correct me if I am wrong. Isn't the function (Proper function) of the thermostat to stay closed untill the coolant temp in the ngine reaches the set temperature(Say 195), then to slowly open& close PARTIALLY to keep the coolant at that temperature ? The last 3 stant thermostats I've bought have 2 settings, full open & full closed ! The temp in the engine goes from about 200 drops to 180, back to 200-220, back to 180 constantly, never really gets to 195 & stay there. This cannot be a correctly functioning thermostat, Can It ? Thanks

Your understanding of the thermostat's correct function is 100% right. If you are having temperature surges, you are most likley low on coolant. Just to be sure, you should probably check/change your radiator cap, and confirm your not dry in the reservoir when its cold, and not over flowing when at temperature.

There is no reason your temperature shouldn't stay in a narrow band for the most part. You shouldn't see any drastic changes at the gage, with exception to the first time the thermostat gets to temperature, there tends to be one quick drop, then it should stabilize.

If you are seeing drastic swinging of the temperature gage, its almost always low coolant. Thermostats tend to fail either open (never reaches temperaute for long) or closed. (very fast overheating)

Fiero's tend to get up to creep up to 220 if sitting still, then it should slowly drop to 195-205 when the fan kicks on.

It is closed most of the time. ALL the way closed. the coolent (water) takes time to gain or loose heat. the T-stat makes the water DWELL in the engine and the radiator. If it just runs through, it will over heat. When the water get hot enuff to open the T-stat the water pump blows it right out. this moves the HOT water to the radiator. This push's the cooler water in to the block,heads, T-stat housing. When the cooler water hits the T-stat it close's, stops the water flow! water in engine gets hotter, water in rad gets cooler.
you will see the temps change if you have a good temp sender/gage.
in hotter weather you need a higher temp T-stat to make the water stop in the block. and the rad. I have to restrick my heater hose to 1/4 inch or less to make the water go to the rad in stead of the heater core.

What do I know? I live 6 blocks from the gulf in miss.
V8 fiero, big block vet, 72 vega.

"Your understanding of the thermostat's correct function is 100% right. If you are having temperature surges, you are most likley low on coolant"

It's also possible to get a faulty thermostat. It's happened to me twice in the last 3 years.

[This message has been edited by weaselbeak (edited 01-31-2013).]

or an 'air bubble' in the cooling system. Follow Ogres instructions to bleed it.

Bled it 4 times, even jacked up the rear. I Too have brought back 4 stant thermostats in the past 3 years ! overflow tank if full to mark (It never seems to go into the tank), just changed the stant Cap. shut it off & it either pushes water or air (humming sound)out of the cap, Not into the tank, actually past the outer seal of the cap. but has never gone above 220. I know it sounds like AIR, But after burping it 4 times....

quote Originally posted by Lou6t4gto: Bled it 4 times, even jacked up the rear. I Too have brought back 4 stant thermostats in the past 3 years ! overflow tank if full to mark (It never seems to go into the tank), just changed the stant Cap. shut it off & it either pushes water or air (humming sound)out of the cap, Not into the tank, actually past the outer seal of the cap. but has never gone above 220. I know it sounds like AIR, But after burping it 4 times....

Possibly a dumb question, but if you replaced the cap Did you use the right one in the right place? Wrong one up front lets air in I think.

Here is a datalog that I took just recently. Notice the coolant temp line that fluctuates up and down from around 195 to a few degrees over 200.

quote Originally posted by Lou6t4gto: overflow tank if full to mark (It never seems to go into the tank), just changed the stant Cap. shut it off & it either pushes water or air (humming sound)out of the cap, Not into the tank, actually past the outer seal of the cap.

Something is definitely wrong.

It is normal operation for coolant to go back and forth into the overflow tank everytime the engine reaches operating temperature and then cools down. That's why there's a "Cold" and "Hot" level indicated on the overflow tank.

I suspect you've got an incorrect (or faulty) rad cap.

quote Originally posted by Fierobsessed: Your understanding of the thermostat's correct function is 100% right...

Sadly no...
Normal Tstat will perform like "The last 3 stant thermostats I've bought have 2 settings, full open & full closed ! The temp in the engine goes from about 200 drops to 180, back to 200-220, back to 180 constantly..." or like image above...

IOW... Stat will close until the engine warms then the Stat will snap open then slug the engine with cold water and closing the Stat again. Depending on weather this can be repeated the whole time the engine is on.

