What happens when a thermostat goes bad?
 

Can they just open partially? My car, with new radiator, all shrouds and seals in place, and clutch fan that spins half a turn by hand, gets up to 230 or higher. The radiator gets hot all the way across and the lower hose does not collapse. A partially open thermostat might explain this. Right?

No, with a partially open thermostat (that is stuck in it's position) the water in the engine would get very hot (not enough flow), the water in the radiator would have all the time in the world to cool down.
I may be wrong but my guess is that you don't have enough air flow across the radiator.

Put a new thermostat in.Cheap and easy to do...you have alot invested in the rad..at least you will have eliminated that as an issue...

I will change the t-stat. Just wondering if anybody thinks that is really the problem.

It's hard to say. If the t-stat is stuck mostly open, then the car will take a long time to warm up. If it's mostly closed, it's more likely to start overheating.

They're cheap. Get a new one and eliminate that question.

Air in the system ..
 

I have a similar problem. I replaced my water pump and now the car runs at 230! I really did not have a BIG problem before, however in my quest for the perfect cooling system I added a high flow pump. Now I have this issue of it running at 230. I think I have not purged all of the air out. I am going to replace the T-stat and purge it yet again.

carbster

In my opinion,no...if your thermostat was restricting flow,you would heat up fast....and overheat above 230...(but put a new one in anyway)..personally,I'd check that fan clutch again...rotate it slowly by hand,and if you feel resistance in one spot and it feels "free" in another,it is toast...I learned the hard way...install a thermo clutch..they operate on underhood temps...
Rich

Edit.... http://www.haydenauto.com/fce2.htm

Pretty sure it's the clutch fan. That is where my problem was about a month ago after replacing the radiator, water pump, t-stat and all hoses. Run the car for awhile then turn it off while someone watches the fan if it continues to spin for 3 or more rotations the clutch is out.
I was able to hold my fan with my hand to keep it from spinning.
Put on a flex fan and spacer and Vette runs very cool now.
:yesnod:

as others have mentioned, 'stats are dirt cheap. If you want to eliminate sthe 'stat as a problem...take it out, and observe if it's cold...place it in a pot of boiling water...if the 'stat opens, it's good, if not, get a new one...oh yea...use some pliers to take the stat out...

Sometimes it helps just to get it out of the circuit and test.

suggest you remove the thermostat completly and run the car, on the road, see what she does. It is either going to get better or stay the same. If it gets better then go down to your local Chevy parts place and but a new thermostat. I learned the hard way to never buy this part from anybody except the dealer.

If it stays the same temp then you have another issue.

FYI.....
I have others that replaced a stock water pump with a "high flow" water pump and have had high temp problems as a result. Nothing else was changed just the water pump. I think the high flow pump creates cavitation and as such slows down the water flow.

With no thermostat in your car (for testing only) the engine will stay as cool as possible. So if you still have high temps then something else is causing it. Like air finding an easier path around the radiator, or a radiator problem, or a fan clutch that feels ok by hand but does not lock up completly. :cheers:

Here is a different suggestion. Do you have the air dam on? Underneath your front bumper? From the picture, it looks like you do, but I didn't know if you took it off or not. I had replaced everything and I was still running hot. Then I found out that you are supposed to have that air dam on there and I didn't. Put one on and the problem went away. It was all a matter of bringing enough air across the radiator.

correct
 

This quote is right, but have it hanging from a thread through the opening. [just push it open cold and drop thread through] and a thermometer in the water to see when it opens. You would be surprised to see how many don't open when they are supposed to. I learned this from an old GM line mechanic at a dealership I worked at a few years back. He said he got burnt on a service call one time because of a new 'bad' thermostat and decided he would check them from then on. :cheers:

I do still have the air dam in place, with all seals.
I think I will try it without the stat to see what happens.
Thanks guys.

Question to anyone here...re testing without thermostat...agree engine would run cooler...but would this "mask" an issue with the clutch fan ? Just wondering...these clutch fans are not easy to check when they are going out...I got lucky as I felt resistance then it would feel free...turned out the bearings were gone...another I had on my Trooper was "wobbly",and that was obvious..
Rich

On a hot summers day the stat opens up and stays open so removing it for testing will not change the engine temp at all. I have a 180 stat and my engine runs at 210 or a little under all the time in the summer.

This does not apply to a winter day of course as the stat will maintain the engine temp at whatever it should be.

Also I had a bad clutch and it was very difficult (for me anyway) to tell that is was not working. I replaced it on a guess and sure enough the new one works alot better then the old one.

Good point on stat staying open on real hot days...and in my case that occurs mostly at highway speeds with temps at 200....in traffic it modulates cause the fan kicks in more often...I have a 180,as well...