91 LS400 heater problem

[jx295c]

Hey gang,
I've been attempting to get my hands around a heater issue. I even went through the laborious task of pulling out the heater core. The heater core heated uniformed when circulated with hot water and there was no restricted flow. The heater control valve opens properly, thermostat is new, and coolant is full. The heater core inlet hose gets fairly hot, but the heater core exit hose doesn’t get as hot. So it seems as if I have a clog somewhere downstream of the heater core. Has anyone encountered a similiar issue?

Not sure if it’s related but I did notice that only the top of the radiator and hose that exits from the top got hot after running the motor at idle over 15 minutes. The bottom of the radiator was pretty cool, including the hose from the thermostat to the bottom of the radiator. The only thing I can think of here is that the coolant isn’t getting hot enough for the thermostat to fully open (thermostat set point is 180 deg F).

Any insights would be appreciated…thx

 

[gloverman]

What temperature is the top and bottom hoses at? Normally top hose is around 80-90 c and the bottom 45-60 c. I like to see a 30 degree c drop down the radiator. The heater core will work in the same way - one hose will be cooler than the other. What is the heater doing wrong? Cheers

 

[jx295c]

thanks gloverman for your response. The cabin isn't warming up. It just blows cool air when the car is fully warmed up. I have a spare heater core, so I plan to hook it up externally in the engine compartment and see how effectively it's heating. If it is, perhaps the flow ducts might be screwed up in the cabin. If the heater core isn't heating with this test, there is inadaquate circulation through the heater core and I'll need to flow check the rest of the coolant system. Is there technique for doing this?

In review of the flow circuit, the coolant flows from the top to the bottom of the radiator so I guess it makes that there is a temp drop, just wasn't expecting it to be that drastic. I suspect the coolant temp has to be pretty hot to recirculate back into the engine block.

 

[gloverman]

Sound like your internal cabin vents aren't working. Try moving your heater controls and listen for the vents moving. You may find the hot / cold vent isn't working so the airflow never switches through the heater core. cheers

 

[jx295c]

yeah I think your right. I hooked up a spare heater core in the engine compartment and it got pretty hot. I'll check the fuses, but if it's not that, there's gotta be something mechanically broken back there or the control module on the left hand side of the heater unit is faulty. That was the only control device that would be suspect that I saw hooked up to the heater unit when I pulled it out of the dash. The one on the bottom, that works fine, just opens/closes the heater control valve when the temperature crosses 65 deg F. Any thoughts are welcome, thanks.

 

[jx295c]

FIXED...culprit: vapor trap

 

[tsirovy]

What is the vapor trap.. Where is the vapor trap.. How does the vapor trap work?