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

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.

Sounds like head gasket has gone. Hot gas from the fuel combustion is being forced through holes in the head gasket into the cooling system, causing hot bubbles in the coolant, sniff the bubbles blowing out of the reservoir, does it smell like fuel?

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.

http://www.ls1tech.com/forums/general-maintenance-repairs/1306398-coolant-bubbling-overflow-fixed.html

Air pocket in coolant system.

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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.

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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!

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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 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

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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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THEN U MOST LIKELY HAVE A LEAKING HEAD GASKET, VERY COMMON PROBLEM ON THIS V8 NORTHSTAR ENGINE.

Check coolant levels in the radiator and the overflow resevoir. You may have blockage in the radiatior and you could have a sticking thermostat.

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.......BEKIND 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 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

===

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!

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.

check fuse and relay, check your coolant to make sure it isn't low, Can you jump the fan to see if the fan will work? If fan works, possibly coolant sensor.