Hello, Jason here to help. This Isuzu uses a plastic plenum spacer/ gasket. These will crack and allow unmetered air into the system causing misfires(rough running) when cold, and also cause the P0171 code. A smoke machine for testing vacuum leaks will find this easily. The rough running is usually worse cold, but can be constant if the vacuum leak is large enough.

This is the most common repair on these V6 trucks.

I hope this helps, Jason

Any reputable shop with a smoke machine can do the test. The truck will need to be cold for the best results. The machine injects a dyed, or colored smoke into the intake system and any leak will be identified by a plume or puff of smoke. Any leaks in the intake system will be found easily. The gaskets are not too expensive and can be done in a couple of hours with a few hand tools.

A smoke test of the intake should cost no more than 30 minutes labor maximum.

I hope this helps, Jason

If it is like every other Rodeo V6 with a po171, the cold really wont matter too much. When a engine is cold, the leak will be larger due to contraction. I have also used water in a spray bottle when idling to find intake leaks. Just spray the gasket area and listen for a change in running, or a sucking/whistling noise. DO NOT use a garden hose to do this test. A spray bottle will add just enough water to change the running of the truck.

Jason