Today the temperature gauge on my 240z is running slightly over half. I swapped out the valve cover yesterday for a polished one, but I don't see why that would have caused any problems. The engine is not leaking any oil whatsoever and it's not low on coolant. Nothing has changed except for the valve cover and I have no idea why the car is running hot. Any ideas??

What did the gauge read before? Slightly over half is normal. However, you'd have to measure the temperature with something more accurate than the dash gauge to be sure.

It usually runs steady at slightly below half on the temp gauge. I can tell by just touching the engine after it's been running that it's running hotter than normal.