Have a bad miss only noticable at idle and loss of power when accelerating

Question

Have replace intake gaskets and timing belt as well as all components and water pump put a new ever valve coming up with power 171,137,156

Answer 1

P0171 Fuel trim lean Bank 1

P0137 02 sensor low voltage Bank 1 Sensor 2

P0156 02 sensor circuit malfunction Bank 2 Sensor 2

The timing belt can be off especially when there is a loss of power. With the fuel strategy of Isuzu a big vacuum leak will cause a lack pf power on acceleration and even at idle, So rule out the vacuum leak , disconnect the MAF sensor this will cause the engine  management to go rich, and use a carburetor cleaner of brake cleaner and spray at the plenum gasket  and fuel injector seal (which tends to leak vacuum) to ruel out the vacuum leak, Also the timing belt has to be set a certain way.