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Old 03-30-2021, 01:32 PM   #1
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LN TStat Failure

So just got done overhauling intake and exhaust, car has been sitting on stands since Thanksgiving. Finally got my 996 row tune this morning and took the maiden voyage this afternoon. I stopped midway through and noticed clear smooth fluid POURING out from rear passenger side. I figured it was coolant so headed home. This was a short drive, 1 mile or so, and most of the way home the temp needle was straight up. Got home, jacked the rear end out, and it looks like it's coming from the expansion tank vent hose (pic below). Popped the trunk, there's a bit of coolant in top of the tank, and when I opened the burp valve it actually pulled air into the resivoir. The resivoir is still full though.

My brain tells me this is failure of the TStat to open.

BUT, I just switched to the LN TStat about 2 years ago (probably a little less). That seems pretty crazy that a 2 year old TStat would fail. Has anyone else experienced this?

I plan on pulling the TStat and reassembling without it in place to verify this is the issue. If it is, I will likely just put the OEM TStat back in place. Does anyone have any other suggestions?

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Old 03-30-2021, 02:11 PM   #2
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And my mind tells me that there is nothing wrong with the thermostat, but you had an air pocket in the cooling system, and may have aggravated the coolant tank in the process of overheating the engine.
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Old 03-30-2021, 04:11 PM   #3
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And my mind tells me that there is nothing wrong with the thermostat, but you had an air pocket in the cooling system, and may have aggravated the coolant tank in the process of overheating the engine.
An air pocket in the cooling system for 2 years?
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Old 03-30-2021, 04:12 PM   #4
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And my mind tells me that there is nothing wrong with the thermostat, but you had an air pocket in the cooling system, and may have aggravated the coolant tank in the process of overheating the engine.
I would love it if you are right. But why would it "aggravate the system 2 years later? You've probably installed more of these than anyone. What would you do at this point?

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Old 03-30-2021, 04:28 PM   #5
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Since it didn't overheat, perhaps there's nothing wrong with the t-stat and the hose just sprung a leak? Maybe while doing the exhaust work if got pinched and weakened? Has it ever been changed? If not, it's a 21 year old hose.
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Old 03-30-2021, 04:40 PM   #6
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Mine puked it’s fluid out the overflow last year. When I investigated I found that my water pump had been leaking. New cap, pump, LN thermostat, flush, and fluid and all’s well.
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Old 03-30-2021, 05:01 PM   #7
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Since it didn't overheat, perhaps there's nothing wrong with the t-stat and the hose just sprung a leak? Maybe while doing the exhaust work if got pinched and weakened? Has it ever been changed? If not, it's a 21 year old hose.
My understanding of that tube is that serves as a vent for the coolant expansion and is supposed to be an open vent to atmosphere, so I don't think a crack in it caused the leak. If it was a main line, that would be a different story


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Old 03-30-2021, 05:02 PM   #8
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Mine puked itís fluid out the overflow last year. When I investigated I found that my water pump had been leaking. New cap, pump, LN thermostat, flush, and fluid and allís well.
What do you think was the culprit for the puking? Externally my water pump is ok and not leaking. Do think you tstat was bad? If so, was it an oem or ln?

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Old 03-30-2021, 05:05 PM   #9
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My understanding of that tube is that serves as a vent for the coolant expansion and is supposed to be an open vent to atmosphere, so I don't think a crack in it caused the leak. If it was a main line, that would be a different story


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Ah, vent hose... duh! Thanks.
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Old 03-30-2021, 05:10 PM   #10
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Ah, vent hose... duh! Thanks.
No worries dude, this is the most convoluted hose pattern on a cooling system

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Old 03-30-2021, 05:26 PM   #11
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My guess is air in the system caused the puke after the engine was shut down at the end of a hot day. I didnít let the fans cycle off before shutting it down.
My pump looked ok but removing the belt and putting some force on it I could hear and feel play. Plastic impeller looked new after almost 20 years. Bearing not so much. A few drops of coolant and dried residue at the bottom showed it had leaked. I didnít check the original OEM thermostat as I was changing it anyway. Physically it looked fine. As far as I was concerned I had found my culprit so didnít heat water to check itís functioning.
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Old 03-30-2021, 05:28 PM   #12
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My guess is air in the system caused the puke after the engine was shut down at the end of a hot day. I didnít let the fans cycle off before shutting it down.

