I have a 1996 Monte Carlo with a 3.1.
I have bubbles in my coolant overflow only after I rev above idle and then let it go back to idle.

I have preformed 6 Block Tests at various times,liquid stayed as blue as the sky. I have had a leak down test done and was told it was good.

New Radiator,Radiator cap, Thermostat, I have checked all hoses for holes and clamps are on snug.

Not a drop of water in the oil,not a drop of oil in the anti-freeze.
Engine runs perfect,engine does not over-heat and has not over heated since I have owned the car(seven years)
I have not lost a drop of anti-freeze and it stays exactly on cold mark in overflow when cold and on the hot while running.
Water pump is about 1 year old.

Engine Vacuum is a steady 17hg.

I made a radiator cap type tool out of a old radiator cap that I put on that allows unhindered flow to overflow to help purge air out whenever I do work on it.

It only bubbles when I give it gas and let it come back to idle.

Everyone says Head or Intake gasket but I would hate to do all the work and it be something simple.
I am at a point of just driving it until/if/when it shows more signs.

 

I had an 03 eclipse that done it,when I punched the gas hard the coolant would shoot out the radiator and just bubble when I let it idle,i ended up having the heads replaced,water pump,and timing chain,they never said what caused this but when I don't a radiator flush I pulled 5 lbs of engine block sealer out of it,so my guess would probably be the water pump or small leak in one of your gaskets

 

I will just change the Water Pump since that is a easy and cheap change and leave it alone from there,I'll be sure to get a good one. No sense worrying about it until it shows more.

 

I had a friend that had bought all the gaskets and such that decided not to do it and just traded his car in,he sold them to me for $50.

I decided to go ahead and change the head gasket and intake,etc.

Low and behold the head gaskets were bad on both sides,I don't see how it didn't have any symptoms and passed all the tests.

I guess you live and learn.

 

Glad you got it fixed!!!

 

Was it the nylon gaskets? Did you change to metal?

 

a shop told me it was the gaskets when i had it happened and i thought they was BS'n me but they was right,mine would get to the point if i stomped on it while the cap was off it would shoot a few inches in the air

 

When I originally bought this car the Dealer said he had put new gaskets in it. It had the metal Felpro Intake Gasket but the Head Gaskets were actually rotten in front of both banks(#1 and #2 cylinders).
The fire rings were the only thing between the water jacket and combustion chamber. I can't believe it passed those tests and even ran as great as it did.

 

Oh wow. Yea sometimes the car will try and trick you.