Heat but no Indication

Got a 1995 Toyota Tercel. As the temps have dropped, I have started to use the heat again.

The heat comes on, but not nearly as warm as I remember. And now the temp gauge barely nudges - but does seem to nudge - above stone cold.

I checked the coolant today while the engine was cold. The reservoir is empty but the radiator is filled to the brim.

This isn’t the first time this season it has been cold. My best recollection is that, until this week, the heater and the temp gauge had both been working fine.

So, I’m confused! Any good ideas? Good tests to try?

Baltimore, MD

Thermostat may be stuck open.

In addition to the most excellent post you should have a pressure test done to find the cause for loosing coolant, and my guess pressure loss due to bad hose or loose clamp etc.

Thanks for the suggestions. Question - pressure test - on the coolant/radiator?

And is there a difference between the red- and green- colored antifreezes? OK to mix?


Pressure test is done on the cooling system, then on the radiator cap. My preference is not to mix colors, dump it all and go with a universal coolant, peak global for example.

It isn’t unusual for a thermostat to corrode and become stuck during the summer, but not noticed by the driver until the winter. I expect that is the problem here, as mentioned above. If you think about it, given the environment they are in, and their delicate nature, it’s a wonder that all thermostats don’t quickly become stuck. The thermostat manufacturers must know some secrets, as the one on my early 90’s Corolla is original and still works as good as new.

The red coolant is the genuine Toyota coolant, I believe

The green coolant is most likely a universal coolant

Go with one or the other. Either would be fine

Does anyone know if it is OK to mix the two?

Don’t mix. Some coolants react badly when mixed, since there are lots of different types of coolants out there. If the coolant type in the system is unknown, best just to change it out. The Tercel should only use a bit more than one gallon of coolant, and a full strength coolant will make 2 gallons of 50/50 once mixed. More than enough for a coolant change. It is best done now, since you are going to change the thermostat, correct?

Stay with red Toyota fluid. You would have to do a flush to get all the old fluid out to switch. You can buy red fluid that is compatible with Toyota fluid.

Just wanted to thank you kind folks for the suggestions.

11/15 - it was a cold night - upper 30’s- and the thermostat went from dead stone cold to tepid:

and then:

11/16 - a much warmer evening - low 50’s - the Tercel blew warmer air - as warm as ever.

But I’m used to this car warming up & blowing warm air pretty quickly, even in snowbound temps in the MidAtlantic region.

So, I suppose it’s time to do a flush n fill, replacing the thermostat as well. And a pressure test, for good measure.

Not sure why those photos don’t show - but here they are in the order that they should’ve appeared above.

These photos, with comments, as I’ve posted to Flickr: http://www.flickr.com/photos/29460752@N03/sets/72157637785581855/


Thermostat is stuck open. I’m certain of that.


Have you considered that your heater core may be partially plugged?

@BustedKnuckles is dead on. Replace the thermostat. You also have to find your leak.

Don’t know if your Toyota is equipped with one but it could be your heater control valve…

OK - I have my new thermostat (in case you wondered, I often move slowly). I need some direction.

This is what I’m thinkin’:

Engine cool - then drain radiator from bottom hose. Reattach hose.

[Replace thermostat, in my case]

Fill with distilled water, run engine til warm, turn heater on full. Run for 5 minutes (more? less?)

Let engine cool - then drain again.

?? So how if it cools do you get the flush/coolant remaining that might be in the heater core? Or is that not a problem?

Fill with approved 50/50 mix antifreeze & distilled water.

Have a beer.

Please feel free to add/augment/correct these steps.

BTW, the heat continues to come on more when the car is idling than when running at highway speeds. I’m guessing a thermostat that is stuck open.

The lazy man’s solution was to slip a piece of cardboard over part of the face of the radiator. If the engine ran warmer and the heat was hotter, you can be sure it’s the thermostat. One winter I just used the cardboard system until the weather got warm enough to work on the car (no garage).

When you run the engine to hot as part of the flushing routine, turning your heater to max will run the rinse water through the heater core. When you add fresh coolant back into the system, I tend to add it at a 70/30 mix to add to the remaining water in the block and heater core.

Can’t wait to remove the old thermostat and do a boiling water test of the old & new.

Of course, that means figuring out how to get to the thing. The top hose goes to a piece on the engine with wires and other doo-dads going there. Any chance the thermostat is on the bottom hose end of things?

This radiator had a drain plug with a flexible hose connector pointing down. No hose was really needed, it drained nicely straight down.