'91 OCC No Heat suddenly

Well, the hits keep on coming. After chasing my oil leak, now suddenly the heat stopped working. Luckily it's the end of the season but that also kills my defogger and it gets pretty wet here in gloomy western Washington. The car sat for three days last week and now the system comes on and blows but just cool air no matter what temp and blower level. The AC hasn't worked for 4 years but I've always had heat/defrost. The car is a '91 OCC. Both heater hoses are hot past the HCV into and out of the core. I actually replaced the HCV three years ago because of a leak and have had zero problems until now. I've disconnected the battery and let the system reset that usually fixed it when this happened in the past but has not worked this time. I've actually tried four times. The water pump, fan clutch, and thermostat were replaced around the same time. (195 degree thermostat) She warms up normal, heats the gauge to 195 and drops to 140-150 after five to eight minutes of driving. I planned to dump the old coolant and flush the system next weekend anyway but this seems more controller related. It is the automatic climate control system. Any suggestions of what to check next?

Check for vacuum at the control valve. Check hose temperatures around the control valve by the passenger side valve cover when the engine is running.

I believe all air is blown across both cores, so if you are getting blown air that isn't hot, I'd guess the heater core is plugged if the pipes are hot past the control valve...
IF you been using any stop leak products, they are very good at plugging up heater cores....

I cut my heater hoses and flushed the heater core with the garden hose from both ends. put the hose back together with t's and hose clamps. Its been working ever since

When should there be vacuum at the valve? I have no vacuum at 90 but full vacuum at 60.
I'm going to flush the core today and keep my fingers crossed.

If the heat stopped suddenly, it is normally something like the heater hose bypass valve failing or the blend door that controls to it having issue, allowing the blend door to stay in the cold air position. The temp control could be the cause if it broke and if it is a cable or manual setup, but if it has a motor control on the problem could be electronic, the motor, or the linkage.

Plugged cores are commonly more slow and just get less heat over time.

Flushed the core, flowed okay but no change in temp. All the hoses are still hot, so I guess I'll dig into the dashboard when I get a chance.

Disassembly is relatively easy. Note routing of vacuum lines (that control blend door, etc.) They run outside the lower case.

Check for vacuum at the control valve above the right valve cover.

It has vacuum when off but not when the heat is on. When should it have vacuum?

Do you have a factory service manual? If you get one it will have the diagnostic technique to determine the correct procedure to determine what is wrong with the system.

The temperature control is via cable by moving a door between the AC evap core and the heater core. "For maximum cooling, the hot water bypass valvediverts coolant flow away from the heater so it won't add any radiated heatto the chilled air flow from the evaporator."

Walterf wrote:

The temperature control is via cable by moving a door between the AC evap core and the heater core.     "For maximum cooling, the hot water bypass valvediverts coolant flow away from the heater so it won't add any radiated heatto the chilled air flow from the evaporator."

He wants to know whether vacuum closes the valve or opens it.

Vacuum is applied to the bypass valve when the temperature control lever is in the full "COOL" position. If you want heat, there should be no vaccum to the bypass valve and it will allow hot coolant to flow into the heater core. "The hot water bypass valve is operated by vacuum from a two-port vacuum switch on the control assembly. The vacuum switch is operated by the temperature lever."

If you can get to the valve and hoses, you should be able feel for hot areas on the valve or hoses to tell if it is opening when the call is for heat.

All he hoses feel hot. The vacuum is present at 60 but not at 90. I pushed water through the core and it flowed but made no difference. I guess it must be an under dash component.

smparr wrote:

All he hoses feel hot. The vacuum is present at 60 but not at 90. I pushed water through the core and it flowed but made no difference. I guess it must be an under dash component.

If the ducting layout is the same as on our 96 RMW, the blend door actuator is right in the middle of vehicle and on the back (toward the seats) of the ductwork. You should be able to disconnect the cable and move the lever and feel if the door is still connected and moving. The ducts should change temp in different areas also. I don't know offhand how much of the ductwork has to be disassembled to get to the blend door if it does need repair.

the 91s had that funky plastic valve with a butterfly by the firewall. look it up for the 91 Buick Roadmaster for the part number. I had one fail on one of my 91s. was a 1 year only part for the cars with auto climate control.

I have a spare for sale.

I finally had a chance to try some fixes. I replaced the heater valve because it was the easiest to get to and voila, heat. I just replaced the leaking original one three years ago. This one was no where near as good but was GM genuine. Guess they don't make them like they used to.