Yeah, I've done all the usual stuff... lift the front end with the bleed funnel, rev it, etc. then let it cool down, repeat...
I've been running the car up to temp and using the Nissan procedure of holding revs at 3k for 10 seconds, repeat 3 times, then let it cool off, check and fill coolant, and repeat until no more air is trapped in the system. No luck so far, every time I drive it for a little while and come back, the reservoir is bubbling.

Like I said before though, I do have heat at idle now and it doesn't seem to want to take much more coolant at this point. Maybe I'm getting closer, but I still find this absolutely insane that it could still have air trapped in there. Any time I've seen this before, there was a leaking head gasket that showed itself either by a combustion leak test, or scoping the cylinder to see coolant dripping into it.

 

Ran it again today, drove it up and down the road a couple times, and got it nice and hot. Monitored the coolant temps on my Autel scan tool, and it stays around 190-195ish at idle. Temps go up to 208 or so and the fan kicks on, brings it back down and goes off. Everything in that department seems to be working perfectly.
The only problem, like I've said, is that the coolant level shoots way up. The reservoir was about halfway (over the max level already) full and then by the time it was hitting 208 degrees, the coolant level was at the top of the reservoir about to overflow. You can see small bubbles coming into the coolant, so there is still air in the system. No idea how, or where it's coming from.
In every case I've ever seen an issue like this that isn't solved by bleeding the cooling system or replacing the cap, it's been a head gasket leak. But there is zero evidence of that here.

All I can think is that there is a blockage of some sort (where and how?) or the water pump is causing cavitation, but how? Makes no sense.

 

Damn that sucks. Sounds like your doing everything right. Have you checked the coolant crossover pipe that connects to the bsck of the heads? Those gaskets can leak. The small drips can burn away on hot engine before it drips to the ground. Happened to me before. Other then that i got nothing. Goodluck man.

 

I did check that crossover, but I'll try to take a closer look at it just in case there's a very small drip that I haven't been able to see. This is really throwing me off, it makes no sense that air is getting in and no coolant is getting out, given that the cooling system is PRESSURIZED! All that makes sense to me is a cylinder leak, but there is zero evidence of that. No coolant visible in the combustion chambers, no cylinder leakage into the cooling system, and no combustion gases present in the coolant.

 

Back at it again... I have been slowly trying to get the front end put together (fixing a bunch of wiring, broken core support, etc.) but want to try to get this coolant issue sorted before putting in too much work. Then again, kinda late for that at this point lol.
I lost heat while idling the other day, and I noticed that the coolant wasn't pulling back into the system from the overflow after it cooled down. I clamped the line on the bottom of the reservoir, as I saw a trace amount of coolant at the fitting, so hopefully that's all that's about.
I tried yet another combustion leak test just to make sure, and yet again, no sign of combustion gases. When I did this though, the coolant seems to slowly push up when I have the radiator cap off. I had sucked the level down to perform this test, and after running for a few minutes, the coolant is back up and just about overflowing.It's not bubbling though.

 

I’m having the same issues with my 2010 G37. I’ve tried everything you’ve listed in this thread as well. New radiator, thermostat, water pump done in 2021, extensive bleed etc. I’ve done a block test and have no signs of a head gasket leak. The car after driving for a while usually 20-30 mins will start to push coolant out of system into overflow and eventually will fill the overflow tank pushing out coolant and cause the car to over heat due to low coolant. The radiator hoses upper and lower seem to hold a lot of pressure even after hours of the car being off which is strange in my opinion. Cars I’ve had in the past will have soft hoses after the engine has cooled. But even overnight with the g still holds the pressure in the hoses. I’m planning on taking the car to Z Fever in Tampa to see if they can find out what’s going on with the car. I’m doing a coolant flush with blue devil radiator flush to see if there’s a clog in the system somewhere. Will keep a eye on this thread and upload any updates I have to see if they may help aswell.

 

Thanks for chiming in! This sounds very similar to what I’m dealing with. Definitely update this thread with any findings!
Since clamping the overflow hose fitting at the bottom of the reservoir, it’s been better about maintaining the coolant level (rises when hot, goes back to normal when cool). Before it would stay high after cooldown. I feel like air may have been getting in at that hose.
I hear a little gurgling in the heater core when I start the car or rev it, but there may just be air working its way out still since I haven’t driven it much. Fingers crossed…

 

Hey mate! I’ve been having this same issue with my 2003 G35 sedan, I would bleed it with a vacuum, on a lift at the front wheels with a bleeder screw bleed and spill free funnel, literally everything in the book. However I’ll have no bubbles (I think) after 2 hours of bleeding and revving and would drive it right? I’d drive for a good 15 miles with no issue. On my way home from the shop THAT’S when I’ll start overheating like what? I would lose ALL heat and the temps would start spiking. I also noticed that the overflow tank would also start bubbling and overflowing just like yours, radiator hose to the firewall (heater/bleeder pipe) would be rock hard as well. I’ve already replaced the thermostat and radiator cap with bleeder screw. Now I’m stuck on if it’s head gasket or a heater core. I have a mechanical fan since it’s the early 03 models and it spins as well, however everytime I try to do a combustion leak test I keep sucking up coolant even if I remove coolant from the radiator so I’m at a loss of words. Any tips or verdicts?

