So I am stumped

So I run a 180 thermostat and out of the blue my temperature gauge runs low
I’ll have doing all these tests everything checked out
check out the wire for temperature gauge in the dash with ground shot up to hot like it should
Check fan clutch runs around 400 rpm
change the thermostat
change the sensor for the temperature gauge with oem Toyota part
I checked the temperature with a temperature gun The thermostat sits at 180 Like it should
so when I hit the coolant tube under The intake for the temperature gauge sender that sits on top it reads 140
why is that doing that
it’s a 1990 22re been running 180 for 20+ years The gauge has never been this way
would air in the coolant system causes
this ?

 

Did the drop in temperature happen all at once??

Did you change all the goodies trying to cure the problem?

If you had drained the coolant and refilled without bleeding the cooling system air pockets do many strange things. These engines are known for air pocket problems.

If the sending unit is not in the coolant it will not read correct. Any sealer or teflon tape will also cause reading to be off as the sensor is grounded into the manifold

Wires get old and tired after all these years as the resistance goes up the reading goes down.

 

Ya the drop came all at once it happen way passed I opened up the coolant system and I burped it I don’t think it’s the temperature gauge because the Gauge goes up to H when I ground the wire so that makes me think it’s working correctly it’s weird I get a 140 reading around the temperature sensor and a 180 around the thermostat
and I tested all of the ground wires in the Engine Bay all zero resistance and the ground wire in the driver side kick panel

 

Ohm test the sensor body to ground and check the resistance, should be at or near 0 ohms (200 ohm setting is best if not automatic).

If you disconnect the temp sensor, ohm meter between the terminal and the housing (or ground) and here's the specs at different temps just to validate the actual sensor is good.



Assuming the wiring from the sensor to gauge is good (could test the ohms directly there to see, but pulling the gauge cluster is a bit of work), I'd guess air in the system. If there was no coolant work done around the time the issue came up, was the coolant level low? If not, maybe head gasket leak? If low, coolant leak of some sort maybe, like hoses, water pump, etc. It's possible to blow a head gasket with out loosing cylinder compression, just not too common that I'm aware of and only is a coolant leak in that case. My dad had a camry that did that before. If you do a compression test on all 4 cylinders, that would check to see atleast if the cylinders are sealed and not pumping air into the cooling system via blown head gasket. Could also smell the exhaust, coolant burning has a bit of a sweet smell, I've grown up knowing the different smells though (oil, coolant, rich, etc), so not sure if it's too easy to translate to other people though xD.

That's everything I can think of off the top of my head, hopefully just a lower radiator hose leaking or something simple like that. Also note, the coolant overflow is only one place to check the fluid level, with the engine cold, check the radiator too, it should be real close to the cap level. It's possible to open warm/hot, but there's risk of burns etc and the system is under pressure (squeeze the upper hose to check how much pressure it has).

Good luck with the diag work

 

Ive seen them go up to abt 1/3 of the gauge display then max out the resistance and stay there when the wiring or sender is bad.