View single post by Jim Sohl sn 11210
 Posted: 07-19-2016 02:07 am
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Jim Sohl sn 11210

 

Joined: 01-20-2015
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 For modified cooling systems, this story may be worth a look, otherwise, it’s just kind of funny: I was asked to diagnose and hopefully repair a friend’s 1975 Fiat 124 spyder. I have owned two of these in the past so I gave it a try. Same symptoms: temp goes nearly from max hot to min cold without rhyme or reason. Cause? Air trapped in the cooling system. This model Fiat has a bypass thermostat rather like the 907’s except that it was added by Fiat rather than a part of the original design. The problem was that with the ‘new’ plumbing, the radiator cap was not the highest point in the system. Thus, removing the cap only revealed that there was coolant up to that level, but the ‘new’ hoses and ‘new’ thermostat housing were above that level and probably dry (when the engine was not running). When running, the coolant pump would alternately try to pump air/steam/coolant as one or more of the three passed by. Result? When pumping coolant, all was well. When pumping air or steam, pumping efficiency goes to near zero until some other event (more boiling) forces coolant back into the pump. Then, whoosh, the coolant that had been sitting in the electric fan-cooled radiator rushed in and the temp gage fell quite a bit until the whole process started again. The period of the whooshing depended on driving habits, ie. low rpm gave the most pronounced swings, fast driving lessened all symptoms. The ‘fix’ I used was one-time-only: I jacked the front of the car up so high that the radiator cap was the highest point, purged the air, and all was well. Anyway, it seemed funny at the time.
Cheers,
Jim