We are all quite familiar with how annoying it is to bleed a VQ35 cooling system, but I'm having a really difficult time here. I can't figure out if there's a leak somewhere, or possibly a blockage. I bought a 2006 G35X a couple months ago, and the seller told me that it was diagnosed as some sort of blockage that needed further teardown. I assumed a head gasket leak, but I haven't gotten a positive result on a combustion gas leak test. Maybe it's a super small leak and that's why, but who knows.
When I pressure test the system, it loses pressure slowly. I cannot find ANY leaks ANYWHERE. I vacuum filled the system, then topped it off with the bleed screw open. Then used a bleeder funnel with the front end raised. By the time it's hot and the thermostat is open, you can see bubbles rushing in through the stem of the funnel. That's what happens while driving, it just pushes the coolant into the overflow until it eventually pours out of the cap. It actually melted some of the core support since it was gushing over so badly. I've followed all of the suggestions of revving it, letting it idle, heat on high, etc. and that has not helped. I still have bubbles rushing in.
Interestingly enough though, the car has never overheated on me. I haven't driven it more than 20 minutes at a time though.

I think to be sure, I'm going to do a leak down test, but I just wanted to post this to get some ideas as to anything else I should check first.

 

I did a leak down test and can confirm that there is NO air escaping into the cooling system. Is there anywhere else than can be leaking and not show? Maybe a strange water pump issue? It definitely seems to be working, given the way the coolant level rises and falls as it's revved.

 

Drained and filled the system twice, tried to bleed it again, and MAYBE it's better? But it's still bubbling coolant into the reservoir after driving. I'm at a loss here. There may potentially be a leak on the top right of the radiator (under the intake) but I can't get it to show itself under pressure, whether while hot or while pressure testing. I just see a trace of coolant there in the crimp, not sure if it trailed down (no trail to be seen except for coolant crust) or if there's a tiny leak.
You can see the wetness at the right end of the radiator.

 

Update on this situation... I've driven the car a few times, making sure to drive it hard and try to force out any air, and so far nothing has changed. I slammed on the brakes good and hard to try to dislodge any air, as I've seen suggested on here. Each time after it cools off and I check, there is no coolant at the bleeder screw. I top off the radiator, and get the bleeder screw/heater hose full, then close everything up, and it still starts bubbling coolant back into the overflow once it gets hot after a little bit of driving.
I've kept an eye on the potential radiator leak, and that dried up and is NOT a leak.

All I can think is that the cooling system is still allowing air to enter, but from where? I have checked or replaced every hose, gasket, and connection. My cooling system pressure tester slowly bleeds off when I apply pressure to the system, however that seems to be the tester itself, as I did this on a known good cooling system (friend's G35) and had the exact same results.
Maybe the radiator cap isn't sealing well enough onto the radiator, and that's allowing it to boil over? It's an aftermarket radiator from the looks of it, so I guess something like this is not unheard of. I don't want to just throw parts at this car though, that doesn't make any sense to me.

 

I ended up replacing the radiator, as it WAS actually leaking. But we're back to square one... still boiling over the coolant once it's hot. I checked AGAIN for combustion leaks, and the fluid did not change colors. I have a really hard time believing I have a head gasket leak if there is no exhaust getting into the cooling system, especially considering how much it bubbles in the funnel once it's nice and hot and I give it a few good revs.

I did a compression test when it was hot, and these are the numbers I got, laid out in the order of the cylinders while looking at the front of the engine...
160 155
170 180
155 160

The center cylinders are both a bit higher, particularly cylinder 4. I don't know if the heads warped, and that's why the outer cylinders have slightly lower compression, but I have no idea how that would be the case with no combustion gases getting into the cooling system. That's seriously baffling me.
The only other thing I can think is that air is getting into the cooling system elsewhere, but where? I cannot find ANY leaks anywhere. Anybody have any ideas? Or is there just that much air trapped in here?

