Blowing cold air at idle?

[TDavidson]

When idling my 93 only blows cold air. When driving it heats better than any other vehicle I have ever had. Does not matter if it's highway or just around town. Anybody have this problem?

Thinking it's the thermostats but other threads on poor heating say the reverse. Heats well when idle, cold air on the highway.

Going to replace all heater hosing and thermostat this spring once my wife gives it up. She will not drive the minivan in the winter now that she's tried the LC.

 

[WarWagon]

Can't say I've experienced this with my '93. Heat AC is all good.

 

[Squash]

Check coolant level.
If low check for leaks...

 

[MrMoMo]

Water pump going south? Not enough flow at idle to push through the partially clogged heater core?

Heat from the cabin relies on the coolant circulation, so your problem likely lies in one of the following places....

Air pocket or low coolant level preventing flow through the core
Partially blocked heater core, reducing coolant flow, reducing heat output at low volume circulation
Water pump damaged or worn, doesn't have sufficient flow at idle to push coolant through core

That's where I would start looking...

 

[Danimal92sport]

Can't say I've experienced this with my '93. Heat AC is all good.

Yeah, my '97 is fine and so is my wife's 2011 minivan.

 

[TDavidson]

Spent some quality time with the FSM and on Mud looking at the water pump. Can't say I hear any noises from it or see any leaks. Coolant levels are good but in need of a flush and change.

Any quick way of testing the pump in place? May I'll just replace it when I do the hoses and forget it.

 

[TDavidson]

Found it!!! I think.

Tested the radiator cap and it failed at 8 psi. Started up engine and pressured up cooling system to 12 psi with hand pump. Hot air out of the vents!!!

Guess that at low pressured and rpm cooling pump was not able to pump through heater.

Off to buy a new radiator cap in the morning.

 

[biggmudder]

This is what my thermostat looked like when I changed it because I wasn't getting heat.

 

[TDavidson]

What does no. 3 water by-pass hose do?

Just found that the PO replaced the radiator and capped it off. I only found the tread below.

https://forum.ih8mud.com/80-series-tech/513599-what-purpose-3-water-bypass-hose.html

 

[TDavidson]

New radiator cap definitely improved heating at idle but not perfect. It still comes out colder when idling then when driving. 30 mph does it.

Next steps (March or April when snow melts)
-flush coolant
-replace all coolant hoses
-replace water pump (not sure it's bad but don't want to go back)
-replace thermostat

At the same time:
-replace belts
-oil pump o-ring
-crankshaft seal
-distributor o-ring
-Pcv valve and hoses
-cylinder head cover gasket (not head gasket)
-spark plug tube gaskets.

Anything else I should get while I'm in there? Thanks

 

[TDavidson]

For those with similar problems worrying about the HG failing I don't think so (famous last words). I did both a combustion gas test and monitored pressure. Passed both.

I'm going to try burping it tomorrow like other posts suggest. Maybe I'll find a check valve and put it on #3 return hose since it was decommissioned anyway.

 

[Elbert]

thermostat

 

[shootin cans]

Sounds to me like your heater core is plugging up. With your truck at operating temp, idling, feel the 2 coolant hoses going into the firewall. They should be close to the same temp, if one is significantly cooler then your heater core is plugged.