Hello, my 2006 infiniti g35 sedan overheats. It's a VQ35DE.
I have the following: new radiator, fans work, new ac compressor, new heater core, new thermostat, new water pump, new head gaskets, fresh coolant. I can confirm everything is OEM except the head gaskets and heater core, those were replaced shortly before I bought the car so I have no idea what brands were used/who did the repairs

My mechanic did a leak test and a pressure test for the engine and they confirmed there's no leaks or cracks in my engine but my car still overheats, we've tried bleeding the system multiple times but pressure tests show there is still air getting in somehow very slowly.

At this point I am running out of ideas, the car has 124k miles on it and almost everything under the hood is brand new OEM. Does anyone have any other ideas as to what it could be? Or is the engine just a dud? I can plan to swap the engine but before I go spending money like that I'd ideally just like to solve the overheating. I've already spent almost $4k in replacement parts and labor so you can imagine my pain and frustration.

Sometimes the car overheats when I go down the road, other times I can drive 30-45 minutes and my temp gauge won't budge. I have to keep my coolant reservoir at the MIN line otherwise it bubbles over and sprays hot coolant all over my engine bay. Either way, the car hardly lasts an hour before overheating, and if I turn on my A/C it wastes no time overheating and goes straight for the H in less than 5 minutes. The car is completely stock except for the rims.

Any and all help is greatly appreciated.

 

Describe in better detail the "leak test and pressure test" the mechanic performed.

Was it a LEAK DOWN test, and a COOLANT PRESSURE test?

Did both those tests pass? I'm finding it really hard to believe your car passed both a leak down and coolant pressure test within normal specification yet you're still overheating after replacing all those components.

Typically when you have random overheat within the time-frames you mentioned it's because the coolant system isn't holding pressure, if it can't hold pressure the boiling point of the coolant is too low and it will boil over eventually.

Does it boil.over at freeway speeds or just city speeds?

 

Also, does the heater blow hot air?

 

Sorry, my mechanic did the leak down test and did a pressure test on the head gaskets, those passed.

When they did the coolant pressure test that is where we saw it losing pressure slowly.

The boiling over seems random enough that I cant personally pinpoint it, yesterday I drove down the street like 5-10 minutes and I was driving city speeds (35-40 mph) and it overheated, but later that night when I was on the freeway for about half an hour it overheated going one way, and then (after letting it cool) on my way back it didn't overheat at all.

As far as the heater goes, last time I tried to use it with the AC button on it was blowing cold and caused an overheat, same with the cold setting. When I use just the fans it blows warm air, not 90 degrees like my dashboard would indicate.

 

Ok if the coolant system won't hold pressure it will definitely overheat. Focus on fixing that problem.

 

Well yeah, the problem is we don't know where the leak is coming from in the coolant system. With all those parts replaced it seems like we checked everything.

 

That's why you need to do a coolant pressure test, that's literally how you find the leak. Pump the system up to 13-15psi and start going over everything with an inspection mirror. I guarantee you'll find the leak. Spray AC leak bubble water around the top of the radiator where the top flange is connected. Sometimes the seal fails but the coolant level isn't high enough to actually push out water. Likely you're losing coolant as STEAM not as a liquid.

There's also a bunch of coolant pipes that wrap around the sides of the block to connect water jackets, those are more of a pain to inspect from the top of the car so you'll probably need to get it on jackstands and remove the bottom splash shield to see them. Same with the oil cooler sandwich adapter behind the oil filter.

 

Also, sometimes it helps to use a mechanic stethoscope with the coolant pressure tester. You will hear the air hissing out, it's REALLY loud with the stethoscope in. The only issue is you need to be in a fairly quiet environment to use it.