Technical 1978 124 Spyder running hot .... or not? Technical readings were taken

[DrOlds]

1978 124 Spider running hot .... or is it? A friend has a 1978 124 that she feels is running hot. The dash gauge was reading hot and it was going into the overheat range so she had took it to a shop and they replaced the dash gauge but It still shows hot. I met with her and recorded the following:

The car was left overnight cold. The ambient temp the morning I arrived was about 70 degrees Fahrenheit (all readings in this thread are shown in degrees Fahrenheit.) I placed my Snap-On temp probe into the radiator before starting and recorded a temp of 104. The temperature sender in the head (the rear one / verified by disconnecting the wire and noting the change on the dash gauge) read 1.759 ohms.

The vehicle was started and driven and the radiator temp held consistently in the low 100's until the thermostat began to open around 160.... The interesting thing was the gauge had already risen to around 200 by then. (It was about 1/3 of the way past the 190 indicator line to the overheat area.) After about a 15 minute ride at 60 mph (ish) the radiator temp rose to 195-199 depending on speed. It did not exceed that temp.

We returned to her house and I measured the resistance of the temperature sensor hot and recorded 173 ohms at 199 degrees. The thermostat housing temperature was indicating 190 degrees using my heat gun. The heat gun did not find and cold area's while scanning the radiator tubes.

Since bleeding the system is a typical recommendation I did the following... Her vehicle has a "Prestone" garden hose flush adapter installed in the heater hoses looping up above the back of the engine. While the engine was running I slightly cracked the cap and had coolant flow without air and this being the highest point I feel bleeding is not a problem (her other shop allegedly did the indicated burp procedure.)

So. Is 199 degrees coolant temp too hot for a 1978 124 Spyder? What did the dash gauge jump up to 200 degrees when the engine temp was only passing 160 degrees as the thermostat was opening? Are the resistance readings at the temp sensor 1.75 ohms cold / 173 ohms hot correct?

 

[s130]

200F = 94 deg.C

Older Fiats run hotter at around 90 deg.C compared to more modern cars.

I don't have he specific thermostat data for a Fiat 124 but I would guess 89 / 90 deg.C.

A safety check you can do is after a run then at idle and stationary let the engine continue to heat up. The thermal switch on the radiator should cut in at a little above thermostat temperature.

On the Strada 130TC 2000cc Twin Cam then:

o Radiator Thermal Switch opens at 90 to 94 deg.C and closes at 85 to 89 deg.c
o Water Thermostat opens at 77 to 81 deg.C and fully open at 95 deg.C

So for the Strada then 90 deg.C (200F) would be a normal/typical running temperature.

 

[124BC1]

I've never seen hot/cold resistance figures quoted for the older Fiat temperature sender units - the usual way of determining a faulty one was to earth (ground) the wire going to it and if the gauge/warning light then worked ok, replace the sender, (also applies to the fuel gauge/low fuel warning lamp).

Now, to try to answer why the temperature gauge reached 200*F before the thermostat opened? I'm not sure! But...

the forward temperature sender on the cylinder head is designed to send the water temperature gauge to full scale (to indicate excessive water temperature) independently of the rear temperature sender (the one that feeds the gauge). This forward temp. sender is actually shown on the wiring diagram as a switch with an additional separate resistor in series before it joins the green wire from the rear temp. sender that goes to the temp. gauge. Perhaps the forward sender is misbehaving? Maybe try disconnecting the wire to the forward sender and see if the temperature gauge behaves normally as the engine heats up?