Radiator temp on guage goes up about 3/4 of the way and then rapidly goes down to just above cold where it has normally been since I bought the vehicle. It does not get to the red portion of the guage. It climbs slowly to a certain point and then when it gets there, it goes to the lower point quite rapidly but not instantly. I have checked coolant level, changed thermostat and top and bottom radiator hoses. Also, have changed sending unit. Originally had 198 degree thermostat and I even changed that to a 180 degree which was a mistake because I don't have a lot of heat inside the car now. I don't suspect the wiring because the guages are solid and the sending unit connector is solid. My next move is a new fan clutch and fan. My fan hub is cracked and needs to be changed anyways. Any suggestions would be appreciated. Thank You in advance
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You are going to have to find where the leak is first before looking at the temp guage problem. I would pressure test the cooling system and look for leaks. check the heater core...make sure thats not leaking. make sure the engine oil and transmission fluids are not over full and milky. Good Luck
There may be two reasons for your problem. The thermostat may be out of specification and opens much later than it should. As your guage is telling you, the coolant gets extremly hot and finally the thermostat opens and allows cooler fluid to circulate and the temperature at the coolant sender goes down lowering the needle on the guage. Also you may have an air block somewhere in your system. You may try to bleed the system if there is a bleed port or vent. Otherwise leave the cap off and run the car up to temperature watching the coolant level in the radiator through the open foller neck. When the thermostat opens the coolant will drop rapidly if there is an air block. Be careful when you try this one because if the thermostat is not opening in time hot fluid will spurt out of the filler neck. My guess is it's the thermostat if you have not had any leaks and have not changed fluid or done anything else to the coolant system lately. Hope this helps.
It sounds like your thermostat is sticking. You can tell when it sticks when your temp gauge spikes high and your blower won't blow heat and when it finally opens your temp gauge goes back down and you have heat again.
Check to make sure your thermostat works right by monitoring the warming up of your engine. With a completely cold engine, start and run the engine for a few minutes, then put your hand on the top radiator hose to monitor if and when it gets warm as the thermostat opens more and more and note the gauge reading( if it works) the top radiator hose should be unbearably hot within 5-8 minutes. If the therm. opens right it should close right aswell, staying open a little while eventhough the radiator may have cooled may be what you are experiencing causing the rapid cooling and is nothing to worry about. The Hose should not get warm very soon after starting the engine. Regarding you gauge, lets take one step at a time, verify proper op. or the therm.
You have air in the cooling system, it needs bleeding, this is best done from the heater hoses. you cannot have a heating or cooling sysem that is not pressurised. no water no heater no demister no temp guage