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
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