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#4526978 - 09/26/17 06:36 PM Sticky/Lazy thermostat or...?
SirTanon Offline


Registered: 06/23/14
Posts: 2158
Loc: Phoenix, Arizona - USA
Hoping for some input here. I have my suspicions, but I'm not an expert on cooling, and I'm not a professional mechanic, so I thought I'd see what others have to say.

This is for my 2010 Fusion with the 2.5 Duratec. I use Torque Pro on a daily basis on my Galaxy S3 'car dash' to monitor everything so I notice when things seem 'different.' The thermostat in this car is a 190F thermostat, and from what I've read, it's supposed to be completely open at ~208F. The coolant system is 'reverse flow.'

Okay, so I drive a lot. Most of my driving is highway, and I commute 55 miles each way, about 52 of which is highway. I see some rush hour bumper-to-bumper traffic regularly, but most of the driving is pretty clear.. usually cruising between about 70-75 mph. Typically, during highway cruising, I see coolant temps fluctuate between 194 and 201 degrees F. There are times it seems to hover lower (186-192), and then other times it hovers up higher (203-206).. doesn't seem to follow any real rhyme or reason.

Today, on my drive in, ambient temps were between about 70-72F, and my drive was pretty standard.. and shortly after I got onto the highway, coolant temps got up in the 203-204 range, and didn't seem to want to come down from there, whether I was going 75 or slowing down to merge, etc.. Then, at one point, I hit an incline with some traffic, and things came down to a crawl. When I hit the top of the incline, the engine was at 208.4F, and traffic opened up (This is normal in this spot - 101 loop north @ Cave Creek road.. there's a backup because of the exit). So I got to the top of the hill, traffic opened up, and I started to speed up again.. and within about 40 seconds, the temps were suddenly down to 183F.. and rose back up and stayed between 186 and 197 until I got off the highway about 12 miles later.

There are also those days when the same trip never sees the coolant get over about 190-192.

I know the way these thermostats are built, when they fail, they're supposed to fail open.. resulting in a 'temp below operating range' code.. but this is not what's happening.

My gut says that the thermostat is either sticking, like the spring is binding, or the shaft/guides are catching.. or it's grown 'lazy'

At one point, I even thought that perhaps some kind of foreign material had come loose inside the cooling system and was occasionally lodging itself in the thermostat or some other passage.. although this seems unlikely, as anything big enough and solid enough to cause this would probablyl also either sieze up the water pump, or lodge for good in a smaller passage somewhere.

What does everyone think?
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2010 Ford Fusion SE - 2.5 liter/6F35 Trans - 257,000mi
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#4526986 - 09/26/17 06:43 PM Re: Sticky/Lazy thermostat or...? [Re: SirTanon]
Kibitoshin Offline


Registered: 04/19/16
Posts: 696
Loc: Kaiōshinkai
I had a thermostat stuck shut on a Lexus LS430. The e-fans were running at full blast continuously even when driving at high speeds. Pulled the thermostat and placed it boiling water to confirm and yep t-stat was stuck shut.

If the t-stat is easily accessible I would remove it from the vehicle and test it to make sure it fully opens.
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#4526987 - 09/26/17 06:43 PM Re: Sticky/Lazy thermostat or...? [Re: SirTanon]
SatinSilver Offline


Registered: 01/24/08
Posts: 10179
Loc: OH
Yes, change it with a new oem one. They're probably designed to last 150k miles or so. On my 99 Accord that had 166k miles when sold, I think it had a lazy t-stat. Talking to a master tech right after I sold it he would have changed it around the 150k mark or during the timing belt service. Cheap insurance esp if it's easy to change. Don't forget PCV valve as well even if it still rattles.

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#4527000 - 09/26/17 06:51 PM Re: Sticky/Lazy thermostat or...? [Re: SirTanon]
Warstud Online   content


Registered: 09/02/05
Posts: 5402
Loc: MIchigan
Don't expect it to fail in the open position. With it being almost 8 yrs old it would be good preventive maintenance.

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#4527013 - 09/26/17 07:01 PM Re: Sticky/Lazy thermostat or...? [Re: SirTanon]
ArcticDriver Offline


Registered: 01/27/17
Posts: 1139
Loc: USA
The only testing I ever do on thermostats is to see if they go "clunk" when I throw them in the garbage can.

Seriously, if you are going to take it out and crudely test it in your kitchen, then you are tsking the same amount of effort as installing a new one.
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#4527023 - 09/26/17 07:13 PM Re: Sticky/Lazy thermostat or...? [Re: SirTanon]
eljefino Offline


Registered: 06/15/03
Posts: 32651
Loc: ME
I'd replace it and flush the radiator... nothing drastic but a couple gallons of distilled water.

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#4527024 - 09/26/17 07:14 PM Re: Sticky/Lazy thermostat or...? [Re: Kibitoshin]
SirTanon Offline


Registered: 06/23/14
Posts: 2158
Loc: Phoenix, Arizona - USA
Originally Posted By: Kibitoshin
I had a thermostat stuck shut on a Lexus LS430. The e-fans were running at full blast continuously even when driving at high speeds. Pulled the thermostat and placed it boiling water to confirm and yep t-stat was stuck shut.

If the t-stat is easily accessible I would remove it from the vehicle and test it to make sure it fully opens.


Considering how the thermostat mounts on this engine, and the work required to get to it, not to mention the likely age and mileage on it, if I'm going to remove it from the engine, there's no way I'm putting it back on. If it comes off, a new one is going on.

