Truck idles smoothly but runs really rough when gears engaged. Have changed fuel filter, plugs/wires, fuel pressure. Injectors are new.
Check engine light is NOT on.
How did the old plugs look?
All 6 plugs were clean with little wear and no deposits. I have also cleaned the MAF and checked for vacuum leaks, did not find any.
How about valve adjustment?
Maybe a compression test is in order.
Automatic or manual?
Can have that done–installed new motor less than 5000 miles ago. Original engine damaged due to oil pump failure.
Automatic
When you say, “Have changed…fuel pressure…”, do you mean you have measured fuel pressure with the engine under load;, or, do you mean that you ONLY checked fuel pressure at idle, or with the engine off? Whether, you did say that, or not, you need to.
If you changed 6 spark plugs, the engine must be a 6 cylinder engine. Probably a V six, no? Is this correct? And the model year of your Mazda B400?
Fuel pressure measured at idle with engine on