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87 Bonneville Heat Wimps Out

Car has the 3.8 liter v6. I have been having trouble with the heat on the car.

For a while it would only blow out of the windshield defrost vents. My mechanic found a couple bad vacuum hoses, and replaced them. He checked all the other vac hoses and they look fine. This heat now blows out of the heat vent.

The problem now is the heat come out nice and warm once the car warms up, but after a few minutes the temperature of the air coming out of the heat vent goes way down, and it feels like it is about 60 degrees. That is cvertainly a lot better than 20 degrees, which was the outside temp this morning, but far from toasty warm.

When the heat wimps out like this, I noticed that if I turn the heat and fan ciompletely off for about three minutes, and then turn it back on, the air comes out toasty warm again. But again it wimps out after maybe 3-5 minutes. To stay warm I just keep cycling the heat on and off every few minutes.

When the mechanic replaced the vac hoses, he also checked for coolant leaks and pressure tested the system and found no leaks. He asked me if the thermostat had ever been replaced and yes, I had done that a couple months ago.

Any ideas on why the heat won’t stay warm?

Two things, either the water isn’t hot enough when it goes into the heater core radiator or there is not enough volume of hot water going through the heater core. First make sure that there is enough antifreeze in the radiator. Since you already replaced the thermostat, you may have a blockage gunk build-up in the heater core. Put your hand on the two heater hose going to the heater core and see if they are both hot. If not, you may have blockage in the heater core and either need to have it flushed out or replaced. One other thing to check is the water shut off valve that shuts the hot water off when using AC. If the valve is partially closed, you will get insufficient water flow to the heater. For an 87 though, I’m betting on either low coolant level or a gunked up heater core.

You said the thermostat is OK, so I’m assuming the engine is warming up as it should. Low coolant could cause this. Is the cooling system full?

I think the problem may be a vacuum leak, either a new one or one your mechanic overlooked. The air temperature is controlled by the vacuum-operated “blend door” which directs air through the heater core or the AC evaporator, or both, depending on the desired temperature.

If the door is moving when it shouldn’t, due to a vacuum leak, it will not keep all incoming air going through the heater core.