Water temp question

[Trifle]

I am mechanically illiterate so bear with me as to how I describe the issue. Vehicle is a totally stock late 1978 124 Spider 1800 with Cali smog equipment.
Water temp gauge seems accurate. At normal highway cruise, it is steady just under 190. At street cruise (i.e., 35 rather than 65), temp gauge steady a bit above 190.
Problem is that in any sort of stop-go, or even low steady speed, water temp very quickly spikes up. It has never overheated though. Gets close to red line but never quite. For example, I was in terrible (and unanticipated) 2 hour traffic jam last week. I was sure it would overheat but it didn't. Needle quickly climbed but then stayed at roughly 80% of the way to red line. On the other hand, in 5 minute stop-go 2 nights ago, it very quickly spiked up to 90% of the way to red line and if I hadn't shut off engine it looked like it would have gone to red line.
It is also alarming how quickly gauge needle jumps up. As soon as I am in stop-go traffic, needle goes up immediately. When stop-go traffic ends and speed goes back to normal highway cruise, needle just as quickly returns down to just under 190. The above behaviour has only been for past 6 weeks. Before that, needle much less reactive.
In stop-go, and as needle going up, I do not hear fan come on. Fan, in general, comes on VERY seldom. I used to be able to hear it come on while engine running, especially when at a stop. Now, it just about never comes on, no matter what. It does work though because it will turn on on rare occasions. It is so rare that I thought it had quit altogether but last time I drove it 2 days ago, and turned off engine, I was shocked to hear the fan actually come on. Note it only came on after engine turned off. This is not because of any inability to hear fan come on when engine running; I have checked.
No coolant leaks. No radiator leaks. Radiator has coolant up to proper level. All hoses sound. Nothing replaced yet.
Thank you for reading, and for any suggestions.
For differential diagnosis, what is most likely, 2nd most, etc.? How do I eliminate possibilities?

[131]

Checking the fan operation is probably the easiest thing to start with. If it's getting hot and the fan doesn't run, the temp. switch could be stuffed. At the bottom of the radiator there is a switch with 2 wires going to it, if you disconnect them and short them together the fan should run, (ignition may need to be on), do it several times to make sure it comes on consistently. If it does, I'd suspect the switch, or a radiator flow problem, if it doesn't, you may have a fan/relay/wiring issue.

[georgeramos]

My car can tend to sometimes run hot too. Never overheats though. Seems the thermostat (new) opens pretty slowly then needle dips suddenly and slowly rises again. I feel like its normal and as long as it never gets to red line I dont worry too much but I tend to look at it a lot when driving in warm weather and I often wonder what if I lived in Vegas or phoenix?...

Im thinking of getting one of the adjustable fan switches and maybe a thermostat that opens at a lower temp...

[Dougieb]

If your heat gauge "spikes" rapidly I would check to see if you have an air bubble in the coolant system.
Mine did the same thing and a good burping of the system cured that right up.
Search through the forum and you can find out how.
The switch to turn on the fan (bottom of radiator) is completely different than
the one for the temp gauge (between the valve covers).
An air bubble will normally seek the highest point it can, usually top of the head.
Coolant may leek out in very sneaky ways. Sometimes it is hard to tell.

I would suggest doing this first before spending money on parts that may not need replacing.
Hope this makes sense.
Let us know how it goes.
Good luck.

[TX82FIAT]

The coolant T from the head to the heater install is quick and cheap way to help bleed the system if you do not already have it installed. The first thing I would do is run the car in your driveway with the hood up watching the temp gauge and fan to test the fan operation. Does the fan come on and the car cools down? The cooling system on these things can vary greatly. I've found that on 100+ degree days the temperature needle will indeed move much further to the right than I like when in stop and go traffic even with a properly maintained and burped cooling system. However, it does not get into the orange/red.

On my car it is common for the fan to come on right after shutting her down as the engine is hot and the water pump is not pushing coolant around. Some owner pop the hood to help heat dissapate when they park the car in the garage. On my car the spikes in temperature only really occur on the 100 plus degree days. you mentioned the past six weeks you have had this cooling concern. What have the temperatures in your are done during those six weeks.

[toplessexpat]

Some owner pop the hood to help heat dissapate when they park the car in the garage.

... and then we need to replace those snapped hood cables from such frequent use

[adrians]

Is the lower radiator hose getting hot ?

[JEEPER]

Checking the fan operation is probably the easiest thing to start with. If it's getting hot and the fan doesn't run, the temp. switch could be stuffed. At the bottom of the radiator there is a switch with 2 wires going to it, if you disconnect them and short them together the fan should run, (ignition may need to be on), do it several times to make sure it comes on consistently. If it does, I'd suspect the switch, or a radiator flow problem, if it doesn't, you may have a fan/relay/wiring issue.

X2 Mine only over heated at slow traffic. I would turn the heater on to drop the temperature. I had a bad connection fromthe thermo switch plug to the fan from the radiator.

[GeorgeT]

An outside possibility is a bad radiator cap.

[Trifle]

Thank you for responses. I notice, in a crevice in the top of the engine at the front, that there is a little pool of coolant-coloured liquid that has collected. I did not mention it before. It has been there a long time. It never overflows the little crevice but never evaporates away either. I will check on other suggested items.

[RRoller123]

A minor (slow) leak that allows air to slowly work its way into the system will cause problems.

[TX82FIAT]

A very small leak that pools may not get any bigger because of the rate of evaporation and cooling. The fact that there is coolant in a crevice is not a good sign.

I am going to take a look at the pressure rating on my radiator cap. As I understand the cap it is supposed to move up letting coolant escape if heat expansion (presure) goes above a certain level to fill the overflow tank and then do almost the same based on a vacuum effect when the coolant cools down drawing the coolant back into the cooling system. I'm now scratching my head as the overflow tank appeared a little high the last time i checked under the hood. I shrugged it off but thinking all was well in the cooling world.

[RRoller123]

You are right about the radiator cap, think of it as a 2-way pressure regulator valve.