Weird overheating 2005 G35 AT Coupe
 

I noticed about 3 weeks ago that when I got home from the long interstate drive home from work the radiator fans would be running at high speed. I had never noticed that before. So I read about how the fans come on high when the coolant temp gets over 212F in the manual. I have a computer that hooks to the OBD2 connector and can read the ECT sensor and sure enough it was getting that hot. The weird thing is I could let it idle all day and it would get above about 208. But as soon as I start driving down the road at a normal speed it would heat up. So I replaced the thermostat. I put the old one in hot water and confirmed it would not open. I then put in a new one from Oreillys (Murray made in germany). I verified it opened before I put it in. I have also gotten the air out as best as I can. However it now has the same behavior only worst. The old thermostat would let some water by even closed and this one was a pretty good seal. I was driving in 50degree weather and as long as I was stop and go, or keeping it in 1st or 2nd and driving it with high RPMs the temps stayed 196-204. As soon as I just drove down the road normal at 50mph in D the temps would start going up. got as high as 220 in a short period of time.

Is it a bad water pump (it has to be working or I would be getting worse overheating...right?), the thermostat isn't working like it should, something clogged in the system?

For most of my testing the hose going from the bottom (output) of the radiator up to the thermostat was very cool to the touch while the upper hose was almost too hot to touch. However if I let it idle and get hot, then that hose heats up some. Its as if the thermostat partially closes when speed is high and the water pump needs to spin really fast (from high engine RPMs) to get enough coolant to circulate.

Thank for any help. I'm pulling my hair out on this one.

The only thing I can think that might cause this is some sort of blockage or maybe a slipping impeller with the water pump. I have read this is rare but can happen. The fact that revving the engine makes the coolant temp go down is proof that the water pump is working some.

Did you ever find out what was wrong with it, I'm having same issues

If your engine has 125k or more miles it probably needs a new thermostat and radiator, I wouldn't even bother troubleshooting just replace them. Save yourself a future failure and replace the fans as well since they're fairly cheap.

Rad, nah, fans, yeah. Mine went bad at 130k ... on a date lol.

Cleric670@gmail....no car only has 59k miles it practically smells and looks new...and yes Urbanengineer, mine broke while i was going to the movies with my lady. :( care is very clean...https://cimg3.ibsrv.net/gimg/g35driv...e441ba6996.jpg

Cleric670@gmail....no car only has 59k miles it practically smells and looks new...and yes Urbanengineer, mine broke while i was going to the movies with my lady. :( care is very clean...https://cimg3.ibsrv.net/gimg/g35driv...e441ba6996.jpg