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Well it 95 F250 HD with 351W V8

replace whole exhaust system.

Now it get 15 mpg but now it down hill to 13-14 mpg.

I notice new muffler show lot black mark which it show me it been run little rich.


I can't smell anyone and people who I ask won't say but say it FINE.


Is there machine that tell it run rich?

I know that if I fix rich issues then mpg would have improve much I bet it could get 16-18 if rich been fixed.

Check the spark plugs. A rich-burning mixture (likely if cat is removed), will show a black or oily fouling on the plugs. A burned or toasty look indicates a lean condition.

Fixes include regapping, and changing plugs.

Note that loss of catalytic converter will increase SOx and NOx emissions and produce a sooty exhaust.

Makita,

There is a device to check how rich or lean the engine is running. The dealership ($$$) or a good independent shop can plug into the truck's computer and look at things like long term or short term fuel trim numbers and tell if it is running lean or rich.

Since you mentioned that the exhaust system was replaced, I can tell you that leaks in the exhaust system will cause the O2 sensors to see what it thinks is a lean condition and dump more fuel into the system creating a rich condition.

Have the shop that installed the exhaust system check for leaks, especially ahead of the catalytic converters.

There are quite a few other things to check, but given the exhaust replacement, that is where I would start.

Good luck!