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Feeling weak (rebuilt 4.0, 4.56, AW4, 31s...), Why?

I wouldn't rule out a back pressure issue. Whats the vac reading under load at ~2k to ~3k after a few seconds? Idle vac readings rarely show excessive back pressure. I have seen back pressure numbers in the 2-5 psi range that can make these 4.0's feel pretty slow.



Agreed. I have seen rich running vehicles destroy cats in hundreds of miles, not thousands. 14k IS pretty long if there was a slight issue.

Honestly, if everything else checks out I would do a back pressure reading. Most muffler shops can do it pretty cheap.

You can make a back pressure gauge super easy if you have a vac/fuel pressure gauge. Cut up an old o2 sensor and thread a Barb fitting​into it. Under 2 would be ideal and anything over 5-7 would be bad.
 
You can make a back pressure gauge super easy if you have a vac/fuel pressure gauge. Cut up an old o2 sensor and thread a Barb fitting​into it. Under 2 would be ideal and anything over 5-7 would be bad.

Doesn't need to, he has torque. It does live manifold absolute pressure. To test for restricted exhaust, run at 2500 rpm for 3 minutes in neutral/park. If vacuum drops, exhaust system is restricted. If vacuum maintains whatever it started as, no restriction.
 
You can make a back pressure gauge super easy if you have a vac/fuel pressure gauge. Cut up an old o2 sensor and thread a Barb fitting​into it. Under 2 would be ideal and anything over 5-7 would be bad.

Some of the gauges kits come with an adapter that goes into the O2 sensor location. Mine does.
 
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