Weird temp reading

[Dr_Snooz]

So if you were driving along and looked down to see your temp gauge looking like this, what would you think is wrong?



I went through the cooling system about 6 months ago. I replaced the radiator with the biggest one I could find. I replaced the hoses, thermostat, heater hoses and heater core. It's always read dead on at 195 until last week it just up and did this.

[beastie_3]

is the radiator full?

a wire may be damaged or the sending unit isnt working.

[roger97338]

I second the idea that it may be the sending unit or wiring from the sending unit. The needle looks almost comically far to the right.

[Psycho71]

Could be a low water condition. I had a water pump going out on a truck a few years back and weeping out of the weep hole. It would spike like that when the sensor was reading the superheated steam instead of water, but when the stat opened it would dive back down when the wter hit the sensor. Scared the carp out of me when I looked down and saw the temp that high!

Barring that being the issue, I'd be looking at an electrical problem.

[Irish_Alley]

in order for the gauge to be pegged out like that your temp should be about 300 i think a sealed system boils at 260 so you wouldn’t have anything left nor a engine lol i would agree with the others check your wires and what not. is that a mechanical or electric gauge

[1979C20]

I like your tach. Does the temp gauge peg that high when you turn the key on and a cold engine? If ao, sending unit or wire.

[Dr_Snooz]

When it's cold it sits at 3 o'clock. When it's at temp, it points at 4. It's like it still works, but the scale has shifted by +200*. I checked the wire last night and it looks okay. I'll replace the sender and see what happens.

[zieg85]

If you find you need a dash gauge I have several extra I'd be happy to mail you out one.

[Psycho71]

in order for the gauge to be pegged out like that your temp should be about 300 i think a sealed system boils at 260 so you wouldn’t have anything left nor a engine lol i would agree with the others check your wires and what not. is that a mechanical or electric gauge

Not trying to be argumentative, your theory is correct when reading water. A pressurized system raises a liquid's boiling point.  But air pockets in a cooling system are steam, and can be superheated (heat added to a vapor after the change of state). A reading like that is not representative of the true engine temp, but the temp of said steam pocket only. Let the water level drop in a system to the point that the sensor reads an air/steam pocket, and readings like that are not impossible to see.

 I just wanted to throw that out there as a possibility, because I have seen it. When my truck did it, it wouldn't sit at a max reading for very long, but it scared me to death to look down and see my temp gauge pegged out for a few seconds, or more, until it got a shot of water across the sending unit. A stop at the very next station, and a top-off of water brought everything back to where it was supposed to be, and no permanent damage resulted. Of course, a water pump and t-stat change followed closely behind. 

All that said, I agree with everyone, including you, it's likely an electrical issue.

[Irish_Alley]

in order for the gauge to be pegged out like that your temp should be about 300 i think a sealed system boils at 260 so you wouldn’t have anything left nor a engine lol i would agree with the others check your wires and what not. is that a mechanical or electric gauge

Not trying to be argumentative, your theory is correct when reading water. A pressurized system raises a liquid's boiling point.  But air pockets in a cooling system are steam, and can be superheated (heat added to a vapor after the change of state). A reading like that is not representative of the true engine temp, but the temp of said steam pocket only. Let the water level drop in a system to the point that the sensor reads an air/steam pocket, and readings like that are not impossible to see.

 I just wanted to throw that out there as a possibility, because I have seen it. When my truck did it, it wouldn't sit at a max reading for very long, but it scared me to death to look down and see my temp gauge pegged out for a few seconds, or more, until it got a shot of water across the sending unit. A stop at the very next station, and a top-off of water brought everything back to where it was supposed to be, and no permanent damage resulted. Of course, a water pump and t-stat change followed closely behind. 

All that said, I agree with everyone, including you, it's likely an electrical issue.

now you are right if the level drops, thats why i said a sealed system. a sealed system wouldnt drop unless its not sealed

[roger97338]

in order for the gauge to be pegged out like that your temp should be about 300 i think a sealed system boils at 260 so you wouldn’t have anything left nor a engine lol i would agree with the others check your wires and what not. is that a mechanical or electric gauge

Not trying to be argumentative, your theory is correct when reading water. A pressurized system raises a liquid's boiling point.  But air pockets in a cooling system are steam, and can be superheated (heat added to a vapor after the change of state). A reading like that is not representative of the true engine temp, but the temp of said steam pocket only. Let the water level drop in a system to the point that the sensor reads an air/steam pocket, and readings like that are not impossible to see.

 I just wanted to throw that out there as a possibility, because I have seen it. When my truck did it, it wouldn't sit at a max reading for very long, but it scared me to death to look down and see my temp gauge pegged out for a few seconds, or more, until it got a shot of water across the sending unit. A stop at the very next station, and a top-off of water brought everything back to where it was supposed to be, and no permanent damage resulted. Of course, a water pump and t-stat change followed closely behind. 

All that said, I agree with everyone, including you, it's likely an electrical issue.

I'm glad you told us how steam effects the sending units. Personally, I didn't even consider steam in the cooling system interacting with the sending unit. Because of that, I was thinking that if the water level dropped below the sending unit, the temperature reading would end up a lot lower than normal, then bounce higher than normal when some extra-hot coolant finally DID make contact with it.

Now, just because I'm curious, would a mechanical gauge's sending unit and an electric gauge's sending unit act the same way in the presence of steam in the cooling system?

[Psycho71]

I would think it would not matter whether the sender were mechanical or electric. Temperature is temperature. The instance I experienced was with an electric sender, in my 98 chevy. I don't think I'll be testing either of my tucks with mechanical gauges though

[jaredts]

My experience with air pockets is erratic gage fluctuations, not pegged hot.  I believe the wire to the sender is grounded.  Either the sender is internally grounded because its gone bad, or the wire fell off and is touching metal, or the insulation burned off and the wire is shorted to ground.  Take the wire off the sender and verify that it goes to cold.

[Dr_Snooz]

I replaced the sender this weekend. It was definitely bad, with the electrode moving around in the sender body. Unfortunately, the problem is unchanged. The gauge sender screws into the center of the left head. In the center of the right head is another sender looking thing. I finally figured out from reading this board that it's the switch for the auxiliary fan. It was broken off and the green wire that connected to it was lying all burned up on the exhaust manifold. I replaced the switch and the wire and now the auxiliary fan works again! Yay! But God is it loud. I totally expect it to burn up in the next year. Anyway, when I can scrape together a few dollars, I'll hit the local Pick-A-Part for a new gauge. Till then, I hope I don't have any cooling system problems. I guess if I hear the auxiliary fan over the sound of the engine, traffic and a squadron of F-15s taking off right beside me (shouldn't be too hard), then I'll know I'm running hot.

[Irish_Alley]

this dont help you. but there is no difference in left and right heads they are the same thats why you have outlets on both sides just fyi. i tried to reread it but, does it still go at 3 o'clock with the key off?