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2G Coolant bubbles

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Probationary Member
Jun 13, 2012
San Diego, California
A back story to put everything in perspective. I recently re-aquired my 97 GST from a friend that had it for 9 years. I got it all freshened up, flushed the fluids and made it reliable. It is my daily commuter. On my drives home, I am mainly on the interstate and I have a hill that is about 2-3 miles of constant climbing and the engine runs about 3k at 70mph(the speed limit). During this climb the engine temp creeps up. It is only 60°F-70°F outside so I don't think it is Normal for the car to really get over temp(please tell me I am wrong). I'm not boiling coolant out of the car but I definitely let off the gas and slow down being cautious. The dash showed the needle go to the top of the temp symbol but it goes back down fairly quickly after I top the hill. It was concerning enough for me to get a higher flowing after market radiator with a high volume fan shroud kit thinking my old one might just be really gunked up and not the best. The radiator helped, but I am still having this problem and when it gets warmer I'm afraid it will get worse. Since I didn't have coolant loss(boilover or it disappearing on me) or signs of coolant mixed in oil or oil in coolant. I feel everything is good head gasket wise.

One day I decided to look for bubbles in the coolant. The car was coming up to temp and I had the coolant fill cap removed from the spout (not the reservoir) and I see tiny bubbles coming from the engine direction(not the radiator side). The engine is not hot yet but whenever I hit the throttle I see more bubbles, like a constant stream of them as the coolant almost over flows. I get a combustion leak test kit and had to cut an opening in 1 gallon jug that fits the opening so I won't suck up coolant while doing the test. After the car is fully up to temp the gallon jug has slightly filled with coolant and there are no more bubbles. When I hit the throttle, I get a small whirlpool /vortex in the fluid like it's trying to suck down the fluid but it could also be grabbing air.

I used the combustion leak test kit for like 10 minutes and the fluid never changed from blue. I was worried the fluid might be bad so I stuck it near the exhaust pipe and pumped it a few times and it changed green.

So now that is all out of the way here is my question/problem.

Is it possible that without the jug on the fill spout, it was sucking down air and then it was bubbling back up?

I plan to replace the thermostat just to make sure and I might do a few more flushes since I am still getting some brown color to the new coolant after 3 flushes. Any recommendations will be appreciated. Thank you if you got through all of that and can provide some insight.
It sounds (to me) like you have a definite coolant leak, but where it's sucking air, and not leaking the coolant. Get a coolant system pressure tester and pressurize the system. Look for leaks. If none are present, I'd do the same thing as a boost leak test and spray some soapy water around to see if you can see it being sucked into the system. Definitely re-flush the system. Back when I worked on cars for a living, and still with my own cars, when I flush the system and burp it, I wait for the rad fans to cycle on and off twice. Some cars like my DD Kia, this happens relatively fast, like within 5ish minutes. With every DSM I've owned, this has taken at least 30 minutes to happen. Be patient as the old cars are temperamental. Hope this helps.
after doing a cooling system pressure test i highly reccomend buying a coolant funnel cap kit to burp the system of air.

if the car is running hot you should be able to turn the heater to hot and blast some seriously hot air out of the heater that will prevent the car from overheating. if your heater isn't working then you are low on coolant or have a huge air bubble in your coolant lines
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