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Coolant boiling in reserve tank #5429817 05/15/20 11:31 AM
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CNAKitty Offline OP
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My car wasn't overheating while driving. But last night while sitting idling in a drive thru forever, I started hearing a odd noise from the front, and my passenger noticed "smoke" coming from under the hood. I shut the engine off and popped the hood. The water in my radiator reserve tank was BOILING and had popped the cap partway off. Fortunately I had a full bottle of new coolant in the trunk. (bought it planning to drain/flush/refill when I get a night off). That cooled it down well enough for then. After the car cooled down 4 hours in the work parking lot, I removed the radiator cap, and the radiator is still full. I also checked the engine oil level, it's fine. But on my way home I noticed it's getting hot fast while sitting idling, i.e. at red lights, but stays cool ok while driving. I'm used to cars with belt driven fans, not electrical like in this. I noticed that the fans are NOT running when the car is idling in park. Is that normal, or is that my problem? (I figure it is, but want second opinion)
Unfortunately my nursing work schedule is hectic with the virus still going strong here, and I can't get enough time off to do more than a "quick fix" for now until my Monday 5/18 one night off. Any help?


'98 Buick Century
Re: Coolant boiling in reserve tank [Re: CNAKitty] #5429819 05/15/20 11:33 AM
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Your headgasket is gone. I had the same issue once and it turns out the headgasket is leaking. No boiling, no overheating, just a stuck radiator cap.


"You keep asking questions PandaBear and you'll end up a vegetarian like my wife" - Camu Mahubah
Re: Coolant boiling in reserve tank [Re: PandaBear] #5429820 05/15/20 11:35 AM
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kschachn Offline
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Yes, the coolant is not boiling in the reservoir, it is bubbling from combustion gasses.


1994 BMW 530i, 252K
1996 Honda Accord, 288K
1999 Toyota Sienna, 434K
2000 Toyota ECHO, 284K
Re: Coolant boiling in reserve tank [Re: CNAKitty] #5429826 05/15/20 11:39 AM
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The_Nuke Offline
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What is the year/make/mode of car?

What engine is in it?

How many miles?

Any recent incidents of overheating or coolant service?

Has the coolant ever been changed?


2012 Charger Pursuit aka "The Bacon Hauler"
- 5.7L + NAG1 (A5) + 3.07 (215mm)
Re: Coolant boiling in reserve tank [Re: PandaBear] #5429833 05/15/20 11:44 AM
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CNAKitty Offline OP
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It was steaming hot! And the gauge shows it heating up fast when I'm idling. And the fluid level in the reservoir keeps going way down.The main question regards the fans. Are they supposed to be turning when parked and idling? Because mine aren't.


'98 Buick Century
Re: Coolant boiling in reserve tank [Re: CNAKitty] #5429835 05/15/20 11:46 AM
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LDM Offline
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Check the water pump first. I had the same thing happen years ago in a car, water pump failed and the only circulation was by the hotter coolant flowing to the cooler areas. It was boiling in the reservoir just like you describe and overheating on the gauge when at idle, but ok while driving.

If that turns out not to be the issue, head gasket is the next most likely culprit.


2018 GMC Sierra 1500 SLT 6.2 Max Tow
1994 Chevy Camaro Z28 5.7
1989 Chevy Camaro IROCZ28 5.7
Re: Coolant boiling in reserve tank [Re: The_Nuke] #5429841 05/15/20 11:54 AM
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'98 buick Century, recently bought, less than 160K miles, 3.1L, And I have no prior maintenance records from previous owners. I have had to add water to the reservoir now and then past few months since I bought it. As stated, planned to drain/flush and fill soon, as a maintenance thing since I knew it hadn't had new coolant in awhile. I have bought new upper hose (and clamps), new thermostat (just because why not), and 3 gallons Peak coolant toward doing so. I want to get a new cap also just because I noticed mine was gunked up, and the lower hose. It's been a case of finding time to do it all in my parking space.


'98 Buick Century
Re: Coolant boiling in reserve tank [Re: CNAKitty] #5429843 05/15/20 11:55 AM
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mk378 Offline
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Overheating only while sitting still is almost always a fan problem. Electric fans normally don't run much when idling but of course they should start before overheating occurs.

If equipped with (working) air conditioner, the fans should run when the A/C is on. Ironically this can help keep the engine cool if the mechanism that starts the fans from engine temperature is not working.

Re: Coolant boiling in reserve tank [Re: CNAKitty] #5429845 05/15/20 11:56 AM
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Davejam Online Content
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They sell a combustion leak detector. But you can just idle without the radiator cap on and if there are constant bubbles emerging you have a head gasket failure.
https://www.harborfreight.com/combustion-leak-detector-64814.html

Depending on how bad it is you can probably continue to drive the vehicle, just avoid idling after you reach temperature for extended period of time - No drive throughs.
Never keep the temp setting to 100% cool, and keep the fan on. Helps dissipate a little extra heat.
The situation will get worse though, so don't put it off for too long.

Re: Coolant boiling in reserve tank [Re: CNAKitty] #5429846 05/15/20 11:56 AM
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LDM Offline
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Originally Posted by CNAKitty
'98 buick Century, recently bought, less than 160K miles, 3.1L, And I have no prior maintenance records from previous owners. I have had to add water to the reservoir now and then past few months since I bought it. As stated, planned to drain/flush and fill soon, as a maintenance thing since I knew it hadn't had new coolant in awhile. I have bought new upper hose (and clamps), new thermostat (just because why not), and 3 gallons Peak coolant toward doing so. I want to get a new cap also just because I noticed mine was gunked up, and the lower hose. It's been a case of finding time to do it all in my parking space.


Since it is a 3.1L V6, it could also be the lower intake manifold gaskets. Fairly common on these as well.


2018 GMC Sierra 1500 SLT 6.2 Max Tow
1994 Chevy Camaro Z28 5.7
1989 Chevy Camaro IROCZ28 5.7
Re: Coolant boiling in reserve tank [Re: LDM] #5429851 05/15/20 12:02 PM
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What's best way to check the water pump? Can it be done without taking off or needing to drain etc?


'98 Buick Century
Re: Coolant boiling in reserve tank [Re: CNAKitty] #5429853 05/15/20 12:05 PM
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As another poster said check the electric fans. Do this NOW not tomorrow, you will damage the engine or loose a HG and possible warp the head(s) if it already hasn't.
If you cant do it take it to a mechanic.


ASE L1, Master. Deutsch Meisterbrief.
Re: Coolant boiling in reserve tank [Re: Davejam] #5429858 05/15/20 12:08 PM
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If you mean don't keep the AC temp at 100% cool, I'm not running it at all. As for keeping the fan running, do you mean the blower fan in the cab? No one has yet answered my question regarding the engine fans!


'98 Buick Century
Re: Coolant boiling in reserve tank [Re: CNAKitty] #5429859 05/15/20 12:12 PM
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mk378 Offline
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I have answered your question about engine fans.

After prolonged idling, they should start to cycle on and off enough to keep the engine from overheating.

When the car is moving 20-30 mph, the motion of the car pushes air through the grille and the radiator and fans generally stay off.

If the A/C is on the fans will run constantly or a lot more, even if the engine is not hot.

Last edited by mk378; 05/15/20 12:14 PM.
Re: Coolant boiling in reserve tank [Re: mk378] #5429865 05/15/20 12:16 PM
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CNAKitty Offline OP
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Thanks for fan info! I can check that now! Replies coming so fast I almost missed yours!


'98 Buick Century
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