Zx330 lc no throttle

Hello anyone and everyone. I have a 2002 zx330 lc Hitachi that will randomly quit revving up. All the lights, gauges and the heater work but it won't rev up. When this happens, It'll start good but will not turn off the engine with the key. If I leave it sitting for a couple hours usually work again. I've changed the stepping motor (throttle motor ) but no difference. Any advice is much appreciated. Thanks

To the best of my knowledge the stepper HAS TO BE calibrated on the machine..
There is a sequence you have to go thru inorder for the motor to learn the throttle travel..
Its called, calibrating the throttle.. its been discussed here a lot but I never paid attention to the procedure.
You probably have other problems seeing it did it before you changed the stepper, but calibrating the new one wouldn't hurt.
The procedure was discussed mostly in the Kobelco section.. Try an advanced search for throttle calibration.

Before I made up manual hand throttles for both hitachi's ,i would get the drivers to turn on the ignition ,work the throttle to full then back to idle before starting the engine . It seemed to help a bit .

3 things it could be. Firstly, the throttle dial works off a 5v, a signal wire and a ground. You possibly may have an intermittent break in the signal wire, which is unlikely. Or the 5v feed is a common feed with all the other pressure sensors located around the machine. if any of those sensors have an intermittent fault or the 5v feed to any of those sensors are shorting to ground, then that'll cause the eng rpm to drop to idle. This is much more likely as i've seen it many times before. A diagnostic check with MPDr will show any codes relating to the fault. Normally with a 5v feed fault, you'll get around 14 faults stored. If its a signal wire or a sensor issue normally you'll see the code related to the sensor causing the fault. And lastly, as someone else has said, it could be needing the engine learning procedure carried out.
From the manual

ENGINE SPEED ADJUSTMENT AND
ENGINE LEARNING
After removing and/or replacing the components as
described below for repair and/or inspection, or if error
in engine speed is found, readjust engine speed and
perform engine learning.
• Removal of the engine, engine control cable, or
EC motor
• Replacement of MC
Adjustment of Engine Speed:
1. Turn the key switch to the START position to start
the engine.
NOTE: Turn the air conditioner OFF.
2. Rotate the engine control dial to the minimum
speed position. (The output voltage from the EC
sensor is 2.5 V.)
3. Deactivate the auto warming up and ECO control
systems by Dr. ZX.
4. When Dr. ZX is not available, keep the engine
running for about 15 minutes to warm coolant
temperature to more than 30 °C (86 F°) until the
auto warming up control system is deactivated.
5. Loosen the bolt holding the control lever to the EC
motor output shaft.
6. While monitoring the actual engine speed with Dr.
ZX or the engine speed meter, adjust the control
lever so that the minimum speed (slow idle)
matches the specification.
Specification: Refer to T4-2 OPERATIONAL PER
FORMANCE TEST.
7. Tighten the lever holding bolt to secure the control
lever to the motor output shaft.
: 13 N⋅m (1.3 kgf⋅m, 9.4 lbf⋅ft)
NOTE: Don’t lubricate the motor output shaft.
8. Perform engine learning.

And

Engine Learning:
1. Turn the key switch OFF
NOTE: In case the engine doesn’t stop when the
key switch is turned OFF, pull the handle
located under the seat to stop the engine.
2. Disconnect Dr. ZX. Wait for 5 seconds.
3. Turn the learning switch ON.
4. Turn the key switch ON. Wait for 5 seconds.
5. Turn the key switch OFF. Wait for 5 seconds.
6. Turn the learning switch OFF.
7. Check the engine speed.

I normally do the engine learning first, then set the engine rpm. Then do the engine learning again. the learning switch is located in the fuse box. Look closely and you'll see a E on the switch cover. That's the position you move it to. And the slow idle speed is 1050 plus or minus 100

Thank you everyone your advice. I tried a throttle relearn when it acted up, ended up not being able to start it again. The stepping motor went into shut off position and that's where it stuck. I checked the power to the stepping motor, all good . I checked the ignition and throttle switch. , they were good too. So I reset the computer and it worked. Worked good for a week. Had a track pin come out so I welded it back in and went to work. 2 days later, throttle issue again. So reset the computer again, away we go. Never had a machine that acted up from welding on while it was running before. Thanks again.