Mazda Repair Questions? Ask Mechanics for Answers ASAP
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The fuel injector for cylinder 1 can be at fault to where it may not be atomizing properly.This will cause a misfire especially under load when driven when more fuel is needed.The fuel injectors need a pressure drop test involving a fuel pressure test gauge and an injector pulse tool or scan tool to pulse each injector to make sure this is the cause here.
I would also have a cylinder compression test done to see if number 1 is 15 psi or more lower on compression then the rest of the cylinders.If it is then this can indicate a valve issue in the cylinder head which was most likely a fault when the engine was put together before it was installed.In this case then you would want to contact the facility that the engine came from to see about parts warranty on a repair coverage for this repair or another replacement.
In some cases when it is the injector you can add injector cleaner to the fuel tank around 1/4 tank and run to near empty and repeat and this will clean up the injector but in some cases the injector may have to be replaced.
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