Ecoboost running hot or unusual temperatures

[JohnT]

TDLR; 3.5 Ecoboost uses a cold side thermostat, use Motorcraft part number HL3Z8575B

Recently I noticed my temperatures fluctuating a lot more than usual, driving on a straight flat road, no wind, temps would climb up to 107 to 109 (225f to 228f) and I could hear the fan roaring. Then for no apparent reason would drop back to 97 (207f). Slowing to turn off at an intersection the temp would drop to 87 (189f). All good symptoms of a sticking thermostat. I checked my active grills were working, even power washed the radiators inside to out and outside to in. No Change.

Off to NAPA and picked up a Motorad thermostat and a jug of coolant. Swapped the thermostat out and just by squeezing the old one it I could see how it was sticking. The brass cylinder catches on the brass tube.

Drain coolant into a receptacle by loosening the drain plug buried up inside the rear of the radiator support on drivers side. Its red so a flashlight will help locate. Loosen the three band clamps for the air intakes to the turbos and move Y air pipe out of way to give yourself room and undo 3 bolts holding the thermostat housing. Drop one bolt into the bowels of the engine bay and spend longer retrieving it than the rest of the job took.

Make sure jiggle valve is at top!

Reassembled and refilled and tested for leaks and off for a drive. It was worse. Now temps were routinely in 105 to 110 (221f to 230f) range no jumps or resets and slowing down were getting as low as 87 (189f)!!!
And in city driving temps were constantly swing up and down by 10c degrees.

Did some googling and was verifying jiggle pin location (12 o'clock versus 6, 12 o'clock is right) when I stumbled on a thread in ecoboost forum on another site where someone mentioned these engines use a cold side thermostat. I did not know such a thing existed. But armed with a new keyword to search for low and behold its a thing and a known issue with many others suffering similar symptoms using a normal hot side thermostat instead of the proper Motorcraft cold side thermostat.

A quick jaunt to my local Ford parts place and 61 CAD (45USD) later for a thermostat and gasket and a passing comment from the parts guy that the sticking thermostat was a known problem and Ford had redesigned the thermostat. The new one has a plastic cylinder (possibly Teflon?). Apparently some Ford shops are guilty of using the wrong thermostat too when doing the cam phasers work if they don't have a Motorcraft one in stock.

Redid all the previous work minus dropping the bolt but this time trying to refill the coolant with the drain plug open

And problem solved. Rock solid 95 to 97 driving around and back to normal stable temps. Motorcraft part number HL3Z8575B

Hope this helps someone else

 

[GlennSullivan]

Are there 2 different thermostats installed at the same time or is this "cold side thermostat" an upgraded part or one used in different production years?

 

[JohnT]

Only one thermostat, it was always a cold side thermostat on these engines. The upgraded part bit is that the factory thermostat had a habit of sticking after a while so , I believe, since August 2022 a revised thermostat is now used with the plastic part.

But any thermostat you buy from napa, rock auto etc will be a hot side thermostat and sold as suitable for this engine. It isn't. Reische claim their one is right, your mileage may vary

Best description I found was this.............. (note the bottom pic is the original Motorcraft thermostat, the upgraded Motorcraft part looks similar but the bottom brass portion is now black plastic)

Posted this in another thread but thought you guys might be interested:



So in the hot-side system the thermostat is placed pre-radiator: coolant flows in from the side (GREEN/RED) and, depending on it's temp, coolant is directed either up to the radiator to be chilled or if the coolant is not hot enough, the thermostat stays closed and the coolant is directed down through the bypass to recirculate. Of course the thermostat will often run partially open, directing a little coolant to both paths.



In the cold-side system coolant flows the opposite direction and the thermostat is post-radiator: Unlike a hot-side thermostat, the bypass control (brass bottom) is not attached to the upper assembly/diaphragm and does not move when the thermostat opens. It maintains contact so the bypass coolant can only flow up through the inside of the pipe, keeping the heat motor in direct contact with the bypass coolant temperature so the thermostat can function properly at all times. When the bypass coolant is hot enough the thermostat opens (bringing in chilled coolant from the radiator - BLUE); this also pushes the heat motor further down into the pipe which closes off the holes in the pipe for the bypass (RED) and shields it from the chilled coolant. However a small amount of bypass coolant is allowed to flow across the heat motor at all times so it can still monitor and respond to changes in the coolant temp.

A hot-side thermostat can function in a cold-side setup to a large degree but there are some drawbacks: When the thermostat opens some of the chilled coolant will mix in and expose the heat motor to cooler temps then it should be seeing to operate properly. Cooler temps then force the thermostat to start closing prematurely and once this happens, the hot bypass coolant will then start to make it open back up... and the cycle repeats. Another issue is the thermostat will always struggle to stay fully open because as soon as the bypass is completely shut off, the heat motor will no longer be exposed to hot coolant, forcing it to close again. The LMS/Even Flo thermostat attempts to address this by placing 4 holes in the bypass valve but then you always have a good amount of coolant recirculating through the bypass instead of being directed through the radiator as it should be.

In my own real world testing with a hot-side thermostat in a cold-side application I found the primary drawback was the way it handled changing conditions. You could see a nice stable temp cruising down the freeway but as soon as you exited and started slowing down to a stop the temp would immediately spike up and it took a little time for it to re-stabilize and cool back down.

 

[GlennSullivan]

Very, very helpful, thank you for taking the time for the detailed explaination with graphics, makes perfect sense. Do you, by chance, know what year Ford introduced the cold side thermostat to the 3.5TT Expedition and if this new upgraded model T-Stat is recommended for all years 3.5TT. I have a 2017 and am not having any issues, but would like to know if they are recommended for that year.

 

[JohnT]

Sorry I dont know when it became a thing. Not my graphics and blurb, copy pasted from ecoboost forum. Maybe someone there might know.

Where I stole from

But judging by posting date since always

 

[GlennSullivan]

Thanks for info … and honesty LOL