I'm back again with more cooling issues.
Now my car (1996 Volvo 850 2.4L base sedan) is perfectly cooled when driving and fine most of the time, but, whenever it's moving slowly or not at all, the temp gauge shoots up very quickly. The radiator fan still does not work, even when the A/C compressor is on. Cranking up the heater stops the temp rise at idle and it goes back to a healthy temp once it's moving again but the summer heat here is 95+ Fahrenheit on most days so that's not exactly a solution. So far I've replaced; Coolant with Zerex G-05, Thermostat with an OE Temp one with the small valve in it facing the correct way, the coolant temp sensor, and the radiator fan relay.
Any helpor advice would be greatly appreciated!
-Luke
ps. Is Synthetic blend High Mileage 10w-40 good for temp ranges between 75 Fahrenheit at night and max of 105 in the day?

 

I would suggest double checking all electrical connections to the motor and possibly replacing the fan. I would also suggest checking electrical connections to the gauge itself and make sure it is registering properly. And while you are at it check all your grounds in the electrical system to rule out any issues there. A bad ground could also cause issues with your fan not running. I would start with the wiring to the coolant temp sensor first.

It also sounds like it could be a failing water pump. When was the last timing belt/water pump service done?

When you replaced your coolant, did you flush the system with cleaner/water beforehand?

As for the oil weight, 10w40 is perfectly fine. Keep in mind as temp goes up, the oil thins out and thickens as it gets colder. Back before all the fancy advances in oil, a lot of the old head car guys/mechanics would run/recommend 10w40 in the summer months and switch to a lighter weight in winter (5w30/5w40/10w30 etc).

More info on oil weights/viscosity numbers: https://www.motorstate.com/oilviscosity.htm

If you are overly concerned with your oil weight vs. temp, you could jump up to 15w40 (diesel weight) and run that, however its not really necessary. Check your owners manual and it will give you a chart showing what oil to run in what temp ranges.

 

Well, you need the fan to work, to cool the car when you're not driving fast. That is what the fan is for.

The idea that there is a problem other than the fan seems very very irrational to me. Extremely so. Are you okay with that? I'm saying don't fix anything that's not that fan.

The fan is just a simple electric motor. If you get power to it, then it'll either run or it won't. That is is a useful diagnostic test. There are various ways to do that, but if I was going to do it, I would probably just jumper the relay and see what happens. You might also check with a volt meter to see if you have power coming to the relay plug.

 

Temp rises too quickly to be just a non-functional fan. Goes from perfect to 3/4 in less than 5 seconds and before that idles just fine for a short period.

 

No clue when the last timing belt or water pump was. Bought it used a few months ago, timing set is on the way now. Used a chemical flush for coolant followed by running well water for a few minutes. Heater core open while flushing it.