Could be a few things.
Exhaust leak could mean cat or muffler has rusted out inside and partially collapsed, blocking exhaust flow.
When you burn gasoline(H) with Oxygen(O) one of the by-products is H2O(water), this rusts an exhaust system from the inside out, cat and muffler parts can break loose inside.
If exhaust can't easily flow out then new air/fuel mix can not come in, this reduces power at above idle RPMs.
Exhaust leak could be a sign that pressure was building up in the system, so it pushed open a hole at a weak point.
A Vacuum gauge can be used to test for exhaust blockage.
Or simply opening the exhaust system close to the manifold, i.e. remove O2 sensor or unbolt pipe, this will relieve any back pressure, yes it will be loud, rev engine or test drive and see if it runs better.
Another thing to check is the ECT(engine coolant temp) sensor, there are two temp devices on all fuel injected engines.
ECT sensor has TWO wires and is only used by the computer
ECT sender has ONE wire and is only used by the dashboard temp gauge.
The ECT sensor(TWO wires) is use to tell the computer if it should "choke" the engine.
A cold engine needs more fuel until it warms up, the Choke Plate on a carb was used to increase the fuel to air mixture, fuel injection doesn't have "jets" so that can't be used.
So the ECT sensor was added to inform the computer when to choke the engine.
This choke includes a higher idle, i.e. 1,100rpms, when engine is cold.
And idle should drop as engine warms up, if this is happening then ECT sensor is OK, if idle is staying high then ECT sensor may be going bad and once engine is warmed up computer is still adding too much fuel causing poor performance.
ECT sensors rarely fail, not never, but rarely.
MAF(mass air flow) sensor tells computer how much air is coming in to the engine.
These get dirty and can report incorrect air information to computer.
This will usually cause a Lean condition when engine is warmed up, so you get Pinging(knocking) when accelerating, and low power.
Last edited by RonD; 12-29-2014 at 11:56 AM.