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1993 GMC C3500 6.5 ECLB Dually
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So I've got an 04 Isuzu NPR 5.2 Diesel, 94k mi on the clock.
This thing will run fine for ~20 minutes, then have absolutely 0 throttle response in gear and run like garbage for 1-2 minutes(Kind of a big problem because it only happens when driving). If I have it in gear and press the throttle it will not rev up (Reverse, Drive, 2, 1) it will only idle extremely roughly. In park & Neutral it runs fine, throttles up as it should.
This has been an issue before and after I replaced - Fuel water separator (no water came out of the old, just clean diesel), oil change. I have run injector cleaner in it.
 

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So I've got an 04 Isuzu NPR 5.2 Diesel, 94k mi on the clock.
This thing will run fine for ~20 minutes, then have absolutely 0 throttle response in gear and run like garbage for 1-2 minutes(Kind of a big problem because it only happens when driving). If I have it in gear and press the throttle it will not rev up (Reverse, Drive, 2, 1) it will only idle extremely roughly. In park & Neutral it runs fine, throttles up as it should.
This has been an issue before and after I replaced - Fuel water separator (no water came out of the old, just clean diesel), oil change. I have run injector cleaner in it.
Have you changed the fuel filter? checked for any codes?
Check the condition of the fuel lines to/from the filter housing. Old, dry-rotted fuel lines can collapse causing a restriction, air leaks.
 

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I have an '05 with similar. The SCV (Suction Control Valve) is suspect. That's the actual device that throttles the fuel up and down. Mine ended up being a bad wire (intermittently open) to the SCV.

You can paper clip the OBD II for codes.

On an '06, it ended up being the accelerator pedal itself.
 
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Don't forget to check the primer pump inlet banjo bolt screen. Follow line from fuel filter to hand primer. Remove banjo bolt (using care to not lose the two metal sealing washer). Then, spin screen out of bolt and inspect/ clean it.
 
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