Overheating / Air Bubbles

[BellaMacchina]

Hello,

I have a problem with overheating of my CS0 which seems to be caused by air that is somehow coming in the cooling system.
To tell the whole story:

I bought the car last october with a radiator that leaked a little bit. I drove it for maybe 200 miles before the winter break without any temperature problems. The coolant temperature was rather too cold which was probably caused by an "always open" thermostate.

During winter break, I installed a new radiator and a new (replica) thermostate.
I bled the cooling system in a correct way: The radiator went on several times, bringing the temperature down.
All right, so I took it for the first ride which went well for about one hour. After that time, all of a sudden the temparature gauge raised within seconds and jumped to the max.
I turned off the engine and felt the coolant hoses. The lower one was cold, so it looked like the thermostate did not open.
Maybe stuck ? Bad quality ?

I brought the car to a repair shop (which is specialized for those old cars). They installed a NOS (Savara) Thermostate and bled the cooling system agin. (Because of this, I am pretty sure a wrong bleeding is not the cause of my problem).

Afterwards I drove the car (with the NOS Thermostate) again two times, each around 2 hours.
And all of a sudden while driving, it happend again: Gauge raised to max. Lower coolant hose is cold.

After cooling down, I checked to fluid level:
The coolant reservoir was still half full, but I was able to fill around 1/2 Liter of coolant in the radiator filler cap, before it came out of the bleeding screew on the "T-Piece" (where the water leaves the block).
So it looks like the cooling system is somehow collection air over time which collapses the cooling flow and/or prevents the thermostate from opening.

I know that one reason could be a blown head gasket, but I want to make sure that there is no other possibility.
Strange thing is, that this behavior started with the change of the radiator...

Maybe a leak somewhere that sucks air in the cooling system ?
Did someone experience something similar and found the problem ?

Thanks !
Markus

[SteinOnkel]

I'm starting to sound like a broken record here

Get a radiator test kit and pressurize the system every time you service it or it does something weird. If it can't hold pressure indefinitely, all sorts of strange things start to happen.

Y'all like the VW guys, just slightly different over there it's always "My oil pressure light comes on hmm what should I do" and after ten pages of "Well it could be [insert 50 different things here]" somebody finally goes to measure the oil pressure.

[rridge]

A sudden rise in gauge temperature is often a sign of a faulty overheat switch. When it is working properly it overrides the normal temp sender in order to peg the gauge needle at 260F to get your attention when the coolant temp is approaching its boiling point. In old age some go out of calibration and peg prematurely.

Disconnect its sender on the head and see if that makes a difference. The regular gauge sender is usually in the rear head port. The overheat switch is in the center port. Each has a single wire with a push on connector under identical dark brown plastic boots. If you disconnect either and the gauge still works you've removed the overheat switch. The gauge will continue to show the correct temp with it disconnected.

[BellaMacchina]

@rridge - that's a really helpful tip. Thanks for that.
Nevertheless the problem is not with the gauge since so much coolant is missing.
But I like the idea of having the real temperature instead of a pegged needle.

Thanks also for the pressurize tip - I will try that.

[SteinOnkel]

@rridge - that's a really helpful tip. Thanks for that.
Nevertheless the problem is not with the gauge since so much coolant is missing.
But I like the idea of having the real temperature instead of a pegged needle.

Thanks also for the pressurize tip - I will try that.

I did not know that about the extra switch. Another piece of overengineering on these cars

If you see the needle on the gauge drop, start looking around. The leak should be obvious. If it isn't, it could be leaking into a combustion chamber That would be not so good.

[18Fiatsandcounting]

But I like the idea of having the real temperature instead of a pegged needle.

Many people agree with you and remove the connection to that "overtemp" switch. I've always felt that if you aren't religiously glancing at your temperature gauge every couple minutes, something other than a 40+ year old Fiat is probably a better choice.

One other note: That overtemp sensor can "halfway" go bad, in the sense that it isn't either on or off but somewhere in between, and that can throw off your regular temperature indicator as the two are wired electrically in parallel.

-Bryan

[davebdave]

... I've always felt that if you aren't religiously glancing at your temperature gauge every couple minutes, something other than a 40+ year old Fiat is probably a better choice.

One other note: That overtemp sensor can "halfway" go bad, in the sense that it isn't either on or off but somewhere in between, and that can throw off your regular temperature indicator as the two are wired electrically in parallel.

-Bryan

I was just driving and glanced at the temp gauge (which had been just right of the middle tick like it usually is) and it was touching the red! Oh no! "My fan must have quit" The weird thing was that when I pulled over the fan was running but then stopped on its own at idle? Didn't smell hot either. The needle just stayed on the line making me really nervous as I drove it 20 miles home. I was going to investigate the fan but decided to check the forum. Once again you guys save the day. Credit to the Italians, the needle in the red did get my attention, but I'll keep the middle probe disconnected from now on.

Edit: It just occurred to me that the thing might actually have been trying to tell me I am running too hot. What is normal on a 90 degree day under spirited driving?

[davidbruce]

Markus, Stein has a good point about doing a pressure test. It is kind of the low hanging fruit of cooling system troubleshooting. You could ask the shop if they tested it when they did the other work. If not you may be able to borrow or rent a tester from your local auto store. Did you flush the cooling system before installing the new rad? If the PO put some kind of stop leak in you could have all kinds of interesting symptoms. If a pressure test shows no leakage, perhaps there is some stop leak blocking a cooling passage making it difficult or impossible to bleed properly. Are you positive the thermostat is oriented correctly? It's easy to get wrong. Ask me how i know. If it was in wrong initially they may have just copied it's orientation if they thought it was faulty as oposed to installed incorrectly. Good luck and plase let us know the outcome.

[18Fiatsandcounting]

Edit: It just occurred to me that the thing might actually have been trying to tell me I am running too hot. What is normal on a 90 degree day under spirited driving?

I think you're running fine, Dave. Most of what I consider normal (optimal) is 180 to 200 oF, no matter what the conditions. Except for extreme cold where it might not get over 170. Down around zero degrees ambient, that sort of thing.

In one of my instances of stupidity, I drove my '69 through Death Valley in July in the late 1980s. The outside temp was near record highs at 128 in the shade, but the car pretty much stayed at 190-200 the whole time. It did rise a bit climbing over the pass to exit Death Valley, around 5000 feet, but even then I don't recall it getting over 210. For which I am eternally grateful to that car. It fared better than I did: black top, black interior, no A/C....

By the way, a sign of a temperature sensor switch going bad is a gauge needle that twitches, or goes full tilt when the engine is fine, as you apparently experienced. Whenever I get a high temperature reading, the first thing I do is disconnect that switch and see if it then reads normal.

-Bryan

[davebdave]

Edit: It just occurred to me that the thing might actually have been trying to tell me I am running too hot. What is normal on a 90 degree day under spirited driving?

I think you're running fine, Dave. Most of what I consider normal (optimal) is 180 to 200 oF, no matter what the conditions. Except for extreme cold where it might not get over 170. Down around zero degrees ambient, that sort of thing.

-Bryan

Thanks Bryan,
I'll continue driving it like I'm being chased.
Dave

[Nut124]

I would never ignore a gauge indicating high temp.

But spirited driving should not do it no matter what outside temp is.

Actual high coolant temp will result in head gasket failure, at least when a traditional gasket is used. Not sure about MLS gaskets. Fire rings of a traditional gasket get pushed into the bore by the excessively high coolant pressure and get smashed by the piston, then fail to seal - destruction.

[BellaMacchina]

To solve the mistery:

It was a blown head gasket - unfortunately

Regards Markus