Question about 1999 Chevrolet Silverado 1500

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Heater in my 99 gmc 2500 is not working

No Heat. Blows really good and has plenty of air but doesn't get hot air. I was thinking that the heater core is plugged. or controls on the dash are not working. I also have a check engine light that comes on at speeds over 70 mph. Code reader says that engine misfire. (intake gasket?) Could these be related

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They could be related but most likely not. There are two water lines which go back to the firewall from the radiator. On one of those lines is going to be a controller that is cable or vacum controlled. If it has a vacum line you need to check the vacum pressure at the connection using a vacum pump. You want to make sure that valve is opening and closing accordingly. If it isn't then your heater will not work because no water is getting to the core. In this case you will need to isolate the vacum leak back to the source of the problem. If there is a cable connections make sure the cable moves freely and in sync with the cab control. If not replace the cable. Assuming the radiator is full this valve is the next place to look for a fault.

Posted on Sep 23, 2009

  • barneycopper Jun 10, 2011

    I had the same problem on my 99 chevy with engine running and hot check to see if 1 heater hose is hot and 1 hose that is colder if so the core is plugged what I did was take off both lines and used a water hose put it over out line on heater core (loose fit) and had someone to turn on water and back flushed heater core it cleaned it and is now working perfect.

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Well i wood open the hood and see if heater hose at bulk head are hot both of them if they are then i would see if my blendoors are moving under dash at heater box when you move the controller if they are not moving could be controller or vacuum leak if you have vac-um leak from in take it could cause a miss or if you have low coolant and it leaking in at intake could couse miss any way hope this helps

Posted on Sep 23, 2009

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I've had this problem. 2000 GMC uses a closed coolant system. Meaning takes and returns coolant from and to the Reserve Bottle and has little air in the engine and heater systems.
If everything appears ok but no heat. "Air Lock" is the most likely cause. (With interior heater set to full heat) Solution; 1. Make sure water pump works. 2. Drain about 1/2 engine coolant into container. 3 Remove both heater hoses from engine side and flush heater core in both directs through heater hoses with garden hose. 4. Reattach heater hoses in correct locations. 4. Refill engine with coolant. 5 Start engine and run for 2 min., use caution when topping up coolant.
6. Most 2000 GMC have twist off caps with plugs located mid way on heater hoses. with engine cold, start engine these caps may be loosened to allow trapped air to escape. Do not attempt with engine hot due to hot coolant injuries. This may take more than one attempt to remove all air from heat system. 7. Lastly most 2000 GMC water pumps have a bleeder plug,( 3/8" square driver plug) located on the water pump. With egine cold this plug should be removed to remove any air. Reinstall plug with thread sealant.
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