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After driving for about 30 min then stopping I hear bubbling coming from coolant reservoir. Also if I drive for longer period and come to stop lights at traffic intersections temperature gauge starts moving up, does not reach red, then as soon as I drive it goes down. I ran car on idle for 20 min and there is no bubbling on coolant reservoir after stopping or no overheating at all (gauge moving up). What could problem be? Car has new Radiator and thermostat.

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    Check your radiator fans for proper operation at idle. Also include year, make, model, engine and mileage
    – user9181
    Commented Jul 30, 2019 at 17:38
  • Is the radiator cap ( pressure) new also ? Commented Jul 30, 2019 at 21:57

5 Answers 5

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As other posters have already mentioned the issue will most likely be air trapped in the system.

To fix this, you need to find the highest point of the system (in most cases it will be on the radiator) and unscrew the cap, then set the A/C's temperature and fan speed to the max (also set the direction of the fans to the windshield) while the engine is running and let the car sit for 10-15 minutes. If the issue truly was the trapped air you should see the coolant "disappear" from the top of the fill hole as it flows down to your engine and the pipes that it's supposed to fill.

After this is done you should top up your coolant up to the "MAX" marking which can usually be found on the coolant cap.

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Unfortunately, this is usually a sign of a blown head gasket.

Check the oil for coolant contamination. Check the coolant for oil contamination.

Another test that you can do is a compression test for tall cylinders.

Depending on the age of the car, you may want to consider a head gasket sealer but keep in mind it doesn't always work.

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You mentioned a new thermostat and radiator were installed. Air can get trapped in newer engines during such repairs and unless purged (usually in a vehicle specific manner), can cause a number of odd symptoms.

That's the easy one...

The bubbles and overheating at idle do also point to a head gasket leak or a crack in the cylinder head. If the old radiator failed and the engine overheated significantly, its much more likely to be serious.

Easy checks...

Check for a milky deposit on the underside of the oil fill cap and discolored antifreeze (assuming you have fresh coolant). If your engine is pushing exhaust gasses into the cooling system while running (the bubbles in the overflow tank), some coolant may going the other way into the combustion chamber when the engine is off. If so, look for white smoke (not the light, wispy normal kind) and a somewhat rough idle upon starting up after sitting overnight. Coolant has a very distinct odor when burned too, your nose...knows.

I have seen success with stop-leak products, if you choose not to fix the head or head gasket.

You will be on borrowed time...

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It sounds like the head gasket. I currently drive an Opel Astra and have the same problem. The mechanic told me I’ve a few days / weeks with my car before it fully goes. But I’ve been driving the car months now. I can’t be kept in heavy traffic. And at traffic lights I switch off my car. I keep the car coolant topped up fully at all times. You can keep driving the car but won’t survive in heavy traffic. My coolant looks brown now. All the signs are there.

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I defer to the previous answers but it is such a 'car-specific' issue as in the case that the head gasket is not blown nor is the block cracked and there are no compression issues after testing then it could simply be trapped air in the cooling system. Porsche water cooled rear-enders are especially suspect of this problem as well as some similar exotics which have large cooling systems with lots of bends and tubes at different heights. But bottom line is that IF your car need more and more coolant then something is wrong. You might double check that the repair put on the correct coolant cap. I've seen a car vomit hot water before after a repair only to find that the mechanic when cheap and bought a generic radiator cap that did not have the right seating design nor spring pressure to hold the hot coolant of the properly operating engine.

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