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Discussion Starter #1
’99 Eldo ESC with 82k miles -new radiator, surge tank. This past weekend 1 hour on Interstate traveling at 70 mph – msg came up -engine hot -idle engine or something similar – the temp gauge slowly moved from middle 1 notch forward to HOT, then within a few minutes returned to middle. Happened several times – wanted mechanic to replace temp solenoid (on the engine) -but resists saying never had to replace one in 20yrs as Cadillac mechanic :(
 

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-Administrator- 2002.5 F55 STS 2014 FWD Explorer
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The chances that the ECT sensor (coolant temp sender) is bad are slim and none.

New radiator and surge tank = recently drained system. The Northstar cooling system is very intolerant of air locks in the heater core circuit - a distinct possibility in this case, and that circuit is a main player in correct cooling system operation.

Study all the threads on Northstar cooling/head gasket problems in the Engines; Northstar forum. There are many checks to perform in order to pin down an intermittent overheat situation, beginning with the coolant purge line to the water pump drive belt system to the thermostat to performing a cylinder block exhaust gas test.

https://www.google.com/search?q=amazon+lisle+75500+test+kit&rlz=1C1NHXL_enUS730US730&oq=amazon+lisle+75500+test+kit&aqs=chrome..69i57.12788j0j8&sourceid=chrome&ie=UTF-8

Harry's cryptic post refers to the sticky thread poll on Northstar head gasket problems - in the Engines; Northstar forum - recommended reading, as with any sticky in any forum of interest.
 

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Being a '99, I'd do a block test after an overheat episode before replacing anything.

Assuming that your purge line is clear and flows coolant, any air left in the system after cooling system work should purge itself in one or two drives.
 

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Discussion Starter #6
Thanks for replying Harry -not sure what you mean about -"be aware you have a 99"-
re coolant level -the poorly designed surge tank -makes it difficult to check coolant level-shame on Cadillac
 

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-Administrator- 2002.5 F55 STS 2014 FWD Explorer
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Read Posts # 3 and #4 again - the 1999 Northstar was one of the worst years for failed cylinder block head bolt hole threads. If you don't study the stickies in these forums you don't have the big picture.

Block metallurgy and head bolt threads/lengths were changed for the radical redesign 2000 and later engines.
 
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