Ecoboost running hot or unusual temperatures

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

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?

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.

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.

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

Thanks for info … and honesty LOL

GlennSullivan said:

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?

Click to expand...

There is only one thermostat. I as many people on these forums 3.5 was running hot. I would pull over and gauge would drop right back down to normal.You could hear the fans running as fast as they can.Most all of the people on these forums had work done on their engines involving replacing timing chains, cam phasers, water pumps etc. Most tried replacing the thermostat again and still had the same issue. Some said it had to be the computer screwing up, some like me sent it to the mechanic and had the radiator replaced still same problem, then I replaced the block sensor still no different I couldn’t even pull my small trailer on flat ground without it running hot. Dealer said everything was fine no codes detected and that’s all these mechanics are trained to do if no codes there can’t be a problem. They for the most part are useless anymore.
The fix is crazy, guys it’s the Thermostat
You can go to Orlielys, Auto zone, etc. They will sell you a the thermostat. The one they sell is not the right thermostat. Not sure why they all sell the wrong thermostat. It has to be a factory ford thermostat for the 3.5 ecoboost engine.
It’s absolutely amazing the whole problem is a simple thermostat.
My hope is this reaches thousands of fed up 3.5 ecoboost engines.
I just happened to run across a video of a mechanic on youtube solving this most aggravating problem.I can pull my pontoon and camper once again and watch the temperature gauge stay right where it should be and not hear the big fans trying their hardest to cool the engine,

My fans are running at full speed most of the time when I'm just driving around town 20-40mph. My temps go from 198F to 220F near instantly and my fans run all the time. Once I come to stop the fans slow down, which I find weird. It's only 40-50F so why would this be running the fans only when moving. AC is off by the way.

Maybe a thermostat issue?

NDspd said:

My fans are running at full speed most of the time when I'm just driving around town 20-40mph. My temps go from 198F to 220F near instantly and my fans run all the time. Once I come to stop the fans slow down, which I find weird. It's only 40-50F so why would this be running the fans only when moving. AC is off by the way.

Maybe a thermostat issue?

Click to expand...

Mileage would be helpful info. Do you have way of monitoring coolant temps? I would start with that. Also check your coolant levels.

JasonH said:

Mileage would be helpful info. Do you have way of monitoring coolant temps? I would start with that. Also check your coolant levels.

Click to expand...

Turned on the option in Forscan to view on my dash. 48,000 miles and it already had the coolant and water pump changed by the previous owner.

FYI if you have your HVAC set to anything windshield related the AC kicks on anyway regardless of AC button unless outside temperatures are around freezing or below. You can check the call by the BCM for AC operation on forscan. And as you are probably aware once AC compressor is engaged, the fans kicks in.

Also in forscan you can set the coolant temp to be displayed numerically above the gauge, few posts in here about it if you search, very useful.

NDspd said:

Turned on the option in Forscan to view on my dash. 48,000 miles and it already had the coolant and water pump changed by the previous

Click to expand...

This is the solution for all the 3.5 Ecoboost engines overheating​

Thermostat - Ford (BL3Z-8575-B)​

This thermostat is specifically designed for these 3.5 Ecoboost engines. The one that the parts stores are selling is not designed for these engines.I searched countless hours, as we all have. I was lucky enough to run across a mechanic on You-tube that figured this problem out. I wish I could remember exactly how I ran across his video. I absolutely take no credit for this solution I am just blessed to have found the answer. As soon as I replaced my thermostat I hooked up to my pontoon and headed to a highway with steep hills. The temp gauge stayed right where it was supposed to be. I just about came to a complete stop at the bottom of one of the steepest hills and took off. The joy was absolutely amazing. After all that time a simple thermostat.

JasonH said:

Mileage would be helpful info. Do you have way of monitoring coolant temps? I would start with that. Also check your coolant levels.

Click to expand...

This is the solution for all the 3.5 Ecoboost engines overheating​

Thermostat - Ford (BL3Z-8575-B)​

This thermostat is specifically designed for these 3.5 Ecoboost engines. The one that the parts stores are selling is not designed for these engines.I searched countless hours, as we all have. I was lucky enough to run across a mechanic on You-tube that figured this problem out. I wish I could remember exactly how I ran across his video. I absolutely take no credit for this solution I am just blessed to have found the answer. As soon as I replaced my thermostat I hooked up to my pontoon and headed to a highway with steep hills. The temp gauge stayed right where it was supposed to be. I just about came to a complete stop at the bottom of one of the steepest hills and took off. The joy was absolutely amazing. After all that time a simple thermostat.

A new Motorcraft thermostat purchased at the Ford dealer solved my very frequent running cooling fans issue. It's worth mentioning that the coolant temp gauge position never changed from the middle after engine warm up. A few years ago, around 40K miles ago, I noticed the cooling fans running more frequently. In a forum search it was said that was part of a software update from Ford. This winter they began running a lot more frequently, even on 20 degree days. No stored codes, shutters open normally, coolant level normal, no heat issues. Long story short, a new thermostat solved the frequent fan running issue. The original thermostat had a rubber coating over the center portion that moves to allow coolant to flow through. The rubber was torn and clearly restricting coolant flow when the thermostat was open.
Why didn't the coolant temp gauge show any indication of the engine coolant temp running hotter? I wish I knew. I'm thinking that all that time the engine was running hotter than it was supposed to.
The replacement thermostat does not have the rubber coating. Thermostat was replaced at 122,000 miles. It was easy to replace.
The thermostat as an issue was mentioned by others in this forum.
Thank you for that!

I just replaced the thermostat in our 2018 Exp with the updated Motorcraft part. I was watching the transmission temperature and it was always over 200* and upwards of 214* or higher when towing our travel trailer. After the thermostat change, transmission temps are an average 20* cooler. 180* to 192*. I have not towed our trailer with the updated thermostat as yet, I am curious to see what the transmission temps run while towing. The updated thermostate is stamped 190*. The old one was 195*.