MY 84 manual, as a rule, runs cool.
I just replaced my thermostat with the proper Wahl and coolant. I noticed that my temp Guage went down one space from 3/4 to 1/2 which confirmed that my old thermostat probably wasn't opening all the way. Everything operated as expected and designed....Now after a month of non driving, the temp guage is at the 4/4 line and drops 1/8 on occasion with the car off and engine warm. I surmise the temp sensor being at fault.
What say the gang?

 

Get a heat gun and measure at the hoses. Make sure that actual temps aren’t the problem

 

how old is the radiator cap.

 

Driving today, the gauge read normal temperature right in the middle. The gauge would suddenly spike you the uppermost line, suddenly drop to 7/8 and fluctuate between the two marks for about ten seconds before returning to steady middle for about 5 minutes before doing it again. I recently cleaned and put electrical grease on the contacts to the temp sensor.
I have a spare sensor that I can try.
How could the radiator cap contribute this this? I'm interested.
The gauges themselves have been upgraded with the solid state boards.
The temp at the water neck/thermostat housing is 185 f.

 

replace the fuse for the instruments

 

That comment was based on an understanding the car was actually getting hot. Low pressure in the cooling system will reduce its efficiency and it will run warmer than it should. Sounds like you have identified the issue as electrical. I have a similar issue except my temp guage won't oscillate accurately with engine temp; my fans come on when its almost at the white line and shut off before they begin fighting the thermostat...I need to do a pod refresh is my problem....and clean the CE panel...car is in good nick but I know those are 2 chores that need my attention in the early future.

 

Walt,

The gauge behaviour you are describing can only be caused by an electrical gremlin of some kind.

The radiator cap has no practical influence on the temperature your system normally operates at. If the cap does not hold pressure the coolant will boil at a lower temperature. According to the temperature you measured the system is not remotely close to boiling.

You state the operating temperature at the water bridge is 185F which in real world units is 85C. Before you believe that number may I suggest you boil some water in the kettle or a pan and measure the temperature- assuming you are at sea level you should get a reading of 100C [212 silly British units]. I suppose it is conceivable that the engine temperature may be correct but I am a bit suspicious of that number unless you have a 75C thermostat fitted- in which case rip it out and get the correct one fitted.

I suspect your ac system is either not fitted or not functional to be able to get such temperatures. Presumably your ambient temperatures are currently in the 25C to 30C range. You could also measure the resistance to earth of one of the temp 2 terminals- indeed if you do, measure the resistance to earth of both terminals when full heated up and we can then check that against the published temeprature values for comparison.

 

The car in not overheating, I'm sure. The temp at the thermostat housing are accurate and possibly a product of my having removed the AC components (evaporator/compressor, etc.) I have unobstructed air flow to the radiator. I changed out the temp sensor with a fresh one and haven't gotten the same erratic gauge so far.
I am one that likes you engines to run cool. Lord knows the car runs great, so I don't think know that 185-200 f is a problem for a 84...I am nowhere near seal level, probably about 1700 feet above.