My pump looked ok but removing the belt and putting some force on it I could hear and feel play. Plastic impeller looked new after almost 20 years. Bearing not so much. A few drops of coolant and dried residue at the bottom showed it had leaked. I didnít check the original OEM thermostat as I was changing it anyway. Physically it looked fine. As far as I was concerned I had found my culprit so didnít heat water to check itís functioning.
Do you think the air came from the bad pump? I've not heard of such a thing.

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Old 03-30-2021, 05:32 PM   #13
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This is interesting - I was just looking at LN engineering TStat webpage, - not only is there no warranty listed but the fine print at the bottom says that there is absolutely not a warranty! Hah!

Turns out that Borg Warner also make a low temp TStat for our cars. Fcpeur0 has them for sale for $57, and that includes the housing and gasket....

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Old 03-30-2021, 06:07 PM   #14
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Do you think the air came from the bad pump? I've not heard of such a thing.

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That or? Bad cap? The cap was new a few years prior and ending in 4. I changed it out anyway.
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Old 03-30-2021, 06:11 PM   #15
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You are saying the car had been jacked up since Thanksgiving, right? Did you jack up only the rear end? Perhaps you did have an air pocket in front which moved up to the rear end while being jacked up? Just a thought...
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Old 03-30-2021, 06:33 PM   #16
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You are saying the car had been jacked up since Thanksgiving, right? Did you jack up only the rear end? Perhaps you did have an air pocket in front which moved up to the rear end while being jacked up? Just a thought...
No, it was jacked on all 4 corners and level all around. It certainly has been at an angle while going onto and off the stands, but no more than driving up a hill or loading on a trailer.

The more I think about this the more I'm convinced it's a failed TStat. There are two (broadly speaking) types of these, traditional ones and "fail safe" ones. What's the difference? When fail safe stats fail, they snap open and stay there. Why? Because when the traditional ones fail, this is what happens and the car overheats. Unfortunately, I don't not think that any boxster TStat is a "fail safe" design.

If I had just changed the fluid that would be a different story, but piper's point is hard to ignore.

I will say though that I asked my friend who is a professional mechanic and he has on multiple occasions seen tstats fail after prolonged storage - essentially they just get stuck. I have mixed coolants in the system (all OAT of course) but it will be interesting to see what everything looks like when it all gets drained and the TStat is out.

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Old 03-30-2021, 06:43 PM   #17
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Well, it`s very easy to test your theory, pull it out and drop it into boiling water, it has to open. Yes, thermostats get stuck sometimes, I had the same issue last fall, I had to turn on the heating to keep the needle straight up.
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Old 03-30-2021, 06:48 PM   #18
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Well, it`s very easy to test your theory, pull it out and drop it into boiling water, it has to open. Yes, thermostats get stuck sometimes, I had the same issue last fall, I had to turn on the heating to keep the needle straight up.
Hey that's any interesting idea! I have a sous vide and I would love to test it ex vivo so to speak.

Your TStat that failed last fall - was it oem?

I guess that what I'm really interested in, is whether I'm the only unlucky soul that's had this failure of if there are others. If there were others I would go back to the oem part, but with a BW alternative now I will probably be going that direction.

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Old 03-30-2021, 07:27 PM   #19
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Hey that's any interesting idea!
That`s the way You can even monitor the opening temperature with the thermometer. But an easier test is to let the engine run at idle to see if the fans at the radiators turn on. This should happen near 100C. If they do, the thermostat is already open.

Sorry, the car was my SAAB convertible not the Porsche. The thermostat was stuck completely but the heater core could keep up with the top down in high traffic even though the radiator was not doing much.

For the Boxster I went with a Wahler thermostat, I`m not a big fan of the low temp T.
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Old 03-30-2021, 08:17 PM   #20
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I just had a horrifying thought - what if I am completely wrong here and I have actually blown an internal engine component that is transmitting pressure from cylinder to the cooling system. I know the head gaskets are rock solid on an M96 but that could do it? Cracked cylinder I would think would cause serious intermixing of fluids and very slim chance of pressuring the coolant system. Ring failure wouldn't pressurize the coolant system, I don't think.

Oh my....


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