low engine temps on extended downhill driving

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dlb
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low engine temps on extended downhill driving

Post by dlb » Sun Apr 19, 2015 4:59 pm

i first noticed this on the corolla back in the early winter of this year, then the car sat while i drove tercel #7 for several months. i just started driving the corolla again and have noticed the problem again. it's a little worse now.

the engine warms up fine and generally stays at normal temp (just below 1/2 on the gauge) but on any extended downhill driving, whether in gear or coasting in neutral, the gauge gradually drops to as low as 1/4. when this happens, the idle rpm also comes up a bit in the usual cold/fast idle manner, so i don't think it's just the gauge acting up -- i think the coolant actually is cooling down substantially so the ECU is acting as though the car is warming up again.

my first guess is a marginal thermostat. i have had the car for several years and not changed it so who knows how old it is, but i wanted to throw this out to the community and see what thoughts other folks have on it.

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rer233
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Re: low engine temps on extended downhill driving

Post by rer233 » Sun Apr 19, 2015 7:27 pm

Don't see how it be anything but the t-stat.
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Mark
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Re: low engine temps on extended downhill driving

Post by Mark » Sun Apr 19, 2015 7:31 pm

That was my problem until recently on my '81. The gauge stayed in the middle during in-town driving, but when the airflow across the radiator increased like when going down long hills or on flat highways, the temp dropped to about 1/4. I replaced the thermostat and now it's fine. I had a look at the old, suspect thermostat and it wasn't stuck open like I expected. I put it in a pot of water on the stove with a thermometer (that only went up to 60 degrees C) and it didn't open until the water was well over the max on the thermometer so it seemed to be working correctly. Maybe it was somehow installed incorrectly.

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Re: low engine temps on extended downhill driving

Post by irowiki » Sun Apr 19, 2015 8:27 pm

Buck does this too, temp is normal at all times (infact, nothing ever makes her overheat hardly) except when we're going down a hill and trying to run the heater in the middle of winter. Plan is to do the timing belt / thermostat / water pump all at once at some point this summer.
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Re: low engine temps on extended downhill driving

Post by Petros » Mon Apr 20, 2015 11:42 am

the engine is burning very little fuel when idling or running down hill, the extra airflow will chill the coolant down below normal operating temp. Likely an aging t-stat, it still works but becomes less responsive. eventually it will completely fail, best replace the t-stat when it is convenient. You will waste fuel when your cold run cycle come on, and could wash the oil film off the cylinders running rich, and cause premature wear to the rings.
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dlb
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Re: low engine temps on extended downhill driving

Post by dlb » Mon Apr 20, 2015 3:06 pm

i just picked up a thermostat and learned something regarding thermostat jiggle valves i want to share.

i looked at the new thermostat and noticed, as with most new t-stats, no jiggle valve. it's a very small store and the guy there is usually a pretty good resource so i asked him if he knew why some t-stats had jiggle valves and others didn't. he said that instead of jiggle valves, most new t-stats have a small cut out where the opening/closing surfaces mate. he said they started doing this instead of jiggles because it is cheaper. he had to look close but he found it on my new t-stat and showed it to me. i'll take a pic of it when i get home.

the significance of this is that i think i remember reading in the tercel FSM that the t-stat should be oriented with the jiggle valve up to allow air to move back and forth and keep the pressure equal. similarly, i would think that any t-stats with the cutout should be oriented with the cutout up. i mentioned this to the old guy and he agreed that it made sense. i don't know that it will make a hell of a lot of difference but it doesn't make the job any more difficult and can only do good so i think i'll try doing this from now on.

*edit* just found this on the stant website:

"- Many thermostats have a “jiggle pin” or “check valve” that allows trapped air in the cooling system to pass through the thermostat and be released from the system.
- If a Stant thermostat does not have a jiggle pin, it will have a "bleed notch” or other method of removing air from the system."

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dlb
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Re: low engine temps on extended downhill driving

Post by dlb » Mon Apr 20, 2015 7:36 pm

here it is. this is the closest i could get without it getting blurry. i circled the notch in pink. if you hold it up to light, you can see light through the notch.

i looked in my ae95 FSM and it says the same thing about the jiggle valve being position at the top. a few nice things about replacing the t-stat on the ae95 is it is much easier to access, and it uses a rubber gasket that snaps around the largest part of the t-stat. so no awkward angles with the t-stat falling out, smearing all my carefully beaded silicone gasket. very nice.
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Re: low engine temps on extended downhill driving

Post by irowiki » Mon Apr 20, 2015 8:33 pm

The Camry has a jiggle valve on the new OEM thermostat. Also had that nice rubber gasket!
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Re: low engine temps on extended downhill driving

Post by lannvouivre » Mon Apr 20, 2015 11:10 pm

Zach's Tercel warmed up to 1/4 operating temp (that's 1/8 on the gauge) once while sitting in traffic!

Yeah, it's getting a new T-stat.
But...did you try hitting it with a hammer?

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dlb
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Re: low engine temps on extended downhill driving

Post by dlb » Tue Apr 21, 2015 10:46 pm

replacing the thermostat did the trick. all is well again.

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Re: low engine temps on extended downhill driving

Post by Petros » Wed Apr 22, 2015 4:52 pm

I used gasket sealer to hold the new t-stat in place so I can get the 10mm nuts in place without risk of having it slip out of place. You will need a new gasket for the housing as well, I use sealer on that as well to help hold everything in place during assembly. It is hard to tell, but if it shifts before you get it tightened down, when you start the engine up, it will blow all that new antifreeze out the t-stat housing.
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dlb
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Re: low engine temps on extended downhill driving

Post by dlb » Wed Apr 22, 2015 5:06 pm

Petros wrote: It is hard to tell, but if it shifts before you get it tightened down, when you start the engine up, it will blow all that new antifreeze out the t-stat housing.
i replaced the t-stat on the dream terc just before i sold it and was paranoid about this after i got it in. but here's what i found was a good test to make sure it was seated properly by reaching into the housing with a pair of needle nose pliers, i could grip the t-stat and try to slide it side to side. there was a tiny amount of play, which meant it was not clamped between the housing and the block. i'm not sure if every t-stat will fit like that on other tercs but it's something you can try, anyway.

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Re: low engine temps on extended downhill driving

Post by xirdneh » Wed Apr 22, 2015 8:56 pm

i had a jiggle tstat once that jiggled a bit too far, tipping into the opening and wedging it open so tstat could not close all the way
Love those Tercell 4x4 wagons but they sure suffer from road noise.

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