Coolant boiling in reserve tank

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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?
 
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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.
 
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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?
 

CNAKitty

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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.
 

LDM

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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.
 

CNAKitty

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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.
 
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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.
 
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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.
 

LDM

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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.
 
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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.
 

CNAKitty

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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!
 
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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.
 
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CNAKitty

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NO mechanic will touch my car right now (nor could I afford them taking days which they tend to do in my experience). I'm a front line nursing assistant working directly with COVID 19 patients, and therefore exposed nightly to the virus, which means so is my car's interior from my driving home in it. And yes I wear my PPE, but it doesn't cover every inch of me and my clothes. I'm pretty much self isolating since starting working directly with the infected, as I don't want to be responsible for accidently infecting anyone. Fortunately I live alone, and while it's been years since I've worked on my cars, I DO have some knowledge about what I'm doing, and enough sense to have a Haynes on hand.
 
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With the engine off try spinning the fans by hand. My son's car had frozen/rusted fans that did not spin.
 

JHZR2

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HG came to mind first, coolant ratios second. Are your fans working? What does the temp needle show?
 
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Originally Posted by CNAKitty
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!
Whether or not the AC is on is irrelevant. On your cool/heat knob as long as its not 100% cool, coolant will pass through heater block and your fan will pull air dissipating some of that heat. But the fan should be on. So, as an extreme measure if your overheating turn your knob to 100% heat and fan on high. I realize that will sweat you out, but may save an engine.
 
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Originally Posted by Davejam
Originally Posted by CNAKitty
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!
Whether or not the AC is on is irrelevant. On your cool/heat knob as long as its not 100% cool, coolant will pass through heater block and your fan will pull air dissipating some of that heat. But the fan should be on. So, as an extreme measure if your overheating turn your knob to 100% heat and fan on high. I realize that will sweat you out, but may save an engine.
Having the AC on forces the fan on because it brings cold air through the condenser. It's a good diagnostic because the car for whatever flawed reason may not want the fans on. Running heat is a rudimentary, emergency way to get heat out of the engine. If one can run heat *with* AC, (eg defrost mode) it should force the radiator fans. If OP lives somewhere cold, the fans could have sat all winter and become jammed. If lucky, spinning the blades by hand (ignition off) might unfreeze them! Another diagnostic option, under the hood, is to jumper the fan relay inside the fuse box. You'll need a paper clip, unfolded, and the car on or running. Unplug the fan relay, should be a little grey or black box an inch across. Study the little diagram on it, should be 4 pins-- two of which are 87 and 30, with a little "drawbridge". Orient yourself upside down like the relay then jam the paper clip in those contacts in the fuse box. Should see a little spark and hear the fans kick in. A flaw is this won't turn off with the key and will run your battery down. An additional diagnostic, very crude, is to disconnect the coolant temp sensor. This will light the check engine light and force the fans on. It's also not great for the fuel mix so it'll run worse. It's a 2-wire connector near the thermostat outlet which is on the driver's side of the motor in the "valley" of the V6. You may have to take off the cheezy plastic engine cover to find this, but it's easy, just yank the oil filler pipe then pop it up and off. Consider calling a local GM dealer and explaining your situation-- you might be able to arrange a "house call" where they only go under the hood. Some dealers are having sales on labor rates right now.
 
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