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Topic Review (Newest First)
03-24-2013 01:23 AM
amc49 Whynot, I've not pulled one of the cyl head sensors yet. Is it configured to where part of the sending end MUST physically touch the head material? Or does it just read the temp in the air pocket there? Any other cyl head temp units I've messed with had to touch or incorrect reading. Wondering if simple vibration might make one wear say a tip to not make contact and if one were to guarantee it touched on remount would it go back to working correctly?
03-24-2013 12:54 AM
whynotthinkwhynot I have seen this before. I'll bet you end up getting a new coolant temp sensor, or cylinder head temp sensor- whatever you want to call it. It's right between #2 and #3 spark plug. When it goes bad, for some reason, if floors the temp signal. There was a New Zealander who was on here who literally spent all his savings taking the car back and forth to the dealer. Each time it ended up with the same problem. He took our advice, self diagnosed it using the electronic odometer trick, double checked his temps with a cooking thermometer taped to the side of the upper radiator hose, and ended up replacing the sensor.

You can check it out for yourself. The electronic odometer trick should still work on a '11, but there might be multiple readouts- one for C and one for F. Earlier cars only have C. Here's what to do: Key off, press and hold the trip odometer reset button, turn key on, continue to hold until the odometer reads [test]. Release, and now each time you press and release the reset button, you will scroll through one of 30 test functions. Look for the one that reads [ 15 C] or whatever you think the temp of the engine should be at that time. You will remain in test mode until you turn the key off, so you can start the engine and drive around with this temp readout. From what I remember, his gauge would suddenly jump to 200 C, which is a ridiculous temperature for automotive engine coolant. Your hoses would melt in short order- and other nasty things happen. You can also double check this readout by using an infrared thermometer or cooking thermometer taped to the upper radiator hose. Use some good duct tape, and it will last for long enough for you to figure out what is going on. It won't last forever, of course. I'd go with the infrared thermometer, but I have lots of uses for those. Of course you can rely on good sense that a vehicle can't be running at 200C, so something must be messed up.

What I don't know is whether or not this car has the supplemental temp sensor on the thermostat housing. Some do, and some don't. Apparently yours doesn't, or the computer might at least be able (if it had code written for it) to double check the information it was getting.
03-24-2013 12:26 AM
amc49 Excuse my stupidity...............thermistor=thermal REsistor, not transistor........I should learn when to shut up...........

and, to modify my past idiotic statement, I may NOT have seen the gauge drop after it went up then lost water. In short, I lied. Getting quite messed up in that hard drive of a brain after 45+ years. Was thinking it out, the sensor will not read air temp but will after water has steamed up passages. They WILL actually read air temp, but the response change is so slow that you may as well accept they don't. Fluid conducts heat to the sensor so fast the result is quick, but air heats up slowly unless the temp dragged up really quick by super hot steam around it. In that case the air would read on sensor too since it is very hot also.

My bad.
03-23-2013 02:16 AM
amc49 All I can say is that somewhere that was not necessarily true. I personally have watched people torch race engines that had no water in them at all and the temp gauge never moved off the stop. I have also watched the temp peg until water totally gone from system and then the gauge drops with just air, but even the steam must be gone to do that. Now it may be that newer sensors may not necessarily do that, but somewhere they did, as we pretty much knew in the old days the sensor does not read if not coated in fluid. Mayhaps they were not thermistors, or 'thermal transistors'.

I messed up on part of that also because cylinder head temp gauges have no fluid around them at all, they read from the metal itself.

Hell, the more I think about it I'm confusing myself....................maybe I'm just behind the times.
03-23-2013 12:35 AM
mikebontoft Gauges lie. That's all I can say. Keep an eye on it though. You may end up having a faulty sensor.

What is this talk of sensors "not reading?" A thermistor is a thermistor, with the exception of having a positive or negative coefficient. GM has used the same sensor for both coolant temp and intake temp on MANY vehicles.
03-23-2013 12:20 AM
leemanfor Well see thats what I dont understand...the car was not overheating. Ive overheated a couple cars and steam blows out and usually the overflow tank is boiling.

nothing even seemed remotely out of place. The motor wasn't extremely hot, the overflow tank was full and at "Hot" on the meter and the radiator wasn't even hot. maybe it was just a sensor issue, cause the car runs fine since
03-22-2013 10:04 PM
amc49 Having trouble with some of this. Every water covered sensor I've ever seen for 50 years will not read if no water around the sensor. It shows low not high. Think it out, air does not conduct temp like fluid does. The sensor itself has no way of knowing whether it is covered with fluid and can only reflect temp around it, again, if air then no temp since air carries temperature much slower because not as dense as water. Also, cooling systems are under pressure, they cannot suck air except under very rare and specialized circumstances. I haven't seen a car EVER suck air and have worked on hundreds. Even a pinhole leak will spit fluid rather than suck air, only way to suck air is to be out of coolant anyway, once pump hits the airpocket it ceases to suck. meaning you have to lose much coolant before you can ever suck air. These systems are set up to quickly and fully bleed air into reservoir tanks all the time and keep it there only so you can see it, something is wrong if it is not doing that. I have two Focus and NEVER bleed the cooling systems, they are already done by the time the stat first opens.
03-22-2013 03:19 PM
Originally Posted by bile0026 View Post
Not quite. The way the sensor is designed it won't read air at all which I think makes it jump to H so that if there was no coolant in the radiator you would know. Liquid temp sensors only work in liquid, they can't read air temps.
ok. I think i need to see if there is a bubble in there like you said.
03-22-2013 03:18 PM
Originally Posted by FordCustomerService View Post
Hi leemanfor,

I recommend making an appointment at your dealership so they can take a look. They'll be your best bet for getting an accurate, prompt diagnosis. Keep us posted on how things progress; if you need my assistance, I'll be here!

Thanks Crystal
03-22-2013 02:52 PM
Originally Posted by leemanfor View Post
Hello all

I haven't really ever had any issues with my wifes 2011 Ford Focus! its a great car and much more comfortable than mine haha.

Anyway, we were driving last night on the freeway and all of the sudden the temp gauge on the car went to H and the temp light came on saying its overheating. I panic and pull over and shut the car off immediately. Mind you the temp outside was 30 degrees.

I get out, check the car. No coolant loss...i got under the car and there was no wet leaks anywhere...the overflow tank is completely full and not really hot. no boiling water. Basically no sign that the car was actually over heating.

I get back in, and start it up and the temp reads normal. Now my thought is possibly a bad thermostat but the car has 42k miles and this is the first time this has happened...
Hi leemanfor,

I recommend making an appointment at your dealership so they can take a look. They'll be your best bet for getting an accurate, prompt diagnosis. Keep us posted on how things progress; if you need my assistance, I'll be here!

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