stuck thermostat?

[BerniniCaCO3]

The conclusion seems inevitable, but thought I'd double check.

Last summer I replaced the water pump. At the same time I went ahead and did the thermostat.
On our temp gauges, it read between the O and the R of N O R M A L.
It wouldn't take that long to get there; I'd see it rise past the R, then presumably the thermostat opened, and it would drop to a steady home between the O and R.
By the time I got out of my driveway, down a couple streets, and was accelerating on the on ramp, it would be warmed up.

This May I decided to rebuild the engine for shits and giggles and horsepower, and reinstalled the old thermostat. Correctly, too: bellhousing facing outwards away from the intake/into the hose, and the little valve to let air bubbles out is on top.
It may be important to mention that the old heater core was clogged utterly, and I replaced that, too, while the car was out of comission.
On one of the first test idles, I hadn't added coolant yet, and watched the temperature rise past the M. So I know the temp gauge works.

Now that I'm driving it daily again, and have put 120 miles on the car and the engine, I'm familiar with its current habits.
Now, it takes an eternity to get to between the N and O and never gets higher. I can go 10 miles on the high way and it'll still be at the N.

Sounds like a thermostat stuck open, right?
But it's virtually brand new!
Could just sitting out of the car for a couple months unused have caused it to rust and freeze up, or some such?

I just want to consider whether a now-flowing heater core could make any difference, whether a new engine is just lower resistance and generates heat more slowly, or if affected resistivity to the temperature gauge could cause it to now read a little lower, or whether I could have installed the thermostat in such a way as to stick it open....

All this said, it's not much trouble just to do the blasted thermostat and know I've got a new one again...
Couldn't just be the

[1990LTD]

verify the temp gauge operation. the gauges on these cars are basically idiot lights and not calibrated to anything in particular, BUT it does sound like a stuck thermostat perhaps.


i'd look at the gauge first in case i could avoid pulling the thermostat out. it's not hard on these cars but annoying enough.

[clemford]

stuck thermostat

For the past few years my 90 302 tc my temp gauge has been a notch or so higher. I thought the old original radiator was getting plugged, so i bought a new radiator at advance auto for $83 and it ran the same temp. So i put a cooking thermometer in the radiator and it wont go above 165 degrees in hot florida. So the thermostat i installed in 98 must be stuck open, and gas milage has been crappy around town, 15 or so driving like an old fart.

I hear the mr gasket thermostats stick closed or opening way to hot brand new. So im buying a stant, the premium one, and as usual check it in a pan of water on the stove.

I was never charged the $83 for the radiator on my mc.

[86VickyLX]

Those temp gauges are almost as useless as the lights. A thermostat is cheap and easy to replace. Stant is good. Just take note of how it was situated, which way it was facing, etc. Clean the gasket surfaces thoroughly. Use some RTV along with the gasket. Put a little in the detent on the housing for the thermostat so the thermostat stays in place when you try to put it together. Use RTV to hold the gasket in place while you reassemble it. I use blue RTV. Others use other variants. I haven't had a problem with the Blue stuff yet.

[gadget73]

a lot of the failsafe thermostats with that bleed hole will run too cold if the cooling system is in good shape. I would verify the actual engine temperature though, as mentioned, the NORMAL gauge means almost nothing. It just tells you if its in the very wide range of "won't break anything".

[BerniniCaCO3]

Yeah, it is a failsafe model.

I know the "normal" is hardly precise, but I remember where it used to settle, and I remember how it used to rise to a point then drop down and settle between the O and the R, so presumably that was 195F, and I was watching it heat up to 215, open up, then cool to 195 and stay there.

At present however, it never, ever, heats up then drops down, i.e., it seems like it must just be stuck open.
It'll just slowly warm up. If I'm driving on the highway in the cool evening, it never warms at all, and when I was stuck in traffic yesterday at 5pm with the AC on full plast, it got to the N in normal, as hot as it ever gets now.
Sure sounds like it's stuck open.
Got the premium stant from napa this morning, will put it in and see what it does; maybe only Tuesday though.

Are those "failsafes" known to be fickle and break easily?
It just caught me by surprise, since it is only a year old, and all I did was pull it out and leave it in a box while I rebuilt the engine, then reinstalled it. And a non-clogged heater core now, too; although how that would cause the engine to stay too cold, isn't really a possibility.

[86VickyLX]

Yeah, it is a failsafe model.

I know the "normal" is hardly precise, but I remember where it used to settle, and I remember how it used to rise to a point then drop down and settle between the O and the R, so presumably that was 195F, and I was watching it heat up to 215, open up, then cool to 195 and stay there.

