Hey guys, I am new here. I hope I posted this in the right section.
I have a problem with the coolant system on my 06 350z.

Basically the car pushes a finger's width worth of coolant into the overflow bottle each drive but never pulls it back, so after like 4 - 5 drives (about 1h - 1:30h each) the rad is low, the heater goes cold, and the car starts to slowly overheat.

Thing is I am not losing the coolant. I can just pour it back, bleed the system, and be on my way again.

Obviously I am scared the head gasket is on its way out, but I did multiple Co2 Block test and they all came back negative. I also did a pressure test of the coolant system, and it held the PSI for 10 hours overnight. There is no smoke or rough running and no continuous bubbling when bleeding. The coolant hoses only get firm after about 15 mins of running.

I tried multiple radiator caps oem and aftermarket, all with the same result. I replaced a lot of the spring hose clamps around the radiator with proper screw clamps. I checked the radiator neck and overflow hose for imperfections, cracks, or leaks.

Honestly, I don't know anymore. Only thing I can think of is to throw a new radiator in her, but I'd like to get some second opinions. I really want to fix her, but I don't just want to throw money and time at it without even having a real lead on what it could be.

 

Sounds like the cooling system is not getting bled properly. How are you bleeding it? Sometimes it takes awhile to get all those air pockets out of the system. Especially if you are not doing it properly. Not saying youre not.

Make sure you are using 50/50 antifreeze/distilled water. Ive seen people use concentrated coolant without adding water and that will definitely cause overheating issues.

 

Front of the car raised about 15cm/6in. I use a no-spill funnel crack the bleeder till coolant comes out, then I wait for the car to heat up to operating temp. I give it the ocasional rev up to ~2.5k rpm and hold that for 10secs. Repeat that till the heater blows HOT and there are no more bubbles comming out. I leave the funnel on till the car is cooled down.

Sometimes I repeat the bleeding a couple of times before testing again but thus far always with the same result.

 

That's the correct bleed process. I suspect you have a coolant leak, possibly a head gasket problem. I would use a chemical test kit on the radiator to rule out a head gasket problem. If you can confirm there's no combustion gases entering the cooling system, then start checking things like the heater core, coolant hoses, & radiator for leaks. Also keep in mind the water pump is inside the timing case. There is a coolant weep hole on the block, behind the A/C compressor. If the 1st O-ring seal fails on the water pump, it will leak out of the block there. This is important to catch before the 2nd O-ring seal fails, if that happens, then you have coolant mixing with crankcase oil.
-Icer

 

Re-reading your original post, some other things came to mind...are you running any aftermarket silicone coolant hoses? What about your overflow tank, is it aftermarket or OEM? Is your 06 a 6MT or AT?

Aftermarket silicone hoses (the cheaper ones) tend to loose water at a slow rate via sweating. They are great for holding higher pressures and handling high temps in track/race car applications, but are not a good idea for a daily driver due to the water loss issues . You can see the signs of this by the white specs that form on the surface of the hoses. The OEM spring clamps are ideal to use since they can expand and contract with heat cycles while maintaining an even 360* clamping force on the hose. Most aftermarket worm screw clamps do not achieve equal clamping force. The worm screw hose clamps are also easy to over-tighten during installation which can damage/cut the hose and cause leaks.

Have you inspected the hose that connects between the overflow bottle and radiator neck? If that hose is original, I'm wondering if it's worn out (become to soft) and is collapsing as the coolant temps drop and begin pulling a vacuum on the overflow tank. That might explain why expansion flow into the tank is working while the reverse is failing. I've never personally seen this before with an overflow hose, but I think it's plausible, especially if the hose was pinched or kinked at some point in its life. Rubber hose this size should be cheap and easy to acquire, I would just replace it for peace of mind.

I'm a bit shocked that the system held pressure for 10hrs & passed a CO2 chemical test based on your symptoms, what PSI did you test with? This really has me re-thinking the whole idea of a coolant leak, perhaps the issue is something as simple as the reservoir tank hose or some kind of blockage at the radiator neck or cap, but only in one direction somehow, like there's a check valve.

If you do have a coolant leak, it sounds like it's only happening (potentially) with the engine at operating temperature, depending on where the leak is, the heat from the engine could be burning off the coolant before it has a chance to form a puddle or drip on the ground.
Cheers!
-Icer