Gs300 coolant is bubbling in resevoir,but temp is fine ?

check coolant level fan operation and water pump

Refill the system and bleed it properly, if bubbles kelp forming need to perform a leak down test. The test is simple, take spark plugs out then one cylinder at a time test it by injecting air while in top dead center possision, if the head gasket is bad you will see air bubbles rising in the coolant. Also if air leaks anywhere else then it is also bad. Next cylinder , intake, exhaust or crankcase. Good luck, if you pay somebody to do it is should cost you a little more than replacing the plugs. If it is diesel is the same procedure but through the global plugs.

Try changing radiator cap if there is no water in the oil it is not a head gasket could be system just needs a flush. It could also be the water pump is not pumping well try the cap first.

to check if air in system, let cool down and remove radiator / coolant reservoir cap and systematically squeeze coolant hoses including heater hoses, you should see bubbles in reservoir as you squeeze hoses, continue until no more bubbles, then top up coolant to required level.

Hi!

Your coolant temp sensor may be defective, check wiring also, if this senor doesn't send the signal the Cooling Fan won't kick in. Fan will come on when temp reaches half way on gauge.

Good Luck!!

Check your radiator cap...the rubber seal maybe bad...and as for your guage...check the coolant temp switch or sensor...One controls the fan, and the other controls the guage..

install a manual guage and forget it.

When the car is completely cool,check the electric fan(s) for smooth rotation.

Clean/check/change the thermo sensor,contact and wire.

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Excavate air pocket in coolant system / check for head gasket leak

This test will kill two birds with one stone.

===

MAKE SURE THE COOLANT SYSTEM and ENGINE IS COLD!

RAN THIS TEST IN A WELL VENTILATED AREA ONLY!

You will spill some coolant during this air pocket purge test.......BEKIND TO THE ENVIRONMENT and ANIMAL please clean up after the test!

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Put the front end on a pair of ramp or park your car on a VERY STEEP HILL (radiator facing top of the hill) .

Top of the coolant reserve tank

Let it ran for 10-15 minutes.

Monitor for air pockets escaping from coolant reserve tank.

Small amount of bubbles is OK at 1-5 minute mark

After the thermostat open up (after 195 F warm up) at
5-12 minute mark or after high idle you should see less bubbles.

If you do not see any in rush of bubbles then your thermostat may bepartially stuck or rusted badly inside the thermostat hosing.

Give the thermostat host few gentle taps.

If you see larger bubbles surfacing after 15 minutes then should do ahydrocarbon (HC) dye test to test for potential head gasket leak.

Let engine cold down and top off coolant reserve tank.

Start monitor for coolant lost

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A coolant flush is require every 2 years or 24,000 miles.

I recommend the thermostat that has a relief pop-let to reduce the change of burst radiator and coolant hoses.

Make sure you get a new thermostat gasket,black RTV and fresh coolant for the job.

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Gas bubbles in the coolant resevoir while the motor is running indicates a blown head gasket.
If in doubt, take it to a radiator shop and ask for a combustion gas check on your radiator.

I think you need the head gasket replaced.

When the car is completely cool,check the electric fan(s) for smooth rotation.

Clean/check/change the thermo sensor,contact and wire.

===

Excavate air pocket in coolant system / check for head gasket leak

This test will kill two birds with one stone.

===

MAKE SURE THE COOLANT SYSTEM and ENGINE IS COLD!

RAN THIS TEST IN A WELL VENTILATED AREA ONLY!

You will spill some coolant during this air pocket purge test.......BE KIND TO THE ENVIRONMENT and ANIMAL please clean up after the test!

===

Put the front end on a pair of ramp or park your car on a VERY STEEP HILL (radiator facing top of the hill) .

Top of the coolant reserve tank

Let it ran for 10-15 minutes.

Monitor for air pockets escaping from coolant reserve tank.

Small amount of bubbles is OK at 1-5 minute mark

After the thermostat open up (after 195 F warm up) at
5-12 minute mark or after high idle you should see less bubbles.

If you do not see any in rush of bubbles then your thermostat may be partially stuck or rusted badly inside the thermostat hosing.

Give the thermostat host few gentle taps.

If you see larger bubbles surfacing after 15 minutes then should do a hydrocarbon (HC) dye test to test for potential head gasket leak.

Let engine cold down and top off coolant reserve tank.

Start monitor for coolant lost

===

A coolant flush is require every 2 years or 24,000 miles.

I recommend the thermostat that has a relief pop-let to reduce the change of burst radiator and coolant hoses.

Make sure you get a new thermostat gasket,black RTV and fresh coolant for the job.

===
Please post more information by clicking the comment link on the top right corner.

DON'T FORGET to RATE ME if my tip is helpful to you!