So I have about 50K on my 2008 G35S sedan (auto), and I've been having the following issue for quite some time now.

The car will start to overheat if it idles for 3+ minutes. I'm not talking about letting the car idle for a long time. Going through a fast-food drive thru is an issue. It's that bad. Idling for 10+ minutes will easily trigger the high temp alert. I recently failed a California smog check due to them leaving the car idling for a moment and it throwing the high temp alert.

The strangest part of this, is that if I take the car to about 10mph, the temp immediately falls back to normal. This is not airflow cooling as it seems to be more related to a water pump kicking in or something. Some initial limited testing shows that revving the engine in park/neutral to an RPM above idle does not assist in cooling (to check if a pump kicks in at an RPM range when not moving) but is not conclusive.

Putting the car in neutral or park (vs. park) seems to help slightly as the car is not producing the same power as being in "drive but not moving", but it does not eliminate the problem.

Any ideas? It doesn't seem like something is actually failing, since it will cool down at the 10mph mark, but certainly something is wrong or not operating at full capacity.

 

When your car is on idle on the drive way or when it over heats, do you hear your fan going? It sounds like a broken fan to me.

 

The fan is definitely running.

 

My dads Honda had the same symptoms, it turned out to be the coolant temperature sensor.

When he was sitting at a light the temperature needle went up, but as he started to move, the needle went back to the normal range.

Just changed out the senor and did a drain/fill for the coolant. Hope this helps.

 

Hmm maybe do a complete coolant flush, also run a engine oil analysis at black stone, check if there's any traces of coolant in your engine oil. A small leak can be super heating your coolant. Other than that it's probably like the coolant pump or solenoid or something.