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Discussion Starter · #1 ·
I recently installed a new 195 thermostat which came included with a Hughes Plenum kit. Now my truck ('99 1500 5.9) only reaches an operating temp of 140 F. Dashboard gauge and infrared thermometer both read 140.

When I was draining the coolant for the Plenum job it came out clean.

Is there anything else I should check before I come to terms with the fact that I got a bad thermostat out of the box?

Could I have installed the thermostat wrong? it is my understanding that if you install the thermostat upside-down then your truck immediately overheats.

Thanks,

Filth
 

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yeah that seems odd. Truck should be getting significantly hotter than that. How long are you letting it run? How cold is it outside?
 

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Discussion Starter · #3 ·
Today I let it run 15 min before I used the infrared thermometer. It's about 30F outside.
 

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Ok you need to run it longer than that probably, in that temp anyway. Are you just letting it idle in the driveway, or actually driving it? On a cold start and idling in the driveway, my truck will not get up to operating temp in 15 minutes, even in florida summer heat.
 

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Discussion Starter · #5 ·
Hmmm... I'll go check again. right now.

Also, should the fan run continuously? or should it cycle on and off?
 

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Discussion Starter · #6 ·
Alright, I just let the truck run for another 30min. It is not reaching full operating temp. Just as when I drive to work, the dash indicates that the temp only gets to ~150 and a hand held thermometer reads 140.
 

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Ok, so you are in fact actually driving it then? On a 15 to 30 min drive I'd think it should get up to temp.

Do you have an electric fan? If you have a clutch fan it will always spin, but will only have the clutch engaged above a certain temp(around operating temp).
 

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Discussion Starter · #8 ·
Yes, in all cases, the truck does not reach 195F
 

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You have a factory clutch fan, correct?

The tstat could be bad, maybe stuck open or something.
 

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Discussion Starter · #10 ·
Yes I have a factory clutch.

I'm almost positive that I have stuck thermostat. I'm just wondering if I should check anything else before I replace it. Or, if there is any way to free a stuck thrermostat with out removing it.
 

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Stop second guessing yourself and go after the stat. Your suspicion about an upside down stat causing an almost immediate overheating condition is correct and not getting up to temp implies that the stat is stuck in the open position.
 

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Discussion Starter · #12 ·
Thanks for the direct answer!

I'm thinking I must have gotten some old gasket material into the cooling system while I was installing the new intake gaskets. That's why the new thermostat stuck open so quickly. Maybe it was bad out of the box, and I just didn't notice until the temp dropped this past week. Or, maybe the gasket crud has been slowly getting caught in the tstat over the past month.

I'm going to flush the system and replace the thermostat.
 
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