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My 79 L48 just started idling rough all the sudden....It was fairly cool outside this evening, but I ran the car for about 20 minutes, and it's like it never warmed up and the temp never got above 1/4....It also hesitated when accelerating from a dead stop, but ran pretty well once the car was moving.
I did just change the neck and gasket that was leaking above the thermostat....is it possible the thermostat's bad and causing the rough idle?
You need to open the hood and have a good visual inspection to see if you have disturbed something while you were working. Check that you didn't loosen a spark plug lead. Also check that the alternator or drive belts didn't get wet with anti-freeze.