 

If you filled it properly and have no big air pockets, air can't get in without coolant getting out and you'd see that as a leak or steam coming out the tailpipe. It's a pressurized system, not a vacuum system that will suck air in during operation. The only way air gets into the system once you've closed it up is by sucking it back in from the overflow after it cools down. Is your overflow ever empty after running and then cooling down? Can you run the car with the pressure tester attached to see what kind of pressure it holds? Wondering how well the radiator/cap is sealed so the system is under proper pressure. The coolant will boil at lower temperature if the system isn't pressurized. What is your reported coolant temp when the thing is pushing coolant into the overflow? Your fans should kick on at 212 and go off at 205 or something like this. Are both your fans running full speed? You don't mention fan operation anywhere....

 

I'm pretty sure the fans are running properly, but I will double check to be certain. That is a good point as far as being pressurized, but if there's a leak, it can't hold pressure (obviously).
The overflow is never empty, usually overfilled slightly after it cools off.The coolant temp was about 190 at the upper hose when it was boiling over into the funnel. The fan had not kicked on until a little after that.

I'll run the car with the pressure tester and see what happens. It may just come down to still having air pockets, as crazy as that seems to me with how many times I've bled it at this point.

 

That's too low a temp for the coolant to be boiling in your funnel. Something doesn't make sense. How do you know the coolant temp was 190 at the upper hose specifically? Are you getting that from a scanner that can read live data from the coolant temp sensor on the car? That's what really matters. The car needs to know what the temperature is in order to operate the fans properly.

There's a test mode you can put the car in that will run the fans in low and high speed so you can observe that they're both spinning properly. If they BOTH run at low and high speed during the auto active test, you know the car can control the fans properly as long as the ecm gets the right info (coolant temp). But if you're pushing coolant into the overflow before the fans even come on at 212 (or something around there), the fans aren't going to help.

 

I was checking with an IR thermometer, I wasn't looking at scan data at that time. I lifted up the front end today and have been running it and letting it cool off with the bleed funnel and so far it seems to be slowly improving, and bubbling up less each time. I have managed to get a good bit more coolant in each time it's cooled off as well, so I think I'm finally making progress.
Maybe it's because I come from BMWs which are easy to bleed, but as a mechanic I have never seen a cooling system that gets air locked worse than these VQs.

 

These cars can be tricky to bleed the air out. This is what i do and ive done it several times with good results. Raise the car up. Use a spill free funnel. Unscrew the coolant bleeder. Pour coolant in very slowly. Dont let it chug chug heh. Once coolant spills put close bleeder. Start car with heater on low. Its a good idea to have a scan tool that shows you coolant temp. I use cheap bluetooth obd reader for phone.

Let engine warm up. Keeping eye on temp. Rev engine to 2-3k rpm for 10sec. Keep doing that. You fans should kick on at 203degrees then shut off around 195degrees. Sometimes the temp will shoot up to 220-230 and thats when it will start to overflow. Shut car off before that happens.
Just keep doing that. It can take 30mins to an hour sometimes for me. Hope that helps goodluck.

 

Thanks! I'm probably going to have to do this once again. I drove the car today for a little bit and the overflow level was quite a bit higher and I could see bubbling inside of it by the time I got back. Still some air in there I guess... ugh...

 

I've gone through the bleeding process several times at this point, and I do actually have heat at idle now, however every time I take it for a hard drive and come back, I'm getting a high level in the overflow and it's bubbling.I've managed to get a good bit more coolant in there, but I have no idea where these air pockets are coming from. Does it really take that many hour long run cycles to bleed these things? I've never seen anything like this in my life.
As I said, there are no signs of a head gasket leak besides slightly lower compression on the middle cylinders. No leak down into cooling system or any exhaust gases in the cooling system.

 

Have you been able to monitor coolant temps with an OBD scan tool? Or verify the fans are turning on around 203 degrees. If you say there's no sign of a head gasket leak, then maybe it's a fan issue. Has the thermostat been changed with OEM? Maybe it's the coolant temp sensor. Coolant temps while driving around should usually be in the high 180s to mid 190 degrees.

 

Coolant temps are normal, fans are running as they should. The car has a new OEM thermostat.

 

Agreed, you gotta lift the front end up high as you can to get the rad cap the highest point in the system. I recently got my G35 and it took at least 3 tries until I got it on stands to lift up the front.