 

Yeah, this sounds like air or a head gasket leak. It’s crazy to me to think that this many people have a head gasket leak on these cars at any given time though. Typically there’s something that causes it (severe overheating), not just a random failure.
I wish I had something more to recommend, but basically go through everything that’s been brought up in this thread. And maybe eventually we’ll all get our cars to hold and circulate coolant properly 😂

As far as the combustion leak tester, pull back as much coolant as you can, warm up the car with the cap off, and keep sucking out coolant if needed. Carefully use the tester once the thermostat opens up, just make sure you don’t suck in coolant. You may have to hover it over the radiator neck just to ensure you’re not pulling too much suction on it and sucking up coolant.

 

Little bit of an update. Drive from Tampa to west palm for thanks giving.(3 hr drive). Before the trip I flushed the cooling system twice and filled with radiator flush by blue devil. Coolant jug at the min mark with distilled water. And the system bled with a no spill funnel topped off to allow bubbles to escape, ran the heater full blast and let the car run for about 45 mins before the trip. Made it halfway on the drive and had to pull over because it began to over heat. Same thing coolant jug full to the brim and no coolant in system. Bled it again and made it the rest of the way. While I was over there I took advantage of the Black Friday sale z1 was having. I purchased the coolant bleeder that replaces the fitting on the heater hose line and a high pressure radiator cap. Oem is rated for 14 psi while the high pressure is 17.5psi. Bled the system a couple times during the weekend in my spare time and had the car parked on a slope with the radiator at the highest point. Drove alllll the way back to Tampa with no overheating thank god. But again the jug was full. It was still hot so I haven’t checked to see if the system had coolant in it yet. I’m thinking if the 2 things I bought don’t fix it, it’s time to bite the bullet and take it to ZFever for a full diagnosis. Will update as soon as I get all the parts on and get a good drive in this coming weekend as I’m going to do the same drive again. By the way I have heard vqs are very picky about thermostats. I’ve gone through 3 and the one on it now is a cheap one I put on as a test. I may end up putting on a oem thermostat as well.

 

Wouldn’t the easier solution for all of us just be to buy a different car? Lmaooo 😂

 

That would certainly be the easiest solution 😂

 

I'm having similar issues with my 2008 G sedan. The passenger side heater hose connector broke for a second time, had it replaced while out of town at generic import shop, and apparently they didn't know how to change coolant properly. Later in the road trip the reservoir filled up so I siphoned it off and put it back in the radiator (gradually). Running around town isn't much of an issue (most trips 5 miles or less). Last weekend I parked on an upslope and squeezed the upper radiator hose repeatedly to get bubbles out and it may have helped. The bleeder screw (allen wrench style) wasn't cooperating and I was afraid to try too hard since it's original and known to be fragile (planning to replace with metal option soon). I didn't want to buy coolant funnel since I won't be changing coolant myself. Will go to local specialist shop soon but hoping to fix with minor troubleshooting if possible. Note that I replaced radiator cap already and it seemed to help, but not 100% fix. Haven't been on any long trips since replacing the cap.

 

It sounds like your issue is just that the system wasn't sufficiently bled. I would definitely recommend getting that bleeder fitting and properly bleeding the cooling system.

 

Been a while… so I’ve driven the car a little bit, and so far I still hear the coolant rushing noise at the heater core when I rev it. The coolant level has stayed relatively steady at the max line, however today it went up a little bit (maybe an inch above the max line) once it was mostly cooled off. Obviously the level is higher when the engine is hot, but it has gone back down to where it was (max line) except for today. I drove it for about 20 minutes, let it idle another 45 minutes or so, and that’s the result. Heat was still blowing warm after idling all this time, but I heard the coolant when I revved it with the heat on.
No clue how there could STILL be air in there, but I guess anything is possible. I can smell coolant too when I stand in front of the car, but there is no visible leakage anywhere. Just residue on the fan shroud from when I had checked the radiator level the other day. Maybe the smell is venting through the reservoir vent?