93 with little/no heat from HVAC

[DanC]

I purchased this car late last year and only drove it a little while before storing it for the winter, but I noticed the heater performance was pretty poor. I never owned a car with auto temperature control and just attributed it to the control system.

Since the weather turned cool here in New England, it reminded me the heater doesn't work very well.

Symptoms are fair heat for a minute or so, then it tapers off to not much heat.

Under the hood the supply hose to the heater core is hot but the return coming out of the heater into the coolant tank is cool, indicating poor flow. There is an in line check valve in the supply that I'm suspicious of.

The engine coolant runs at about 195 degrees, so the thermostat and the rest of the cooling system appears normal.

It seems either the heater core is blocked or the check valve is not opening very much or is just stuck mostly closed.

Does anyone know the purpose of the check valve? I can't find any mention of it in the factory service manual. I'm thinking of removing it as a troubleshooting tool.

Any advice or experience with this kind of problem will be appreciated.

 

[DanC]

Did some troubleshooting today. Here's some additional information.

The device I thought was a check valve in the supply hose has a spring loaded orifice disk that will move in the direction of flow. I can assume the orifice is there to control the flow of coolant to the heater core but I don't understand the fact that it will move about 1/4" in the direction of flow.

Maybe in the center position coolant will be able to flow through and around it providing more flow at idle and slow engine speeds.

When the engine speed picks up the disk will seat itself against the housing and only allow coolant to flow through the orifice.

Just educated (?) guesses here. Anyone know the facts?

I installed a straight through fitting in its place and at normal road speeds the heater really works but it is still weak at idle.

Any Corvette HVAC gurus out there?

 

[Coupe89]

Does the air flow thought the correct vents when switched ?
Even thought I have 89 with they same type of unit. It might be the programmer.

Also do some searching on C68 (Auto HVAC)

 

[Ken Anderson]

It is most likely the HVAC module, an expensive part.

 

[DanC]

Thanks for the responses, but the entire HVAC system works perfectly (modes change, blower speeds up/down in response to temperature changes, etc.) except for the poor heater performance.

I'm certain it's a coolant flow problem through the heater core. The return hose is barely warm at idle with the system calling for heat. I have just never seen an orifice/check valve in the heater coolant circuit in any other vehicles. Control valves and isolation valves, yes.

If anyone can provide an explanation for this device, I'd appreciate it. I'd also like to know what to do about the low flow at low engine speeds. The engine doesn't come close to overheating at slow speeds or in traffic, so the coolant is flowing adequately through the engine and radiator, just not making it through the heater core in sufficient volume at low speeds.

Thanks again.

 

[Bob Chadwick]

I had the same problem on a Acura Legend. Turns out it was caused by an air bubble in the heater core that was not allowing hot water in the core in sufficient quantities to heat up.

 

[GeorgeT]

If you have a bad pressure cap on the radiator (overflow tank), you may not build enough pressure for the system to work properly.

 

[gearshift]

Heater buttons flashing numbers and buttons do not work.

My heater works on and off. The digital readouts will not stay constant and just flash numerous numbers. The buttons only work some of the time. If I hold down auto and another button, the heater comes on for a few minutes. The rear window defroster button will not work.

Either the face plate on the heater panel is shot, or the heater box needs to be replaced. Has anyone experience this?

 

[jmccloud]

Heater effectiveness

DanC,
It's common for crud to build up in the heater core. Flushing the core with water at modest pressure will clean it out. The core will only withstand 20psi or less so don't make a direct connection to city water pressure. Also, take great care removing the heater hoses from the core as the connectors will pull out of the core very easily. It's easier to use a spiral cut to get the hoses off then intall new hoses.
Flush forward, backward, forward, backward, etc. until the flow increased. It usually takes 5-20 cycles to clean one up. It's very obvious when the flow increases.
Corvettes and many other GM cars had antileak powder (pellets) added at the factory. It's a vegetable product and can accumulate in the heater core. You should add some back in if you drain and flush the whole system, especially if you have aluminum heads, to prevent gasket leaks. This is in spite of it's bother in the heater core. The pellets come 6 or 8 to a pack and are about $5 at the Chev parts counter. I forget the part number but there are several threads that include the p/n. Use the 'search' tab above.
Good Luck

 

[DanC]

jmccloud,

Thanks for the advice. It sounds like that may be my problem. I read in the service manual where Chevrolet states to use the pellets. I guess the heater core must have the smallest passages. Looks like time to flush.

I'm still curious about the orifice/check fitting on the inlet hose. Anyone have a clue what this is for?

Thanks,

DanC

 

[SKIP780]

check valve

that check valve is to limit the pressure when the engine is revved up.it is there to keep the heater core from bursting.