I might test it after I have it off, just to see how bad it actually was, but that's as far as I'll go with that. A brand new Motorcraft RT1193 thermostat is only $35 from Rock Auto.. and if I really wanted to cheap out, I can get a Gates thermostat, which is still far better than the used one that's on there now, for less than $18. At that price, you don't even question whether the old one is worth testing.
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2010 Ford Fusion SE - 2.5 liter/6F35 Trans - 257,000mi
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#4527126 - 09/26/17 09:08 PM Re: Sticky/Lazy thermostat or...? [Re: SirTanon]
zrxkawboy Offline


Registered: 11/30/04
Posts: 1761
Loc: SD
Originally Posted By: SirTanon

My gut says that the thermostat is either sticking, like the spring is binding, or the shaft/guides are catching.. or it's grown 'lazy'


This would be my guess, too.
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#4527143 - 09/26/17 09:37 PM Re: Sticky/Lazy thermostat or...? [Re: SirTanon]
zorobabel Offline


Registered: 01/27/11
Posts: 479
Loc: Anaheim, CA
I'd be more inclined to leave the old OEM t-stat in, than replace it with a Stant.
Since you're always monitoring the coolant with the Torq, leave the t-stat alone for now, you're worrying too much about 10F.
If you want to replace the t-stat with an OEM as PM, go ahead.
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#4527279 - 09/27/17 05:53 AM Re: Sticky/Lazy thermostat or...? [Re: zorobabel]
nwjones18 Offline


Registered: 11/30/09
Posts: 1052
Loc: Warner Robins, GA
Originally Posted By: zorobabel
I'd be more inclined to leave the old OEM t-stat in, than replace it with a Stant.
Since you're always monitoring the coolant with the Torq, leave the t-stat alone for now, you're worrying too much about 10F.
If you want to replace the t-stat with an OEM as PM, go ahead.


The OEM thermostats I've replaced lately (ACdelco and motorcraft) have had the Stant S right on them.. On the acdelco it did have a wobble/bleeder valve in it that the superstat doesn't, but other htan that i'm sure it was just a normal superstat.
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#4527289 - 09/27/17 06:23 AM Re: Sticky/Lazy thermostat or...? [Re: zorobabel]
HoosierJeeper Offline


Registered: 11/23/16
Posts: 1276
Loc: WI
Originally Posted By: zorobabel
I'd be more inclined to leave the old OEM t-stat in, than replace it with a Stant.
Since you're always monitoring the coolant with the Torq, leave the t-stat alone for now, you're worrying too much about 10F.
If you want to replace the t-stat with an OEM as PM, go ahead.


x2. I've read a lot of bad things about the Stants. Wouldn't run one myself.

Those temps don't sound out of the norm. My Cherokee has a digital display for the engine temp and on a normal day, it'll range from in the 190s to 213, maybe even 221 if it's real warm out. Normal. I'd be more inclined to think it were stuck open if you got the code or even if it just topped out at 185 or something. If it were closed, I think you'd be overheating in a bad way.


Edited by HoosierJeeper (09/27/17 06:23 AM)
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#4527321 - 09/27/17 07:23 AM Re: Sticky/Lazy thermostat or...? [Re: SirTanon]
Miller88 Offline


Registered: 04/09/08
Posts: 12704
Loc: Onondaga County
Replace the thermostat. Which is a major PITA on the Duratec 4.

I had similar issues on my focus combined with no heat in the winter from it being stuck open. I decided that pulling the intake manifold, radiator fans, PS pump and belt were too much for me to tackle in the apartment and it went to a mechanic.
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#4527348 - 09/27/17 08:09 AM Re: Sticky/Lazy thermostat or...? [Re: SirTanon]
SHOZ Offline


Registered: 06/28/03
Posts: 5750
Loc: Illinois
I changed two lazy OE t-stats this year. The symptom was loosing temperature when coasting. Sometimes dropping below the opening temperature rating.
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#4527510 - 09/27/17 11:34 AM Re: Sticky/Lazy thermostat or...? [Re: SirTanon]
SteveSRT8 Offline


Registered: 10/10/08
Posts: 18958
Loc: Sunny Florida
IMO t-stats really do have a personality. Two identically rated units may produce slightly different operating temps in the same car. And the way they open and close can differ even more, especially the time it takes them to fully open from a cold start.

I run a Motorad 176, it gets my sig car just hot enough to avoid codes on all but the coldest days, and keeps it cool enough to avoid major KR.
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#4527665 - 09/27/17 02:54 PM Re: Sticky/Lazy thermostat or...? [Re: SirTanon]
SirTanon Offline


Registered: 06/23/14
Posts: 2158
Loc: Phoenix, Arizona - USA
I was actually very close to buying a MotoRad thermostat assembly from RockAuto, but according to RA, the MotoRad opens at 180F, and the Motorcraft one opens at 190... Also, the pictures RA lists for the MotoRad stat shows the retainer is plastic, while the MC one is metal.

Yes, the MotoRad one is about $20 cheaper, but give the two differences above, I didn't feel comfortable enough about it. Plus, the MC one should last another (at least 5-7 years), so the extra $20 seemed like a no-brainer. Easy, cheap, insurance.
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2010 Ford Fusion SE - 2.5 liter/6F35 Trans - 257,000mi
2014 Nissan Altima SL - 2.5 liter/CVT - 55,000mi

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