Question about internal thermostat in 77/78

[Odoyle]

Is there an internal thermostat in the cylinder head behind the coolant tee in 77/78 spiders ? The radiator fan switch is turning the fan on at around 210 degrees f. Have tried multiple different low temp fan switches, original fan switches, new temp sending units in the head, different temp gauges, new external thermostat. Pretty certain it is not a bleeding issue as well, have been topping off radiator with car running and front end lifted while squeezing hoses. My 83 runs at 190 f all day, still has original fan switch, thoughts ?

[RRoller123]

There SHOULDN'T be one in there, maybe the gauge is just reading high? Has it always been like this? Given all the things that you have changed, it sounds like maybe there is a resistance problem somewhere along the sending wires to the gauge that is making it read a little high. After all, if you have changed actual gauges, sensors, switches, etc, you have changed almost everything in the system except the wiring.

Since the system does warm up and then cool down, presumably (?), when the fan comes on, it is clear that the external t-stat is at least working. I suppose it is possible that the external t-stat it is opening too slowly, or at too high a temp, and there is some latency period of heat up that makes the head and radiator temp build up beyond what would be expected. That being said, I think the fan switch (at the bottom of the radiator) reaction time should be pretty quick, so it is somewhat of a mystery. Has the external T-stat been replaced, tested or removed?

If the external T-stat is not opening enough, it seems logical the head would run hot, i.e. the closed loop heat up time runs too far, for too long, and the radiator will see a cooler than expected temperature. Thus even if the fan switch turns on properly, the temp sensors in the head would still show a higher temperature than expected based upon the chosen fan switch temp.

[18Fiatsandcounting]

Could it be that your cooling system is just fine and working as it should? 210 degrees sounds about right in terms of when the fan should come on, and I've always considered 190 to 200 degrees to be "normal operating temperature". That would be in the middle of the temp gauge or slightly higher. I only get worried when the gauge passes about two-thirds on the gauge, which I'm guessing would be 220 or so, and there's no obvious reason (long mountain climb, stop-n-go traffic in summer, etc).

I don't have an answer for why your '83 apparently runs cooler, but perhaps it was designed to?

-Bryan

[rridge]

Don't assume a 40 year old temp gauge is still accurate. Wiring issues and the senders themselves create problems. A simple IR gauge pointed at the top of the head behind the coolant outlet tee will tell you true head temp. The same gauge pointed at the bottom tank of the radiator will confirm proper operation of the fan switch. You can also compare the top and bottom tank temps to whether the balance of the cooling system, radiator, hoses and pump are doing their job. A typical temperature drop top to bottom should be 20-30 degrees F.

[RRoller123]

Yes, but he has already changed multiple gauges and sensors, so that would account for that?

[18Fiatsandcounting]

To the OP: Are there any other symptoms? If the radiator cap releases steam into the expansion tank, you definitely have an issue. But, for the Fiats I've had, the gauge hovers around 190, plus or minus 10, given the ambient temperature and driving conditions. If the gauge rises above 210 or so, the fan kicks and brings the temp back down to 190 at which point the fan turns off. In summer and in traffic, the fan might cycle more often, and in winter it might hardly come on at all, and I've always considered that normal.

And, to add to what others have said, the gauges, sensors, fan switches, etc. are not precision devices and I'm guessing their tolerance even when new is at best plus or minus 10 degrees.

-Bryan

[Odoyle]

Thank you for feedback guys, really grateful for the responses. No steam or bubbling in coolant tank at any point, certain its not getting to 250-260. I dug into some old threads on the forum and found a couple other guys describing exact same problem I'm having. Said he replaced the stiff, brittle wires to the sensors on the head and the gauge read accurate after that. The wiring I replaced was as stiff as uncooked spagetti. Used some good wires in the same color from 79 parts car, hope that solves it.

[18Fiatsandcounting]

The wiring I replaced was as stiff as uncooked spagetti. Used some good wires in the same color from 79 parts car, hope that solves it.

Sounds like a good plan, and sometimes it's the connection between the wire and the spade connector (crimp connector?) that has gone flaky. I'd suggest a new spade connector along with your new wire (not that the connectors are easy to reuse in the first place).

-Bryan

[RRoller123]

I am betting that the external t-stat is not operating correctly. When cold, the t-stat diverts coolant to recirculate ~ only through the engine. In this state, the sensors/temp gauge on the head show cold, the radiator fan switch sees cold.

Then the coolant warms with the head, and the t-stat starts diverting more water over to the radiator. The head sensors see warmer, the radiator starts to see warmer coolant but not enough at this point to turn on the fan.

Then when warmed up, the t-stat fully opens and the system is balanced such that when the head is at proper operating temperature, and is indicated as such by the head sensors and temp gauge, the radiator fan switches on and keeps the temperature down there in balance, at the 170, 190 switch point, whatever.

But if the external t-stat doesn't fully open, if it is sticking and only diverting partial water to the radiator, I expect that the head will run somewhat hotter, and the head, even though the radiator fan switches on at the correct set point, will show hotter than it should be, with the system out of balance.

I know I probably didn't explain this well, but it has been a stressful week, lol.

[Odoyle]

I sorted the overheating issue on my 77 by replacing the water pump with a GMB water pump and low temp external thermostat from autoricambi. Impeller design on GMB pump is completely different than the OE style I had installed previously. Temp is now right in the middle of gauge and fan kicks on and off as it should.