* and the heater control valve (controls the flow of hot water into the heater core, not sure of the technical name). The rear heat only blow cool air. Its got a 150k on it, 5.7 engine. I've flushed the heater core, seems to have great flow.... Any ideas?

  2. #2

    Default

    Have you replaced the heater controls, meaning that control the temp, fan setting and direction settings? If not try that, and if that doesnt work, there are a few relays that work in unison with the hand controls found in the cab. Get a hanes manual and figure out how to test all the relays that switch the temperature. Sorry to be so vague but hopefully this helps.
    J. Lord Chevy Truck Man

  3. #3

    Default

    I think we're on our 3rd heater control. But the heater control valve in the engine bay is vacuum powered and when give no vacuum it opens all the way (giving the heater core full flow). I've unhooked the vacuum and it doesn't change anything. I think the engine is not getting hot enough (155*) to heat the air.

  4. #4

    Default

    Using flat jaw pliers, etc clamp off the top radiator hose. Does the engine reach normal temp now? If it does the water must be getting by the thermostat... defective thermostat or coolant is getting by somehow. Benchtest your old thermostat. What temp does it open at in a pan of heated water?

    By the way... any loss of coolant or Check Engine light?
    Last edited by terryny; 09-29-2011 at 02:24 AM.

  5. #5

    Default

    I'll try this tonight. What temp should the engine run at? 190? 195?

  6. #6

    Default

    With the hose clamped off... temp will keep climbing just as if the thermostat will not open. Shut the engine off between 180-190... DON'T LET IT OVERHEAT. This test is to see if you get heat when the engine is a normal operating temps. Make sure there are no signs of a leaky head gasket, you don't want excessive pressure building in the system when you are working on it. In rare cases a hose could blow and expose you to extremely hot coolant... wear eye protection. Consider starting with a cool engine. Remove the radiator cap and start the engine. Any bubbling, gurgling or boil over? You could conduct this test at the same time as the clamped thermo hose... but remember not to let the engine temp rise above the thermostat rating... stop a little before you reach that point to offer a window of protection.

    Note for installing thermostats... insert an aspirin to hold the thermostat open. This will allow the air to bleed from the system while you top off the coolant. The aspirin will then dissolve and normal operation will ensue.

  7. #7

    Default

    Quote Originally Posted by 99ChevySuburban View Post
    I think we're on our 3rd heater control. But the heater control valve in the engine bay is vacuum powered and when give no vacuum it opens all the way (giving the heater core full flow). I've unhooked the vacuum and it doesn't change anything. I think the engine is not getting hot enough (155*) to heat the air.
    That valve has nothing to do with delivering heat. It is only used when you engage Max AC. When Max AC is on, vacuum is applied to the valve. The valve contains a diverter plate so hot coolant is directed right back to the engine rather than flowing through the heater core.

    Use an infrared thermometer to measure the temp of the gooseneck when engine is hot. I think you will find it's right about 195. The dash gauges are not very accurate.

    Next time you install a thermostat, test it on the kitchen stove in a pot of water. Use a candy/baking thermometer to compare.
    When you hear hoofbeats, look for horses not zebras.

  8. #8

    Default

    Do you have an update for us?

  9. #9

    Default

    The radiator had a very small leak. Replaced it. Replaced the fan clutch. Replaced the temperature sensor. Bench tested the thermostat, worked great, opened right at 195*. The coolant was very very dirty, flushed the system twice with Prestone flush kit, then ran the hose through it until it came out clean. Put it all back together and the truck runs right at 160* and the heater blows lukewarm air just like before.

    I'm going to try another thermostat. Maybe the one in there isn't closing all the way after install (seemed to work when i bench tested it). Its the only thing I can think of.

  10. #10

    Default

    If the cooling system was that nasty, time for a heater core or two. You know the thermostat works properly, you tested it. You can keep replacing it, not going to change anything.
    Last edited by 2COR517; 10-06-2011 at 05:35 PM.

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