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1996 Cadillac Deville
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I know these cars blow head gaskets when overheated. Mine gets up to 225 and higher when I turn the engine off. On another car I have the fan stays on for a bit after the car is stopped to cool it down. The Caddy doesn't do that. Is that correct?
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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There's no provision for normal fan operation after Key: OFF.

225 is perfectly OK - the fans don't even go to SLOW until 224 or so. Overheat occurs at 274.

These engines DO NOT "blow head gaskets" when overheated. It was designed to run over 50 miles with NO coolant. The head gaskets fail, primarily in the 1997 - 1999 engines, due to the cylinder block head bolt hole threads pulling out > causing loss of bolt clamping pressure > causing exhaust gases to blow past the fire rings > causing coolant loss > causing overheating.

Our fans go to SLOW at 224 or whenever some A/C function is set and to FAST at 236. If fans start automatically the engine cools to 213, fans shut off, the cycle repeats - summer heavy traffic or extended idling, usually.

Fans don't run at over 30 mph or so and MAYBE after a really hot shutdown - over 250+ or so. The fan circuits are hot at all times but not the ECT (temp sender) circuit.

Make very sure the purge line and its hollow bolt/nipple are clean and flowing a small stream at idle. A clogged purge line is a common cause of Northstar overheating for unexplained reasons.

The entire system operation and temperatures is explained in detail in the Cooling section of the GM/Cadillac service manuals.

Here's your pre-2000 fan circuit -

fan control circuit-early.gif
 
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