Remember Fiero cooling use 2+ times most cars uses... Can be hard to get stable temp with 3.5 gallons of coolant.

You may have others problems too... Air in system, heater w/ problem, etc... but this is normal w/ good cooling system, especially in cold weather.

Use Stant SuperStat to get more stable engine temperature. 195° use Part # 45819
See my Cave, Thermostat and the rest of cooling section.


------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

[This message has been edited by theogre (edited 01-31-2013).]

Last night I took out the 195 stat and put in a 180 I had. the brand new cap that was still leaking, I cleaned the radiator neck & the OUTER seal on the cap, then put a real light coat of permatex gasket maker on Both surfaces & re installed. Today my wife reports that the car stayed below 160, when she got home it hadn't leaked a drop ! usually when she pulls in at the end of the day the car would immediately upon shutoff, Leak. Now no leak.

Stant now has a good FAQ... see ABCs of Thermostats and Frequently Asked Questions

quote Originally posted by theogre: Stant now has a good FAQ... see ABCs of Thermostats and Frequently Asked Questions

Actually, the thermostat IS suposed to be an analog control, not an OPEN / CLOSED operation.

As described on the Stant pages for which you provided links:

From the ABC's page:

quote Thermostat Temperatures ■ Thermostats have a “rated” temperature such as 180F or 195F ■ This is the temperature the thermostat will start to open, give or take 3 degrees ■ The thermostat fully open about 15-20 degrees above its rated temperature

And from the FAQ

quote When does a thermostat start to open? The thermostat starts to open at the rated temperature, plus or minus 2 degrees (F). When is a thermostat fully open? The thermostat is usually fully opened at 15 - 20 degrees (F) above the opening temperature.

The thermostat does partially open but for the most part it's what we call "bang bang" on control theory where it's full on and then full off.

Either way the coolant system has to be able to actually cool the engine. If you have a problem somewhere else the thermostat can be wide open and the engine still overheat.

I have a 160F in my car which stays wide open and Yes I am in closed loop. That way if my temperature creeps up I know that something is wrong.

quote Originally posted by FTF Engineering: Actually, the thermostat IS suposed to be an analog control, not an OPEN / CLOSED operation.

Technically true but Fiero and others normal Tstat open/closes too much/fast/etc and slugs engine w/ cold coolant driving in cold weather. Often can see this on dash gauge and in ECM data streams.

Thermostat, sensors, even engine has some lag time to respond to coolant temp. The Tstat is always playing Catch up to cooling temperature. Remember that WP RPM will affect coolant flow too.

This "problem" shows up mainly winter time. Nothing is really wrong with Tstat and cooling system but Can cause panic attack to drivers. Worse when Fiero owners think cooling problems are normal, Another popular myth. (Problem in summer check fan switch etc...)

All Stant Tstat I've tested... 180, 195, and SuperStat 195 on Very good cooling system w/ New Rad and heater core on 87 DIS L4. (I avoid most other Caps and Tstat brands.) And rad fan is Off and car moving still does it. Using SuperStat will reduce or stop this "problem."

Cold Weather Problem source? Maybe some/all of...
Too much radiator surface area in cold weather. Altering surface area will affect radiator performance. Trucks often block some/all radiator grill in winter for this reason... Many will do this on cars in sub-zero weather. (Be very careful if you try that. Car can/will overheat.)
Huge amount coolant to heat
Heater plumbing design/problems (Heater plumbing problem or heater air is block by dirt etc can make this worse.)

everything in it, the engine (everything in & on it) new all aluminum radiator, heater core. All I expect is it to get to the "set temp", And STAY around that temp like it's supposed to. Am I asking too much for "this design" car ? I did jack up the back of the car to fill, removed the rad cap to let air out. I drove the car for 4 years without this problem.

I have NEVER seen a thermostat that will "snap" open. Take any thermo made and put it in a pan of water, already boiling or bring it up to temp slowly. Either way you will see it open at a controlled even rate. Same thing on cool down.

I have no idea whether it snaps open or shut, But the coolant temp at the" Mechanical "sender ( actually HAS 2 new Mechanical Temp gauges monitoring it) goes from 210 to 170 in 3-4 seconds. then climbs to 200-210 in about 15-20 seconds. It absolutely possitively will not maintain a "steady temp" By Anyones standards. Seems to me, if it would open or close "Slower", and NOT "all the way", I would think the temp of the coolant would remain "steady".