Can computer water temp sending unit register hot air?

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Old 08-21-07, 04:54 PM
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Can computer water temp sending unit register hot air?

Now I need info.

Today my car overheated after I left big box store I practically live at. I had to quickly turn around and head back there after noticing temp gauge reading 3/4 to hot with check gauges light flickering. I knew I needed more coolant and presumed I was low. But when I popped the hood, with the car running, I noticed the fan was not running. I let the engine cool while I went in and got the anti-freeze. Then I did some testing. Ended up going to nearby junk yard and got fan relay switch. While yard attendant was getting the new switch, the engine cooled down and I opened the radiator cap and level was low so I added coolant. Then he came, installed the new used relay, idled the car for a while, and the fan worked.


Then I thought to myself, "Silly me. Telling people not to buy parts if you don't need it." I got thinking (too late of course) that MAYBE it was the low coolant level the whole while. That even though the thermostat area of the engine was most likely pretty hot, that probably no coolant was actually touching the temp sending unit inside. And MAYBE the sending unit needs coolant, rather than hot air to be touching it? But then I also reasoned that perhaps the car overheated FIRST because of the bad fan relay?, and sent coolant bubbling into my resevoir (which it did), and then because the level was low now in my radiator, that this is what caused the overheating.

So which is it?

I could take the new used relay out, if I have to and test again with my old relay. But does anyone else here plain know the answer to my question? I think it is good that everyone knows the answer, anyway, as running low on coolant is something many people encounter, and those sending units are at the high point in the engine, and thus probably all vehicles behave the same way, regarding whether or not coolant touching the sending unit is required, instead of just hot air touching the sending unit.

I also could destroy the relay and look inside it, but I don't want to do that in case it is good, and this would give me a spare.
 
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Old 08-21-07, 05:04 PM
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That's a good theory, BUT, why was the coolant low in the first place? A system with no leaks should not loose coolant.

I do believe the sensor has to be submerged, but without a borescope, it would be hard to determine if it was or not.

I hate shotgunning parts as you said, but sometimes you have to do it.

If the fan relay was the culprit, and the fan was not coming on, this would explain the loss of coolant; evaporation.
 
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Old 08-21-07, 06:23 PM
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About 3/4 qt./200 miles disappears. I have been recording distance traveled/per qt. loss now for quite a few months and the rate is holding steady. Obvious concern is blowing out some area in head gasket. Plugs or cylinders show no indication of coolant down inside. Not one drip of coolant ever spotted on driveway. Exhaust has no sweet smell. (I run 50/50 mix year round). Oddly there have been days where no loss from resevoir or radiator! (Figure THAT one out.) I even top off radiator to be sure as I can't believe how this can be. I have no thermostat in car since I changed out radiator months ago, and it actually runs a good temp and have no issues that way, nor am I suffering poorer gas mileage from running engine cooler. (Nor am I burning more oil.) It actually shows temp gauge in spot usually where another car (like mine or others) WITH thermostat would be!

WHY did I leave out the thermostat? I did that at time of radiator change out to be sure no air bubble was hung up inside engine, as I didn't want something like this confounding diagnosis. So now, when I top off the radiator, I know that there is no air bubble likely in the system with no thermostat in it. Soon I will have to put it back in though as our temps here will start dropping as fall approaches.
 
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Old 08-22-07, 05:37 AM
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Could it possibly be that if hot coolant level is low, it flows under the sending unit when circulating, and the air above has converted to steam, which like boiling water, remains at a lower temperature? Or, in a pressurized system would this not be the case (think pressure cooker)? Or is there some sort of inbetween, with this?
 
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Old 08-22-07, 11:11 AM
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Could be; yes the sender has to be submerged to register properly.

You can still get an air bubble with or without a thermostat installed.
 
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Old 08-22-07, 03:18 PM
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tow guy,

Is there a common spot an air bubble can occur, especially one that can affect the cooling of the engine, if not trapped behind a closed thermostat? Say the head gasket IS leaking, which I think mine is now, irregardless that I have no sweet smell from the exhaust (maybe the cat nullifys it?), and the engine is discharging exhaust gas air into the coolant system. I could see it maybe get trapped up the heater hoses, but that should not affect the cooling of the engine I wouldn't think. Today I had to replace another spark plug in cylinder #3 for the same reason I changed one out a few days ago: Totally clean insulator that is cracked. I guess coolant is hitting the hot insulator and cracking it. Bummer. I had head gasket replaced about 3 years ago. Seems like yesterday though.
 
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Old 08-23-07, 10:47 AM
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Yep, good news is you have nice clean spark plug(s) from the steam cleaning; bad news is it's time for head gasket(s) again. Need to have the head(s) professionally checked for cracks and to make sure the surface is not warpped, too.

Common place for an air bubble is in the heater core/plumbing, but I think that's a symptom and not the problem.
 
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Old 08-23-07, 03:12 PM
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You can determine a bad head gasket by doing a compression test as well. This test does usually find a leaking headgasket among other things.

You can rent the test equipment from Autozone.

By the way, what kind of car is this? Sounds like a typical 3.8.
 
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Old 08-23-07, 03:15 PM
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I suppose I should. I got the gauge.
 
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