90 Yota 3.0L V6, 5-speed:

Engine warms and registers in the middle of the temperature gauge, but the heater blows lukewarm air UNTIL I drive and the heat gets much warmer.

Is this likely to be a symptom of a bad "heater control valve"? Looks like the vent on the top of this part may sense the temp of the ambient air temperature (which would be warm/hot while the engine is warm, and idling when stationary).

Any ideas? Thanks!

 

Try removing the hoses from the heater core and then back flush the heater core, also check that the heater control valve is opening all the way, and see if that helps.

 

Did you work on anything that may have introduced air into the cooling system recently? An air bubble in the system can cause this symptom.

 

not to hi-jack, but i'm curious to see what you find. i have the same problem in an 86 22r pickup, i backflushed the heater core, and even bypassed the control valve. still luke warm heat turning warm much later down the road.

 

If you have back flushed the heater core and bypassed the heater control valve, and your sure there is no air in the system, then the only thing left is a plugged heater core.

 

I backflushed mine like 3 times and it did nothing so I just put in a new heater core and it solved it

 

If you have to replace the heater core its about a 4-5 hour job, ive done heater cores on two 86 4Runners, and its not hard to do, just very time consuming.

 

Antifreeze level was about 1" below filler neck. Added, and will report outcome soon.

Can anyone explain what controls the "heater control valve" is controlled (external temperature?).

 

This sounds like a faulty thermostat to me. my took forever to warm up, it showed the middle on the gauge. I changed the thermostat (12$ at napa) and now she heats right up and the gauge is still in the middle.

 

In my experience, back flushing a bad heater core rarely works... My 2 cents...

 

Be carefull of backflushing the heater core. If its in bad shape you can spring some leaks..

Mine is warm till ive been driving for an hour or so

I will eventually replace the core