Si_chRis

So recently with the cold weather here (20 degrees in the morning), I have noticed that on my hour and a half drive to work, my temperature gauge would fluctuate 1-2 bars below the spot where the needle normally sits in the middle area.

This only happens while going on long straight roads, highways, or coasting downhill (still in gear while coasting, not in neutral, downshifting). The needle will never get close to cold, just 2 bars, maybe 3 at the most towards cold from the normal middle area.

Would a stuck open thermostat cause this? Does it sound like I have a stuck open thermostat?

Some facts:
-In stop and go, city traffic, the needle stays at normal spot and would move down 1 bar (if I hit a long straight) or not at all.
-The needle never goes towards hot.
-My heater is always on and it's always hot and never gets cold.
-On a cold start/fast idle and waiting 5-7 minutes after start up, my top radiator hose starts to get warm/hot, the bottom radiator hose is cold. The heat coming out the vents blows warm-hot as the car warms up.
-Once I get to work after driving for over an hour, both the top and the bottom radiator hoses are the same hot temperature by feel.
-No coolant loss. Reservoir stays the same level.

Other facts:
-Car is a 2000 Honda Civic SI.
-I changed out the radiator last winter with a Koyo OEM replacement (identical to stock), but the core is bigger (maybe twice the size or less) than the old OEM radiator.
-Used Honda Type 2 blue antifreeze.
-New radiator hoses also changed.
-Bled/burped the cooling system with a bleed funnel kit similar to this:

-I made sure the air bubbles came out by "massaging" the radiator hoses.
-Heat was on hot and on high setting.
-Bled the cooling system till the radiator fan kicked on and off at least 5 times.

I am going to bleed the system just to see if it helps whenever I get time. I already ordered a OEM Honda thermostat since it's cheap but I wanted to see what you guys think.

motegicivic

I am not sure what the problem is?

As long as you are getting heat and it running half way or lower you are ok.

To play it safe do the thermostat.

sfar785

came into thread, saw Nissan motor, srsly just wtf'd

motegicivic

He was just showing the type of funnel he used, that is not his car.

sfar785

lol i know, but the pic was the first thing that i saw

MonkeyBoy668

It certainly sounds like your thermostat could be failing open. Usually when they fail the spring doesn't push open the valve as far as it should (or not at all), but sometimes they fail in such a way they they don't close beyond a certain point, so they're running partially to fully open.

I had a similar problem in the winter a couple years ago, around town on side streets everything was fine, but once I got on the highway the needle would slowly drop, and the faster I went the lower it got.

Is there any correlation between your average speed and how low the needle gets?

motegicivic

Can the thermostat fluctuate, say it opens at highway speeds and then close again or does it stay open at temperature?

Si_chRis

If my thermostat is partially stuck open, would the bottom radiator hose warm up the same or slow rate as the top radiator hose in cold start/fast idle? Because when I checked it the other morning after 5-7 minutes of warming up in 20 degree coldness, my bottom hose was cold while the top was getting warm/hot.

I would say there isn't a correlation between speed and needle movement. I can be cruising 65 mph on the highway or 45 mph on a straight backroad and it won't move at all. If it does move, it slowly moves down like a bar or two but it does this over a long period of time.

I do notice that when I am coming to a stop going downhill, the needle moves down 2 bars quickly.

I always thought once it opens after operating temp, it stays open.

Wrona

you are probably looking at a new Honda thermostat...what happened to my car is just that except my temp gauge would be barely off the line the car would not overheat there was just no heat coming out of the vetns got a new thermostat now it sits perfectly at where my integra sits at where it used to be

Si_chRis

My heat coming out the vents warm up normally and stays hot though.

It's weird. On my commute home tonight, the temp gauge went to the normal spot and didn't change until I was almost home coming down a steep hill. Was downshifting and coasting. As soon as I got back on the gas and started driving, the temp gauge started to go back to normal and stayed there.

My thermostat should be here tomorrow so I'll change it out when I get a chance.

motegicivic

Does your temp gauge act normally? Does it gradually raise as the car warms up? Then when you shut the car off does it drop down?

The gauge does not bounce if you hit bumps etc.

Si_chRis

Yes, the gauge gradually rises as the engine warms. This morning, I let it warm up to a 1-2 bars above C before driving off. Heat was was warm and rising as I was driving. Engine was at operating temp 5-10 minutes later when I stopped at the gas station.

The gauge does drop down when the engine is shut off. The gauge also doesn't move when I go over bumps.

Si_chRis

So I changed out the thermostat with an OEM Honda one, bled the cooling system, and the fluctuation has gone away. The gauge stays pegged in the normal spot and doesn't move while driving. The heat seems to be blowing hotter also.

I also took the old thermostat and put it in boiling water to test it. It opened all the way and closed all the way when I took it out of the boiling water. I also did a test of leaving the thermostat in the pot of boiling water then letting it simmer. The thermostat opened all the way and didn't react to the water coming to a simmer. Is that normal or should the thermostat start to contract to the water simmering?

motegicivic

I do not think it is that smart to react it is mechanical, basically opens and closes.

I think sometimes with age they can get lazy, maybe not open fully or fluctuate I recently did mine as well, I would get periodic running hot at idle (like once a month), been a few weeks so I am hoping that was the problem.

Was the one your took out oem?

Si_chRis

Yes, the old thermostat removed was OEM Honda. I don't know if the numbers stamped on it (aside from 78 degree celsius) means the manufactured date, but if it is, it's the original one.

motegicivic

So that is about 170 degrees.

I hope mine does the trick, mine is OEM as well.

vanboxer

I think i have to do the same thing because when it is cold out it takes my temp gauge a while to get to normal operating temp.