Ok. The PTU which is the square black shape object behind your upper coolant hose. It send signal to the ignition coil to fire the spark plugs. When it get hot it will stop working. I relocate my to the front next to my MAF. I suggest you relocate your if it's the problem and you get a new one. TwinTurbo. Net I think have a instruction to relocate it. I guess the designer of the Z didn't realize that the heat will make the PTU fail.