Coolings system help

[scotty64]

So I have a brand new to me truck. It can be seen here:
http://67-72chevytrucks.com/vboard/s...d.php?t=467035

I drove it home (6 hours) and the temp stayed about 180--so I assume it has a 180 degree thermostat in it. Two days ago, the temp gauge climbed to about 220 and then back to 180. (As background, I know that the radiator has a pinhole leak in it. When I checked the coolant level in the radiator it was low, but the core was not quite exposed). Anyway, I topped it off and all was well.

Tonight (trying to recall my high school auto shop class) I took off the radiator cap, started it, and watched what happened as it heated up. Here's the deal:

Between cold and 180 there was no action in the radiator at all (no fluid movement). Between 180 and 220 the radiator "burped" multiple times (meaning that the fluid welled up out the the radiator neck and onto my garage floor). Then, at about 220, the temp started dropping, the burping stopped, and the fluid was clearly circulating (and it started steaming). I also fished out some black RTV (silicone) that was floating in the radiator--kind of a semicircle like it had been used to seal something.

I guessing that this is a "starting to fail" thermostat? Any other thoughts.

Thanks.

Scott

[geezer#99]

Sounds like a normal thermostat. When they fail it's usually stuck closed. I had one that did the same. Don't overfill it. About 1-2 inches down from the top.

[Rannos]

Do you have a temp switch on your electric fans? IF ya do what temp do they come on at? Even if ya have the right thermostat if you dont have air moving across your coils the water wont be cooled. I would look at that. Also it sounds as if you may have some air trapped in your cooling system. if you let it cycles a few times then fill it back up at operating temp. you should be able to get it purged. Hope this helps. Also the water should circulate at 180 if its a 180 stat.

[bama63]

x2 rannos it may creep up 5-10 degrees before fully opening but no more

[scotty64]

Rannos--thanks. I did some research on burping radiators (air trapped in the system) and I think that is exactly what is going on. I do have electric fans, and they have started to run constantly--that is another thing that I'll have to chase down, but clearly isn't the cause of the high temps. I plan to add some more water, burp it again this afternoon, and see if that fixes things.

[66 lwb cs]

Might just replace the thermostat they can't cost but 15.00 or so and should be easy to get to

[scotty64]

Originally Posted by 66 lwb cs

Might just replace the thermostat they can't cost but 15.00 or so and should be easy to get to

Agreed. Although the TPI manifold makes it a little trickier. I am going to continue to try "burping" the system to see if it works. The radiator has a pinhole leak and needs to be fixed--when I have the radiator out I will absolutely replace the thermostat for good measure.

Which brings up a point. Does anyone know if there are t-stats for this engine (350 cu in TPI motor out of a early 90's Camaro) that are the "fail open" versus "fail closed" style?

Rather through a code than cook the engine.

Scott

[scotty64]

So I ran the truck up to operating temperature again without the radiator cap on. A little variation in the level of coolant in the radiator, but none "burped" out. Here is what the temp gauge is doing (truck has 2 electric fans which were constantly on--that is another issue).

Temp slowly rises to about 215, then goes down to 180, then goes back up to about 215, then goes down to 180. It falls faster than it rises. Is it possible that there is enough cooling capacity that the thermostat is opening, then it is cooling down enough to close the thermostat, then it is opening back up, and so on?

Maybe the thermostat is a 195 or 210? (Per web research stock is 195.)

Thanks

[Rannos]

I have an LT1 in my 64 and it had a 195 stat. It will get up to 195 then drops to 180 and stays about there give or take 5 or 10 degrees.
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