ISX fuel mileage help

[Rawze]

Is this an 870 or 871? -- sounds like possibly an 870? if you had a separate intake temp sensor.

870 is notorious for egr valve sticky and problems because they suffer heat issues / poor design. - unplugging egr valve will not guarantee it will stay closed, it may drift open after a while. Also 870 will give turbo derate and make turbo overheat, shorten its life (boost will spike 36 or so psi then fall back to about 31-32). -- It takes proper programming to prevent this.

for troubleshooting it ...

only way to switch off egr temporarily and get engine into proper non-egr mode is to unplug the egr temp sensor, not the egr valve like some social media morons would have you believe.

If you unplug the egr temp sensor and have egr piping pulled away from intake manifold, egr gas flow should stop right away. if gas is still coming through, then the valve is not closing all the way. I think about all 2007 and newer versions of isx (870, 871, 2250, 2350) with egr this test can be done to see if valve is closing properly. Flow will not usually be totally zero, but it should be very little compared to when it is plugged in.

When is the last time you replaced the Delta-P sensor? -- the exhaust back-pressure sensor? - cleaned out the cross-pipes for the Delta-P?