At present however, it never, ever, heats up then drops down, i.e., it seems like it must just be stuck open.
It'll just slowly warm up. If I'm driving on the highway in the cool evening, it never warms at all, and when I was stuck in traffic yesterday at 5pm with the AC on full plast, it got to the N in normal, as hot as it ever gets now.
Sure sounds like it's stuck open.
Got the premium stant from napa this morning, will put it in and see what it does; maybe only Tuesday though.

Are those "failsafes" known to be fickle and break easily?
It just caught me by surprise, since it is only a year old, and all I did was pull it out and leave it in a box while I rebuilt the engine, then reinstalled it. And a non-clogged heater core now, too; although how that would cause the engine to stay too cold, isn't really a possibility.

Air in the system can make it appear to be running colder than it actually is. Does the system get pressurized?

And on the note of failsafes, they like to get stuck open and never close. Fuck them. Get a stant or a motorcraft and be done with it

[Phoenix]

Yeah, I got the same problem you do. Just recently at Scott's John and Scott were trying to get my coolant gauge to read anything other than cold, and we went through three senders (including a brand new one, currently on the car) to no avail. Just like in your case, initially we ran the car with too little coolant and it went down past A in NORMAL, so the gauge works, John also verified this by testing the sender with a multimeter.

It's been about 800 miles since, one BIG effin road trip home and now also some city driving. The arrow, while never even getting to N at Scott's, now resides between N and O once it warms up. I have a failsafe thermostat, a new sender, a working gauge, and a perfect coolant mix... I'm thinking that what Gadget says makes a lotta sense, it simply likes to run on the cold side with everything in good shape and a failsafe thermostat.

[gadget73]

my Mark has a regular 195 thermostat, and it runs between N and O. never verified the temperature but everything seems to work.

[BerniniCaCO3]

OK: found time today to pull the damn thing out and investigate!
Sure enough, it was stuck open.
Damn thing is DESIGNED with 2 barbed tabs that, when the thermostat should open past a certain point, it will get stuck on these and never close again.
Theoretically I could reuse it, having released the plunger (for want of a better word) off of the 1 barb it was stuck open on.

Turns out also that it was a 192F thermostat, and the car had used to hold steady between the O and R (before it failed open). Now I have a stant preminum thermostat (well, napa, but stant makes napa's) in there now,
and it hold steady between the R and M. Take it for what it's worth; maybe one letter = 3 degrees, but probably no such sure thing.

Glad it's running at the right temperature now



Now, it does beg the question... what the f*ck is this failsafe thermostat supposed to accomplish? How do they fail?
Unless there's something more to it than those retaining tabs, there's only one scenario I can come up with, let me know if it makes any sense:
those tabs mean that once the engine has overheated a certain amount, perhaps, when I was running the engine w/o coolant for a few minutes (no, it didn't actually overheat, just the far side of normal),
the thermostat can get stuck open on them once the plunger has opened too far.
Can a thermostat, once overheated along with the rest of the coolant, start to leak whatever builds pressure in the canister to resist the spring and open?
Is a thermostat, once overheated a single time, at risk to fail to open again the next time the engine warms?
If so, then this design makes some kinds of sense of any sort.
If not, then this design was drawn up by idiots.

[BerniniCaCO3]

The surer symptom I had of a failed thermostat, was its inability to stay at a single point, ever. And tellingly, it ran rather cooler still when going at highway speeds at night, than being stuck in traffic in the afternoon with the a/c on full blast, which is as hot as it'll get. The fact that it ran at different temperatures in different driving conditions was a good indicator that temperature was not being regulated.

So Phoenix, if within 10 minutes or so of driving it gets to the N and O and stays there, and it stays there on the highway and in traffic, and at high noon and at midnight, then you should be good.
If on the other hand it reads below the N on the highway and past the N in traffic, etc., then maybe check if your failsafe is caught open also.

I hate that lower bolt on the thermostat housing, hahaha.

[86VickyLX]

Damn thing is DESIGNED with 2 barbed tabs that, when the thermostat should open past a certain point, it will get stuck on these and never close again.

I said that they do that. Maybe not in this thread, but in another thread that I'm sure you posted in. If it overheats, it'll stay open so it'll have the full amount of flow the thermostat to help it cool off. It's a crock of shit. It's not even when it overheats, it's when it gets hotter than the thermostat thinks it should be. Like sitting on the freeway on a 100 degree day, with ac on.