Overheated
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,235
Likes: 646
From: Orange County Ca
Success, hopefully.
I could feel that the bottom of the overflow tank was wet, so I pulled it out. A little bit of a pita (I had to remove the top part of the fuse box, which is always a little scary), and I replaced the O ring on the quick disconnect fitting on the bottom. I did find, as others have said above, that the O ring wasn't a standard size, so I used the closest I could find.
It appears to be working well.
So, what did I learn.
The coolant temp sensor seems to be working well.
The fan seems to be working well.
There are no other leaks.
My best guess is that the overflow tank fitting was leaking a little bit and that the coolant was pooling on a flat plastic filler piece below it, so it didn't leave a puddle on the ground. Then would evaporate because of engine heat.
Why did the cap on the overflow cap fly off and break into 2 pieces?
Low coolant and steam pressure????
I could feel that the bottom of the overflow tank was wet, so I pulled it out. A little bit of a pita (I had to remove the top part of the fuse box, which is always a little scary), and I replaced the O ring on the quick disconnect fitting on the bottom. I did find, as others have said above, that the O ring wasn't a standard size, so I used the closest I could find.
It appears to be working well.
So, what did I learn.
The coolant temp sensor seems to be working well.
The fan seems to be working well.
There are no other leaks.
My best guess is that the overflow tank fitting was leaking a little bit and that the coolant was pooling on a flat plastic filler piece below it, so it didn't leave a puddle on the ground. Then would evaporate because of engine heat.
Why did the cap on the overflow cap fly off and break into 2 pieces?
Low coolant and steam pressure????
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,235
Likes: 646
From: Orange County Ca
Well, not fixed.
I did 3 heat cycles--start cold, bring up to normal temp (230 per the dash), oil temp 200+, idle for 5-10 minutes, shut off, repeat. Not a drip one. I did do a exhaust gas in the coolant tank test, and it came out negative. I drove 8 miles to a gas station, and the coolant was gushing out of the overflow tank. The cap stayed on and was working correctly. The fan was on. After shopping for about a hour, the car had cooled down somewhat and I started it up and added about 1/2 gallon of water. The water immediately started bubbling/squirting out of the tank with extreme velocity.
I'll change the stat as it's the easiest/cheapest part to change, but what's next?
Water pump? It's not leaking and turns freely, so how to check if it's working well?
I did 3 heat cycles--start cold, bring up to normal temp (230 per the dash), oil temp 200+, idle for 5-10 minutes, shut off, repeat. Not a drip one. I did do a exhaust gas in the coolant tank test, and it came out negative. I drove 8 miles to a gas station, and the coolant was gushing out of the overflow tank. The cap stayed on and was working correctly. The fan was on. After shopping for about a hour, the car had cooled down somewhat and I started it up and added about 1/2 gallon of water. The water immediately started bubbling/squirting out of the tank with extreme velocity.
I'll change the stat as it's the easiest/cheapest part to change, but what's next?
Water pump? It's not leaking and turns freely, so how to check if it's working well?
Last edited by Sayfoo; Jun 12, 2026 at 06:38 PM.
Race Director
Joined: Feb 2001
Posts: 11,382
Likes: 2,112
From: Grapevine Tx
2018 C3 of Year Finalist
2016 C3 of the Year Finalist
Dude, it's your thermostat. I just went through this last month. Stuck closed. New 194* thermostat and everything is back to normal.
Melting Slicks
Joined: Feb 2018
Posts: 2,198
Likes: 1,057
From: North Texas
You could always pressure test the cooling system.
Sounds like a thermostat that is stuck or maybe a blown head gasket letting hot gases into the coolant passages. Look for bubbles in the overflow bottle when you rev the engine, bubbles would indicate a blown head gasket.
Sounds like a thermostat that is stuck or maybe a blown head gasket letting hot gases into the coolant passages. Look for bubbles in the overflow bottle when you rev the engine, bubbles would indicate a blown head gasket.
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,235
Likes: 646
From: Orange County Ca
Keeping my fingers crossed.
Thanks for all the reply's.
Last edited by Sayfoo; Jun 13, 2026 at 12:39 PM.
Melting Slicks
Joined: Dec 2004
Posts: 2,541
Likes: 773
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Race Director
Joined: Feb 2001
Posts: 11,382
Likes: 2,112
From: Grapevine Tx
2018 C3 of Year Finalist
2016 C3 of the Year Finalist
When I removed my stuck closed thermostat I got a lot more coolant come out than when I replaced my stuck open thermostat. Kinda like it dammed it up, so be ready for a half a gallon or more to come out.
Race Director
Joined: Feb 2001
Posts: 11,382
Likes: 2,112
From: Grapevine Tx
2018 C3 of Year Finalist
2016 C3 of the Year Finalist
I changed mine out last summer because it failed open which is the usual failure. Then that one failed closed a couple of months ago. Go figure. They aren't hard to change.
Team Owner
Joined: Sep 2000
Posts: 25,029
Likes: 2,719
From: Far NW 'burbs of Chicago
St. Jude Donor '13
Several people here posting about bad thermostats, our 2017 is now on it's third thermostat at 65k miles. Each time the engine was running too cool, each time installing a new thermostat (only) fixed the problem. Since the current GM issue is poor quality, and some of the aftermarket ones are either worse or much more expensive, I'd use your current one until it goes wonky. But once you start getting abnormal readings, replace it ASAP.
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,235
Likes: 646
From: Orange County Ca
I got a new stat and temperature sensor from RockAuto delivered today.. Stat came in a Genuine ACDelco marked box and the sensor in a ACDelco professional marked box.
BOTH are made in china.
What a load of crap.
I'll install them tomorrow.
BOTH are made in china.
What a load of crap.
I'll install them tomorrow.