I have a 2000 chevy malibu LS - when I turn the heat on, it blows but there is little heat to the air plus I am noticing an oily substance and debris in my coolant even though I flushed it about 2 years ago. What could this be?
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Follow the heater hose from the engine, and as you get closer to the firewall, you should run into it. It is in line with the hose, and should have a vacuum hose connected to it. Apply vacuum to the valve and it should move. If not, then replace the valve. If there is no vacuum when engine running and heater turned on, then check the hose for leaks or the vacuum source.
Blow out the radiator core from the engine side with compressed air to remove bugs and other debris. Make sure the cooling fan comes on when the engine gets to operating temp. If the car car has AC, turn it on which should start the fan immediately. If no fan action, try to turn the fan blade(s) which should turn easily with no noises. Check the fan fuse and fan relay. If the fan(s) come on fine, check the coolant level in the radiator core--it should be right at the top. Check the seal on the cap and replace it if it is decayed or damaged. Make sure the reservoir bottle is filled to the line and the connecting hose is in good shape. It is possible that the thermostat is sticking and not opening fully. Also, check the intake and outlet hoses to and from the radiator for firm condition without decay, collapse, or other damage. A hose can sometimes peel away a portion of the hose interior which can cause blocking. Hope some of this helps!
Chevy made some weak intake gaskets on 3.1 and 3.4. My guess is that the intake gasket is weak and/or leaking and it is letting pressure out somewhere. At idle maybe the system pressure is to little to push hot coolant into the heater core.
If the check engine light is on, I would take the car to an autoparts store and have them scan the computer for fault codes. Most parts stores will do the scan for free. It sounds like it could be a faulty idle air control valve, but getting the codes should end speculation and identify the faults that the computer has noted. Hope this helped and best wishes.
You probably need to replace your dual climate control valve (aka Heater valve) This regulates how much hot coolant each heater core gets. When it malfunctions it either doesn't turn on or doesn't turn off. So either you get limited or no heat on one side (or both sides) or you get warm air when your ac is on, usually out of one side and not the other. Mine went out at 50-60K and I had hot air blowing on my feet in 90deg weather. If you are not under warrant I would recommend purchasing one yourself and taking to a trustworty mechanic. It only takes about 45 min to replace but the dealer will charge you several hundered for the unit plus several hours of labor. Rockauto sells them for about 170, I managed to find mine online a couple years ago for 115.
You also could have air in your coolant system. You can't just add coolant on these models after you drain the system or let it run low on coolant. The degas bottles on these models also have a flaw, where they crack along a seam. Sometimes coolant will leak, sometimes not, depending on how high the crack is on the bottle. But what it is guaranteed to do is allow air into your system, which can cause havoc with overheating (air surrounds the thermostat so it doesn't open) and keep coolant from flowing, particularly through the highest point in the system: the heater core. There is a heater core bleeder valve attached to a line near the top of the degas bottle (next to the expension line up top) with a plastic, flathead srewdriver face. Turn the heat on high and run the system at idle for 5 minutes. Then open the valve and keep it open until a steady stream of coolant comes out. (It's like bleeding your brakes.) That will clear any air from the system and at least give you peace of mind that this is not your problem. Then run the motor at 2000 rpms, with the heat still on high, for about 5 minutes or until hot air starts coming out. Release the bleeder valve again until a steady stream comes out. Then let the system cool and check your coolant level and fill as needed. I'd pull the degas bottle and inspect it...should have been a recall.