Different owners  describe the engine stumbling problem using a variety of expressions. For example:: Stalls at idle, stalls when I put the clutch in, stalls when I turn a corner, misfires, surges, no power, cuts out, no power at top revs, runs rough, etc. However we describe it, the underlying result will be that the engine is not being fed the correct fuel/air mixture ratio at that particular moment.

To put it another way, the engine computer, its program, or some other device is therefore not operating correctly.

There are more than 33 items, which if faulty, that can cause engine hesitation and stumble problems.

 Here is the list of items on the 300ZX  that can be sources of this problem (no particular order):
 

Worn or defective spark plugs PCV hose air leak Air flow meter Intake valve deposit Exhaust gas sensor carbon canister fuel pressure fuel pressure regulator Injector injector circuits ignition coil and circuits Transistor ignition unit ECU harness connector ECU power supply & ground ECU Crank Angle sensor and circuits

AAC valve Air regulator clogged idle screw Battery alternator EGR Temp sensor Temp sensor connector Throttle position sensor Idle switch Cone Air filter Crank angle sensor Fuel temp sensor Speed sensor Incorrect timing belt installation Ignition Timing

An engine can have quite a few things not functioning properly and still run- The fuel pressure regulator can be inoperative or the EGR can be non functional and you may not notice it, the exhaust gas sensor may not work , so the engine runs a bit rougher etc.

 So where do you start? If the problem is intermittent, it may  be very difficult to find the problem. As another example, I still had a 1987 Mitsubishi Starion, which ran perfectly for 35,000 miles. Then it developed an intermittent hesitation problem- you would be cruising at 60mph, on a 400 mile trip, ease off the gas pedal, and suddenly for 1/4 second, it was as though someone had switched off the engine and then turned it back on again. The car would then run faultlessly for the rest of the trip. This happened 4 times in a period of 1 year, then never happened again. I eventually sold the car at 70,000 miles running perfectly.