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4.8L V-8. End of last winter, warning light came on saying "Engine Coolant Display Defective", at which point the system totally disabled the Coolant Temp displays and signaled a CEL. Once I stopped, I turned off the car, started it back up, and the display worked and the temperature quickly went to 201F. This seemed like a faulty temp sensor so I had it replaced by an indie shop for $2500 since they had to remove the intake and more to do it. Within 15 minutes the same warning came back on; I was a bit miffed. Shop read the code this time and said that it was an "improbable temperature" or something, meaning that the temperature did not heat fast enough for that amount of time running. They said that it was likely the thermostat sticking open and it would cost another $3500 to fix it! Seems they were just right there a minute ago...
Anyway, that being the case, I just soldiered through to the warm weather (just about a week), reset the codes, and drove the entire summer with no issues and the engine temp warming normally to 201F. Now that the cold weather is back, the problem is back. The Porsche dealer says that it MAY BE the thermostat and wants to change it for $1500. I have serious reservations about this because of this odd behavior: The temp seems to get stuck between ~145-150F for a long drive time, like a stuck thermostat. Eventually, the warning light comes on and shuts down the temp display, cooling fans go on max. Turn the car off and back on and the temperature very quickly climbs to 201F and everything is normal. Is this not a mechanical thermostat? What the heck is going on here? It sounds more like a software issue than a mechanical one.
I would love to hear a description of this cooling system. I'm an OG and used to changing a thermostat with 2 bolts and a hose clamp.
 

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1 - Find a new shop. $2,500 to R&R the intake? It takes about an hour to remove (working slowly) and an hour to replace. The sensor took perhaps 15 minutes to replace. Even figuring $250/hour rate - what did they do for the other 7.5 hours they charged you for? And then $3500 for a thermostat? They're robbers. Get your car away from there.

2 - The thermostat is mechanical with an electronic temperature controller. It has a heating coil around the wax pellet that opens the thermostat. It sounds like your thermostat failed open. Even the dealer is over-charging you at $1,500, but at least they'll give you a nice coffee and a loaner car.

Changing the thermostat is somewhat more complex since it's mounted behind the water pump, and the intake and water pump come off in order to replace it. It sits in a thermostat housing, and depending on the year of your V8 it might be worth replacing the housing if it has glued-in pipes (the new design has bolted-in pipes). A new thermostat housing is around $400, and I believe with a new housing, thermostat, and water pump I was out of the dealer for less than $2,500.

I assume OG = Old Guy - a lot of us are, but we have to adapt to current times. No use crying over complex designs. If you don't like them, for about the same as a Cayenne you can probably find a fully restored 1960s US car... where 2 bolts and a hose clamp and you can do the thermostat.

BTW - when you have time, please visit New Member Introductions and introduce yourself and your Porsche. It's the friendly thing to do, and we're the Friendly Porsche Forum..
 
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