This will probably generate more questions than answers, but sometimes that is what it takes to get to the root of the problem .......
Here is a quick list I put together on the things that can affect power (in relative order of "cheap and easy to resolve" to "most expensive"):
- Boost leak (CAC tube): check for leaks.
- Clogged air filter: check filter minder or just replace the filter.
- Plugged fuel filters: replace filters w/ OEM filters.
- Aerated oil (wrong/worn-out oil OR too much oil): change oil properly.
- Wire chaffing: check problem areas.
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- Bad fuel.
- Clogged fuel lines: Check system (run pressure check if you can).
- Air in fuel: check filter caps and lines for leaks.
- Bad EGR valve or clogged EGR valve: clean EGR valve.
- Clogged EBP, MAP sensor tubes: clean tube / hose.
- Stuck turbo vanes: clean turbo or replace center section.
- Injector stiction: get latest flash or maybe use HotShots.
- Having your truck flashed when you did not return your tuner to stock.
- Improperly completed flash.
- Bad MAF sensor: replace sensor.
- Bad ICP sensor or harness (usually will have rough or no-start w/ this).
- Bad IPR valve (or restricted IPR valve due to solids on the inlet screen).
- Bad VGT actuator. Replace actuator.
- Bad fuel pump (HFCM).(you MUST have a minimum fuel pressure of 45 psig at all times or you risk expensive injector damage).
- Leaking EGR cooler. Replace or eliminate the EGR cooler.
- Bad injectors. Identify and replace leaking injector(s).
- Bad FICM, usually there are quite a few codes w/ a bad Fuel Injection Control Module.
- Bad HPOP. Usually a "no-start" condition will be experienced with a bad high pressure oil pump. It is a fairly big/expensive job to replace this pump.
- PCM defueling due to excessive oil and/or coolant temps.
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