Heater core repair

[BammBamm]

I have a couple of questions. I found what was left of a temperature sensor copper wire. What does it go to? It didn’t attach to the heater valve. I would also like to pressure test the heater core. What psi can I charge it to?

[18Fiatsandcounting]

I found what was left of a temperature sensor copper wire. What does it go to?

Early spiders had a sort of primitive "temperature trimmer" for the heating system, and that copper wire attached to the heater valve to adjust the setting a bit. It didn't work, made no sense in a drafty convertible, and so it was dropped. Unless your heater valve is the original, that copper wire does not go to anything and can be ignored.

I would also like to pressure test the heater core. What psi can I charge it to?

If memory serves, the radiator cap is set to vent at 7 psi, so that would be the maximum pressure of the cooling system and thus a good pressure to test your heater core at. I've seen some folks pressurize the cooling system to 11 psi or more, but this was to check for head gasket leaks. That sounds too high to me for normal system operation.

-Bryan

[18Fiatsandcounting]

To add to what I said above, I found a reference in an old Fiat workshop manual to test the cooling system at 14 psi. That still seems high to me, and my guess is that if you pressurized these older cars to that level, something would pop. It always does...

-Bryan

[davidbruce]

I would double check your manual but I believe 13psi is the correct cap. Both my 73 and 80 had 13s when I got them.

[BammBamm]

Thanks for the replies

[18Fiatsandcounting]

I would double check your manual but I believe 13psi is the correct cap. Both my 73 and 80 had 13s when I got them.

In an odd way, I think we're both right. Sort of. I had to do some research, but part of the issue is that radiator caps tend to be rated in terms of pounds (lbs, #) and not psi. For instance, Midwest-Bayless sells a cap for the earlier spiders rated at 7 lbs, while RockAuto sells caps for the later spiders rated at 13 lbs.

It also appears that earlier spiders did have a lower pressure cap, and 7 lbs would correspond to about 9 psi given the radiator neck opening of 1 inch. I did verify in several Fiat workshop manuals that the radiator is to be tested at 14 psi (not pounds), and one Fiat shop manual for the 1800 and 2L spiders listed the radiator cap as releasing at 11.4 psi. The actual value was in metric, which is why you get the rather odd 11.4 value in English units.

I also verified the release pressure of the caps in my '69 and '71 using a kitchen scale. The '69 cap is marked as 7 lbs and does indeed release at 7 pounds on the scale. The '71 cap (from AR) releases at a higher pressure of about 11 lbs. I also found some references to the fact that the earlier engines did not have reinforced radiator hoses which is why they were kept at a lower pressure. Modern hoses could likely go higher.

So, it would seem that a 7 lb cap (9 psi) is right for earlier spiders, while an 11 lb cap (13 psi) is correct for later spiders. I did not find a consistent answer to when the cutoff date is, but it appears to be in the early 1970s.

More than you ever wanted to know.

-Bryan