Overheating? Maybe?

[alfapop]

1974 Fiat 124 Spider. The car starts and runs great but after a few miles the temp gauge runs up towards the red. Then, just as I start to get worried, it quickly (within 5 seconds) drops right down to normal. I was thinking the thermostat is sticking or of the wrong type but the previous owner put in a new thermostat not too many miles ago. It also sometimes happens just driving along. It suddenly runs up to the red and then back down. Should I put in another thermostat to see if that is it or could it be something else? The gauge? Sending unit? Something with the fan or the fan thermal switch? Electro-magnetic clutch (which I didn't know about until just now as I read about it in an old Haynes manual). I can roll up my sleeves and start testing things but if someone has a good idea on what it might be it could save me a bit of time. I'm thinking to test the electro-magnetic clutch first. Thoughts? Thanks.
Rob G.

[18Fiatsandcounting]

Does the temperature gauge needle "flick" into and out of the red zone very quickly? If so, the overtemperature sending unit* on the cylinder head is probably going bad, or the wire to it is shorting somewhere.

If the needle rises into the red over a matter of 10 or 20 or 30 seconds or so, then you likely have an issue with the thermostat, or possibly an air pocket in the cylinder head. I wouldn't assume that the previous owner put in a good thermostat, or even the right one, or properly "burped" the cooling system to remove air.

*There are two temperature sending units on the head: one for the gauge movement, and one that send the needle into the red once the set point is reached. These sending units do slowly fail over time, leading to erratic gauge readings.

Also, you shouldn't have an electromagnetic clutch assembly on your '74. The electroclutch fan was used only in very early cars up until the late 1969 or early 1970 model years. You should have a conventional electric fan on your '74.

-Bryan

[alfapop]

Glad to hear I don't have to worry about the electro clutch. The car is actually still up on the storage lift so I haven't opened up the hood yet this spring. I think I will start by 1) burping the system, which may not have been done, followed by putting in a new thermostat. Not that expensive and easy to change myself. I do recall looking last fall and seeing the wires coming off the two sending units were suspect but as the gauges appeared to be working I did not touch them. Bottom line, you have given me a trouble shooting plant to pursue. Thank you for the advice. I'll post the results if I solve the problem.

[alfapop]

Okay. Since my last post I have put in a "T" at the high point of the cooling system and burped it. The problem is still there. The needle doesn't "flick" into he red. Just climbs up. But new info is that if I open up the heater the temp comes back down. Works fine as long as I am boiling up in the drivers seat. Also, I pulled over when the needle was well above 190 degrees and popped the hood and the fan was not on. I don't think the fan is coming on at all. It does work though as I hooked it up to a 12 volt battery and it came right on. So could it just be that the sending unit at the bottom of the radiator has gone bad? What I haven't done is let the car run up to the red and when if quickly comes back down pull over quickly and see if the fan has just come on. If it does, that would also mean a faulty switch, right?

[TX82FIAT]

You may be on to something with the T stat. The system takes a few minutes to get warm and runs a short loop in the cooling system to get hot expanding the loop once temp is reached and increasing cooling. If the t stat is not opening all the way or if there is some air in the system it will run hot. The gauges on these cars do fail as mentioned above. A couple thoughts. 1. For relatively cheap you can buy a handheld IR sensor and read the temperature of the car at the top and bottom of the radiator, head, block and hoses. This will give you and idea if car is indeed running hot and may isolate an area of restricted flow in radiator or increased heat in head. 2. You can take an old pot filled with water, cooking temp gauge and place the t-stat in the pot on the stove and see if it opens at the right temperature around 190. Both the switch at the bottom of the radiator and the T-stat have options you can buy that will open the cooling loop or turn on the fan at lower temps such as 175. I installed a manual switch at bottom of rad when in bumper to bumper traffic in Texas 101 heat to proactively smooth the cooling cycle by turning fan on manually in advance rather than wait for fan to kick on via switch in theory reducing the up and down heating/cooling a little. 3. May be best to run through heating cycles in your driveway to test things rather than out on the road. If the car is running hot and you pull over stopping the airflow it may actually continue to heat for a minute on the side of the road. All this assuming no white or blue gray smoke, oil and coolant both clean.

[alfapop]

Thanks. I suppose changing the thermostat is relatively easy and is the next thing I should try. But I still think the fan is not coming on when it is supposed to. I'll have to read up on the system as I understand there is a fan switch on the bottom of the radiator and also a relay somewhere. Maybe one of those is bad. Three things to try. I'll report back. All part of the fun.

[TX82FIAT]

The fan switch on the bottom of the radiator is always hot and I do not believe it has an associated relay. This is why when you shut the car off the fan may still run to cool the engine. you could have restriction in the rad, air or a host of other things going on. This is why the IR temp reader (about $15 harbor freight) helps determine if there are hot or cold spots in the system.

[18Fiatsandcounting]

...I understand there is a fan switch on the bottom of the radiator and also a relay somewhere. Maybe one of those is bad.

<< I see TX82FIAT also responded. >>

Remove the two wires going to that switch at the bottom of the radiator, and with the ignition on but engine not running, short the two wires together (use caution and keep away from the fan blades). The fan should come on. If it doesn't, then you have a wiring issue somewhere. If the fan does come on, then either the switch is bad, or the bottom portion of the radiator (where the switch is) is not getting hot enough. That could be because of blockage, sludge, thermostat not opening as it should, etc.

-Bryan