Originally Posted by my old blazer
I've read and reread here and still wondering if changing the factory injectors will help with MPG. The after market offers from several makers the replacement unit with the true injector for each cylinder instead of the master unit that fires all injectors 3 times for each revolution. These fire only for each cylinder at the opening of the intake valve. My 99 has gone from nearly 20 on the highway to 16 on a good day. It had 44K on it when we bought it 4 years ago. The old girl now has 80K and the MPG is the only complaint........
Sounds like about the amount of mileage that you lose when the TCC is not engaging anymore.
What you have been told or read about injectors firing differently is incorrect. All that was done for the upgrade MPFI spider was the 6 solenoids for fuel flow were moved from a central location to the end of the tubes and the poppets were eliminated. As far as the firing of injectors, the PCM determines this and it knows no difference between the two types of spiders. This is why you can upgrade so easily. Functionally they are identical, but the upgrade MPFI spider is more reliable without the poppets - and that is why they went to it.
Both types of injectors will fire sequentially with the firing order unless there is a problem with the cam sensor determining which cylinder is next. If there is a problem the SES light is illuminated and the injectors are pulsed in two banks as a limp in mode.
Do you have a SES light on, or even any pending codes - especially misfire codes? These can and will quite often prevent the TCC from engaging and will hurt fuel mileage. Can you feel to TCC lock up and see the tach drop a few hundred RPM as you pass 47mph at light throttle in OD? If you are driving a steady level 50mph for >10 sec, does the tach go up a few hundred RPM when you tap the brake pedal and the TCC electronically disengages?
p.s. The PCM will definitely be able to recognize any test pipe installed in place of a catalytic converter. It will set a code and turn on the SES light. It monitors converter efficiency using O2 sensors and will not like zero efficiency. There are ways to try to trick the PCM, but IMHO is not a good way to go.