I have a 1978 124 Spider that is over heating.Had the fan and thermostat replaced but still over heats. The expansion tank is not installed. Could this cause over heating?:bang:

Lack of an expansion tank won't cause overheating initially but can contribute to overheating over time.

The purpose of the expansion tank is to catch any coolant expelled from the radiator as the engine heats up and return this amount to the radiator as the engine cools down, i.e. an automatic radiator topper-upper!

So, if the radiator is full and the engine is run to full temperature, it shouldn't overheat. But any coolant expelled due to expansion won't be returned to the radiator so the coolant level will be a little lower when the engine cools down and still low when the engine is next started. If you were to top up the coolant every time the engine was cool, you could run the car without an expansion tank!

Lots of reasons why an engine can overheat. You've replaced the fan and thermostat. Have you checked for a blown head gasket, choked radiator core/fins, cooling system leak, faulty rad cap?

When you say the engine is overheating, what do you mean?
Temperature gauge reading high? or coolant fan cutting in frequently?
or steam coming out of the radiator overflow?

Need more info!

AL.

There are two temp sending units. One transmits the true temperature of the engine and the other is a "fail-safe" which sends the guage over NEAR to the red zone on the guage if the unit thinks the driver is not paying attention to the temperature! Sounds bizarre but that is how it was designed. A Fiat vendor told me that, if the guage reads almost overheating but does not reach the red zone, I should disconnect the fail-safe one. I did that. (I forget whether it is the front one or the back one). Temp is now within normal range.

Have you carefully bled the cooling system? On a '78 you must remove the top of one of the hoses and park the car with the radiator above the block. Pour coolant into the radiator until it flows out of the upper end of the hose, Be sure to have the heater control open. My '79 has a bleed plug in the center front of the t connector, check for that on your '78. If steam collects under the temp sender due to air being in the system you will get strange readings on your gauge. There are careful instructions online on this bleeding process. Be sure that your sender unit is not "sticking". I had that happen and I was getting readings all over the place. A new sender unit is cheap.