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I've read somewhere on the internet how some mechanic installed inline thermostat off some old Renault 5 GT turbo in the coolant hose of Alfa 147 1.9 diesel that had thermostat stuck open.
This is the one:
(https://a.d-cd.net/7ff59b9s-960.jpg)
The car owner claims it's been fine for 9 months and going.
I am not sure you would save all that much if at all, as a unit on e-bay for 156 JTS is $80 plus shipping. Refer to the following link https://www.ebay.com.au/itm/Alfa-Romeo-Thermostat-2-0-JTS-156-GT-GTV-Spider/263150845861?hash=item3d45033fa5:g:9JYAAOSwuxFYzLMq
I can vouch for the eBay seller from the link above - got selespeed accumulator and brake pedal switch from them. Same day postage, excellent communication.
Also, that thermostat is for JTS, they also have one for Twin Spark (could be the same, I'm not too sure):
https://www.ebay.com.au/itm/Alfa-Romeo-Thermostat-T-Spark-147-156-GT-GTV-Spider/263150843678?hash=item3d4503371e:g:9JYAAOSwuxFYzLMq
The 2 thermostats linked above are different. with the first link being for the 2.0 JTS engine and second for the twin spark engine (refer to their respective part numbers).