Should an external thermostat allow ANY flow from radiator when cold?

[maxm50]

I'm replacing all of my coolant hoses and just pulled off the external thermostat. It looks very old - possibly original from '81? It does not look like the vendor parts I see, but rather like a cylinder + cone with one port at the tip of the cone (to water pump) and 2 ports out the side of the cylinder (to coolant T and to radiator lower hose).

I took the time to clean all the grime off of it, and then noticed that no matter which port I poured water into, it flowed out of the other two. Then I tried blowing air into it (good thing I cleaned it first). Same thing - I can blow air into any port. Sliiight restriction of the radiator hose port, but not much.

This tells me that my thermostat is probably stuck halfway open. Correct? Or are these things designed to flow a small amount of coolant through the radiator even when cold?

Autoricambi's site says that you should not even be able to blow air through the lower radiator hose port: https://autoricambi.us/external-thermostat-oem/

I'm about to click "buy" on this replacement thermostat but just thought I'd check here first. I've seen plenty of people talk about bench testing by boiling the thermometer, but never heard anyone say if "closed" is "air tight closed" or even "water tight closed"!

Thanks!

[maxm50]

Update: I tested it with a heat gun and it does indeed open more fully (I see the internals move) when hot. Not sure what temp. I can do a boiling test tomorrow.

When cold, here's a photo of the water flow. I'm running it "backwards" here to show how there's some restriction on the radiator hose side (causing only a little flow out that port) and no restriction on the coolant tee port (most flow goes that way).

Any thoughts? Is this normal operation or is this thermostat hosed, ie. rubber seals dried up?

I've read about folks drilling small holes in new thermostats to allow air to bleed out... I guess this would function the same way, but I'm concerned that this might be TOO much flow through radiator when cold.

[manoa matt]

Those are the original SAVARA thermostats. I have one and it does the same thing, but keep it as a spare. It would be interesting to see if a new thermostat would seal totally, however I seam to recall blowing in the various ports when I got a new one and it was tight. However if there are tech articles advocating drilling a small 1/8" hole to relieve air pockets, then that thermostat may not be that bad, just a slightly quicker warmup to operating temp. Once its open, its open. For $30 I would probably get a new OEM temp thermostat from one of the vendors.

[maxm50]

Thanks for the input! I measured the water flow this morning when cold. The radiator hose inlet flows at 1/8 of the flowrate of the coolant tee outlet. 18oz poured in to water pump port = 2oz (11%) out radiator port + 16oz (89%) out coolant tee port. So, if only 11% of the water when cold is flowing through the radiator, I doubt this will make much of a difference in engine warmup time. My gut says maybe a 5% increase in warmup time vs. a completely sealed thermostat. And I guess I won't have to worry about air pockets. It seemed to warm up decently and held very steady 190deg engine temps before, so, I think I'll just reuse it for now. Maybe I'll try swapping it in 2yrs at my next coolant change

Oh and in case anyone's wondering I just did the flowrate test using the thermostat held in a sink over two pint glasses. 16oz out one port (filled glass) and 2oz out the other. So, I guess a "beer" out one and a "shot" out the other.

[18Fiatsandcounting]

My only 3-neck thermostat is installed in my car, but I recall when I did the test a few weeks ago that Max did above, that a little bit of water flowed out of the side that was supposed to be closed. Less than his picture shows, so maybe just a slight dribble.

It's likely that aging of the thermostat, in particular corrosion along the internal sealing edges, will worsen its ability to be either full "off" or "on" in either direction. Whether that is a problem in a particular car probably depends on climate, general condition of the rest of the cooling system, and the owner's tolerance for things that aren't perfect. [EDIT based on Max' recent post: If your warmup time is relatively quick, and the engine holds at 190 degrees, you're probably fine. Keep in mind that the issue may slowly worsen over time though.]

-Bryan

[SteinOnkel]

Makes bleeding a lot easier. I always drill a small hole into thermostats.

[maxm50]

Hey Bryan, good point about the sealing edges. When I opened the thermostat using a heat gun last night, I noticed that the sealing surface for the thermostat hose seems like it was never designed to form an air-tight (or even water-tight) seal. It's an off-white plastic disk that pushes up against the aluminum housing. No rubber, no gasket, no O-ring - nothing that would form an air-tight seal even when new, as far as I can tell. I noticed some "coolant crystals" for lack of a better term in this seal area last night - perhaps I'll open it up again and use a small pick/file to try to clear these out before I put it back on the car.

[18Fiatsandcounting]

Yes, you can try and scrape off some of those "crystals", taking care not to damage either the plastic disc or the aluminum. I don't think thermostats are supposed to seal like a faucet would, and in fact the garden-variety disc thermostats in many cars are just a plate of metal pressing against an outer ring. If you hold them up to the light, you can see that they don't seal perfectly even when new.

So, it seems like the thermostat should block most water flow, but not 100% like a valve would.

-Bryan