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Discussion Starter · #1 ·
2011 F-250 6.2L V8 here.
I just clued in the oil temperature gauge always shows ~60% between min/max, even when the engine is obviously stone cold (like immediately after starting engine when it sat all night). I like using remote start to warm up the truck so took me a while to notice, not sure how long it's been doing this. The coolant temperature responds as expected though - gauge starts moving after 5-10 mins of driving and stays on the low side, also not towing or hauling anything heavy so I'm not too worried about anything overheating.
My first thought is a faulty oil temperature sensor? Then I remembered about a month ago I got this error code (before I noticed the oil temp gauge acting up) could it be related?
P0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Haven't seen it again after resetting, but replacing the thermostat valve has been on my to do list.

Would appreciate any insights.

(tried the search but only finding threads about oil temperatures while towing)
 

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I can only address the P0128 code. You don't say how long you let it warm up, or what the air temps are in your area are. With a long warm up and unloaded driving, the engine coolant will not get to the temperature required to open the thermostat for a long time. The time will exceed what is preset in the system, therefore the light. Once you get it to get hot a few times, the code goes away; as it apparently did. My engine will do the same thing since it is only 12 miles to work and in the winter the ECT may never even get to 180. I put a sheet of cardboard in front of the radiator to reduce cooling air flow.

If you decide to pull the thermostat, test it by putting it in a pan of water and letting the water heat until the thing opens. Check the temp it opens at. Check the new one before installing.
 

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Discussion Starter · #3 ·
meteor thanks for the info/tips.
Actually I saved the freeze frame from the P0128 code:
Time since engine start = 14min, coolant temp 76C/169F and ambient temp 7C/45F.
Speed = 60mph, 1675rpm, calculated load = 93.7%, 16.9% relative throttle.

The "thermostat housing" (90 degree elbow with a flange at one end) looks like it would be mounted on the block, not at the rad or in the hose, right? Hopefully it's not buried too far..
 

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Yep, what you have is simply too long at too low ECT. It's better for the engine to run warmer, but not cause for panic.

The thermostat will be directly underneath the triangular flange. I always recommend OEM replacement, but that's because the gasket on the OEM is the only one I've found that seals properly (my engine is different).
 

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Remember that the thermostat has a bypass valve on the bottom side. When it's closed, coolant bypasses the radiator and circulates back thru the block and heads. As the coolant warms up to operating temp, the thermostat opens and allows coolant to flow thru the radiator. At the same time, the bypass valve closes off the return. Most of the time, the thermostat fails cold. The rubber seal falls apart and jams the thermostat open allowing coolant to flow thru the radiator. When that happens, the engine can't get up to temp and stay there.
 
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