Hello forum. I’m Jeff.

I have a very nice 2003 G35 sedan (209K miles, runs great) with no rust and a body in like-new condition but I’m experiencing persistent overheating with boiling at the cooling reservoir. All the parts you’d expect to be replaced have been. New radiator, water pump, thermostat, heater core, and temp sensor. It’s not leaking. It has no codes. No CEL illuminated. It’s been to the dealership and they cannot resolve my issue. This car is too nice to let go. I still love her and want to save her but I don’t know who to turn to. I’m not a mechanic so it’s pointless telling me how to try and fix it. I need a G doctor. If there’s such a guy. The dealership says the engine block is blocked and they can’t flush/clear it adequately. Could be true I guess. They say I need a new engine. Nah. They blame my use of a radiator stop leak product a few years ago. I don’t know what to think about that. Another mechanic who hasn’t seen the car yet (he’s swamped) thinks it’s the head gasket but won’t do the job even if it that’s the culprit. I don’t think I have any of the symptoms of a bad head gasket. But I do need to have a mechanic test for that I guess. I can still drive the car when it’s cold for a while. Heat will work and the temp gauge will stay normal. The more heat that builds up with driving, the more likely the gauge is to start climbing, heat to be lost and the coolant reservoir is to fill up and start boiling. I’m actively monitoring these and not pushing the car past its current limits. When it’s too hot I stop driving it, open up the radiator cap and overflow reservoir cap to let the heat out more quickly and I’ve even taken to sucking the coolant out of the overflow reservoir and putting back into the radiator rather than waiting for it to return on its own. Does anyone know of a mechanic who’s dealt with this before and can repair my G?? Thank you in advance. I can answer questions about the symptoms but like I said, I’m not a mechanic so I’ve not done anything to try to fix it myself. Who can help me??

 

You probably have the single mechanical fan and single electric fan yes?

Good chance the mechanical fan clutch is worn out and needs replacing. You can test it yourself by getting the engine up to operating temp, shut the engine off, now try to spin the mechanical fan by hand, it should feel pretty stiff and when you try to SPIN it it will immediately stop. Compare to what it's like when the engine is cold and you'll be able to spin the mechanical fan pretty freely but probably not make more than a half a rotation. Go to a parts house and feel how stiff the mechanical fan is by having them pull one off the shelf for you, then you'll get a good reference for how much tension should be on cold spin. A worn mechanical fan will have a LOT of free spin to it, sometimes even when hot, it has a spiral spring that presses on a clutch plate, the hotter the spring gets (from soaking up engine heat directly through the metal-to-metal contact) the harder it presses that clutch.

Also possible the electric fan isn't even turning on, let the engine run for 30seconds then turn your AC on max high, that will force the electric fan to turn on, make sure it does and look at it with a flashlight to make sure it's spinning fast.

Sometimes the electric fans spin slow when they're getting worn out, sometimes the relay in the IPDM fails and the electric fan just doesn't turn on.

That's where I would start just to verify it's not a fan cooling issue since what you're describing "heat will work and the temp gauge stays normal, the more heat that builds up with driving the more likely the gauge is to start climbing" sort of indicates that the engine is cooling, but you hit a threshold somewhere that causes it to boil over.

Another likely culprit is you have a small leak in the system that's not allowing it to maintain proper coolant PRESSURE. You might not be leaking coolant but that doesn't mean it's not escaping as a gas, a coolant pressure test will verify that the system holds the roughly 13psi needed to maintain lower boiling point. You can rent the tester yourself (Stand pressure tester is the industry standard, look up a youtube video how to do it). I always start by pressure testing the cap, the release pressure for the cap will be listed on the cap in either PSI or BAR, makes sure the cap passes, then apply the CORRECT pressure to the coolant system based on what the cap pressure is. Normally it's something like "if the cap pressure is 13psi then you test the system at 15psi" type of thing. It's super easy to do, if you know how to install a radiator cap then you have the skills to do the pressure test yourself. It's a hand pump like for a bicycle tire, but designed with adapters to fit your cap and radiator. Usually your local auto parts house will rent you a coolant pressure tester for nothing more than a security deposit because they want to sell you repair parts.

Or you can pay a shop to do it, just make sure your coolant temp has gone down before taking the cap off, or at least know how to do it with a rag and a firm grip.

If the fans both pass inspection, and it passes a pressure test then it's time to schedule an engine leakdown test to see if the head gasket is blown, tell the mechanics the reason for the leakdown test so they know to look for bubbles in the radiator.

 

Thank you for the reply. I will definitely look closely into the fan/fans issue to see if that’s the problem and I think pressure testing the cooling system is a good idea, something I can probably handle. I am presently into a “new to me” car (2013 Lexus ES350 Florida car with 71K miles) so I’m looking to move this G car regardless. It’s still so nice and has so many new and recently installed parts into it, I’d love to find someone who can save her. Until this overheat took her off the road this was my regular daily driver since 2016 that I loved driving. Even with my new car on the road, I miss the novelty of driving a 20 year old car in such fantastic condition. If anyone is interested in this car, you can text me at nine seven 8 four 2 three one 1 seven four. Thank you!