My 2001 Chevy Malibu 3.1L Is Overheating

I have a 2001 Chevy Malibu. Bought it new. Very low maintenance car so far. Now my heat will only work if driven down the road or RPM’s raised. Replaced thermostat. No leaks any where. Heater core not clogged. Full of coolant. Does NOT overheat. What could it be?

What size engine? How have you determined the heater core is not restricted?

3.1 engine. Flushed core to see if was clogged. Both directions. Water was very clean. Also checked water pump today. In excellent shape. Flushed radiator today to see if is partially clogged. Didn’t seem to be so, but I’m no expert.

Well, would not suspect the radiator, or it should overheat. Must be a restriction somewhere, or possibly air pocket in the system. You may be aware that this engine is good for rear head gasket problems causing combustion gases to be forced into the cooling system. This would cause air pockets, may not overheat, but will see bubbles in the coolant jug when running. Check with your hands that one heater core hose is hot and the other warm with the fan on high. The temp should be different in a properly working heater core, since the fan on high would take heat out of the core, and the return hose would be cooler than the hose going in.

Well, one side is hot while the other side is warm on the hoses going to the heater core. This time when I started it up, it got warm as usual. I then drove it down the road. After parking back at home it started getting cool again. It is an odd thing to me for water to just not circulate into the heater core and make warm air. Can water still flow through heater core, but not go through coils? Don’t know what else to do. No bubbles in reserve tank.

What can happen is a heater core is restricted, so heat transfer is poor, but this does not seem to be the case with your car. My Chevy Malibu is in a very odd condition. It does act like air in the sytem. It is possible that the thermostat fixed it, but still have an air pocket in there. Really need to see exactly what the coolant temp. is doing to determine if this is an actual condition of coolant temperature goes too low. If not, then must be a circulation problem. Does not sound like a problem with a temp door in the dash.

When I flushed radiator, changed thermostat, and flushed heater core again, when refilling system it only held about 5 quarts. To me, it sounds like a lot more should fit in there. The only weird thing with that is one would think it should overheat. I keep opening the little screw on top to allow all air out of the system. It gets to where there are no bubbles coming out with the coolant. Where can you test the temp of the coolant?

I think there in is your problem- you keep loosening the bleeder and keep getting air out. Air pockets cause the problem you are having. Goes back to checking for signs of that rear cylinder head gasket as stated above.

What I meant was I loosen the bleeder while filling the system and after I start it to make sure there are no air pockets. After that, no more bubbles. I went ahead and replaced the water pump. It was leaking out the hole just a little. It has more pressure now, but too much for the flow through the system. Backed up to coolant jug. Took radiator out to inspect and it looks like that may be the problem. Not enough flow for the heater to work at all times. Not sure yet, but the thing will only hold a half gallon of water and I can see sludge in there that will not come out with anything I have tried. There is no coolant in oil and no oil in coolant. We shall see tomorrow after installing the new radiator.

Thanks to you MW for all your help. As you said in your 3rd post, it was a restriction problem in my 2001 Chevy causing it to overheat. The radiator just could not handle the flow needed to heat. It would flow just enough to keep the car cool. Even after soaking radiator overnight with cleaner, I could see gunk was not coming out. Bought new one and now all is fine. The new water pump and thermostat were needed, (they made it 240,000 miles) but were not the main problem. May replace heater core later. Probably has some of what radiator had. Hope this helps others. Thanks again.