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Discussion Starter #1
FYI, after owning this 9C1 LT1 Caprice for more than ten years, I can say that if you want factory behavior, get factory spec parts. Some parts are critical. I replaced my original thermostat with Duralast twice. I noticed that the gauge would give wide swings between 1/3 and 2/3 scale. In winter, the engine never warmed up unless I was sitting idling first with no air moving through the radiator. Then on the highway the engine would get so cold it would barely register on the first tick. The Duralast design has a little hole drilled in it which is not original design. I suspect it was allowing cold coolant to flow all the time. This winter it became intolerable so I installed an ACDelco 12TH10D. What a difference! The coolant now comes right up to temp whether I'm driving at highway speeds or sitting. The gauge goes right up to half scale and stays there unless I'm sitting. If I sit for more than a moment, the temp will creep up to 3/5 scale when the fan kicks on and the temp goes right back to center. Heat is good now even at 5F. These motors need to run hot for best fuel economy and low cylinder wear.
 

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Near every TS I've ever thought to look at had a teeny relief hole, but hardly considered large enough to affect cycling. You didn't mention, but you likely weeded out any complications with the first replacement that coulda been caused by not burping the air enough. In any case good to hear you're running right with a (comparatively) rock steady AC. I run the MotorRad 160* like most guys with a program, but agree it's possible to run a SBC too cool.

To your point, parts house aftermarket runs the gamut from 'value-added' to 'one step from failure when new'. But Duralast especially makes me flitch like 'Niagara Falls' with the 3 Stooges though. (not very) Funny story; replaced a marginal performing DL battery that finally failed 3 months before free replacement warranty expired, only to have the brand new one kick the bucket after just 6 months.
 

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Problem is on these cars the hole allows relatively cool radiator coolant to pass through,and around the thermostat. Affecting it's operation. On most every other cooling system the thermostat is on the discharge side of the engine's coolant system going to the radiator. The small amount of coolant the same temperature having little,if any affect on thermostat operation.
 

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Discussion Starter #4
That confirms the reason. Reverse cooling.
 
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