What is wrong with my cooling system?

Cool car- That has a remote thermostat- is it still there? sounds like you are trying to do custom work- a good idea is to just try to keep it factory- hook the heater back up- anyway, if the remote thermostat is still there, those cars are hard to get all the air out of the cooling system- what always happens after cooling work is that an air bubble gets trapped behind the thermostat, keeping it from opening- it takes a lot of patient "burping" with the radiator cap off (engine off)- squeeze the radiator hoses repeatedly and watch the air bubbles come out of the radiator filler- man, I have spent many hours doing that! this will work if EVERYTHING else in the cooling system is righteous- you did say it was a NEW thermostat?

First, I like the older 124s with the thermostat where they belong in a housing on top of the head- Fiat complicated their simple little machines on an regular basis- part of it had to do with American regulations- but the more complicated they got, the harder they were to work on- I still love them, especially the 124s, but I don't like working on the later ones especially- that's why I have a really good FIAT mechanic who knows more than I do and who loves them, too- Like I said, keep it stock, that's how they were designed to work, and now: find a GOOD Fiat mechanic- let them do the heavy lifting- don't blow that engine up-

Especially the fuel-injected ones like yours- the first picture is my '74 124 Coupe- the sedan is the first 124- a '68 124 with a pushrod engine- I had one and boy, was it easy to work on- a dream machine- that's what I wish I had again, a 124 Sedan with a pushrod engine- haven't seen one for years- that was a pinnacle of automotive engineering- if you don't know, that fuel injected engine is difficult to put a cam belt on. too- I would rather eat glass than do one of those- if yours is not cherry and completely original, I would just get rid of it- there are others out there, and one with a carb is a lot better to own- like I said, lose the frustration and just get a good mechanic- it is too difficult to talk you through things on the fuel-injected cars especially- here's a photo of one of my favorites- the first year, 1974 X1/9 with the 1300 sohc and the tiny bumpers- good luck-

The fan on my Spider quit working one day in traffic. I noticed it when the temperature gauge climbed. Traced the problem to a ground wire that had come loose. Reattached the spade connector to its terminal and the problem was solved. Hope your problem is simple to fix also.

Very simple fix. Look on YouTube for Fiat 124 thermostat. You need to drill a very small hole in the bottom of the thermostat as when you fill antifreeze air will be blocked from coming up from the bottom of radiator and passing into block. Also remove small antifreeze hose on top of engine and fill antifreeze at this point. You are low on antifreeze. Simple physics to fix liquids don't flow up hill. I had same problem.. Thought it was a water pump then thought it was thermostat. If you have the type of thermostat you can't drill very small hole in simply remove lower radiator hose to bottom of thermostat. Fill radiator or lower radiator hose when full put hose back on lower thermostat. Then fill small hoses on top of engine. No more overheating...