The thermostat would not work if coolant entered the engine from the bottom radiator hose.

 

These engines have "cold side" thermostats. The coolant enters the block at the bottom where the thermostat is located.

 

If that was true, the thermostat would close, as soon as it opened from the "cold" coolant

 

My bad! I had never heard of a cold side thermostat: https://www.ecoboostperformanceforum...p?topic=2490.0

 

This is in fact exactly what happens. The thermostat does a lot of partial opening and closing before it fully opens with a cold side configuration.

 

Essentially, it’s probably not a good idea to run a 170° thermostat without a tune, correct?

Also, in reading through the warranty document… Changing to a performance/aftermarket thermostat does not void your factory warranty, or did I read this incorrectly?

 

Yep...pure analog!!!

 

Resolved this on my truck (2018 F-150 Raptor) today!

Short Version: Removed and replaced the thermostat with a new OEM thermostat (Motorcraft 195 deg), in accordance with Ford's workshop manual.

Long Version:
1. My questions were: "Why are temperatures spiking AFTER sustained applications of boost from the turbochargers and not DURING?; Why is the temp spike so repeatable by applying atleast 5-10 seconds of boost, then abruptly letting off of the throttle?"

2. From studying the system, I learned how the coolant flows from the coolant pump to the engine block, heater core and turbochargers before being sent to the radiator. The flow of newly-cooled coolant being sent to the engine is regulated by the thermostat. Importantly, the coolant pump is engine driven which means that depending on pump design, pump output could correlate to engine RPMs.

3. This means that one possibility is: As the turbochargers introduce a large amount of heat into the system, the cooling system is able to compensate because engine RPMs are able to sustain a certain cooling pump output. As soon as the throttle is released, engine RPM decreases, and pump output decreases leading to a spike (increase) in coolant temperatures. I tested this in the truck by spiking the coolant temps with a sustained boost application, then aprubtly downshifting to decrease boost and increase engine RPM (without increasing airflow) - observations from this test showed that coolant temps were able to be easily decreased by increasing engine RPM (pump output). This means the cause of spiking coolant temps is directly related to either a pump output flow malfunction or an obstruction of coolant flow in the system.

4. The following led to a decision to replace the thermostat as a start point of troubleshooting:
- Inexpensive and relatively simple to remove and replace.
- Symptoms had a slow onset, repeatable AFTER high heat loads from the turbochargers, and no abnormal temps were noted during idling or no-boost operations - likely not a blockage in the radiator as radiator issues would likely manifest DURING heat loads.
- With throttle application / high RPM, the coolant pump could be overcoming a restriction in flow caused by the thermostat not fully opening to allow cold coolant to the engine. Conversely the thermostat could be allowing too much hot coolant to bypass back into the engine which is being compensated for by pump output. Either of these would explain the symptoms.

5. Carried out the thermostat procedure per Ford's workshop manual:
A. Drain coolant via the radiator drain valve. OBSERVED LARGE AMOUNT OF WHITE PARTICULATE SEDIMENT IN COOLANT - at 40k miles on OEM Motorcraft Orange Coolant. Drained ~1.2 gallons.
B. Remove and replace the thermostat and gasket with OEM Motorcraft replacement. OBSERVED SUBTLY WARPED INNER SEAL, AND ASYMMETRICAL WEAR MARKS on bypass valve - thermostat was likely no longer seated properly and allowing too much hot engine coolant to bypass!
C. Fill the system via the de-gas bottle either with a vacuum filler or pour method. I used Motorcraft 50/50 Prediluted Yellow Coolant without a vacuum filler. I followed the bleed procedure precisely: Involves cycling the engine to 3500 rpm for 30 seconds, 30 seconds idle, 1 minute off for a total of 10 TIMES.
D. Test drove the truck and observed no more issues with coolant temp spikes. Temp remained 200-215 on average, and repeatably peaked at 219 after high turbo loads. (Previously would peak at 234).

 

I think it's not a good idea at all. The thing is meant to run at the temps it runs at, and other things, for example the transmission and oil, have temps that are heavily influenced by the coolant temp and shouldn't be run colder full time. Besides, the grill shutters and fans do an awful lot in determining the operating temperature.

There's no such thing as "doing X voids the warranty". If Ford can prove (or claims and your lawyer can't show otherwise...) that your modification caused the problem, they don't have to cover it under warranty.

 

I know some had no luck with replacing the thermostat... my thoughts are:
1.) Did you do the work yourself or have a dealership do it? If a dealer did it, did they provide you with the old thermostat? I have had dealerships tell me they did the work and it never actually happened.
2.) Was the Ford procedure followed? Corners could have been cut to include: Replacing the thermostat without draining the fluid via the radiator. Not using a vacuum fill tool or not performing the bleed procedure 10 times.
3.) Was the fluid drained and then REPLACED and not REUSED? After finding so much sediment in my OEM coolant, the old coolant could have been a contributing factor.
4.) If all of these were followed, I imagine your next components to troubleshoot or replace would be the radiator then the coolant pump.