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Discussion Starter #1
Bought my 99 4.6 in may and it had a lot of power. Came with 3.90 gears, catted h pipe into a cat back. Also has CAI. July I bought a bama tune and an o/r x for my car but didn't change the donut gasket between headers and mid pipe bc it didn't come with one. One of the bungs was stripped on the passenger side o2 but I tightened as much as possible. About a couple days after, I drove 15 miles to a friends house and on the way back the o2 came out and dangled till I got home and again put it back in. This happened 1 more time till I eventually added high temp thread locker. I also ran my car on low fuel a lot because I didn't make a lot of money as a 17 year old "young and dumb" driver. Fast forward to October I'm really starting to question my cars performance. it's slow until 35k rpm. After that it really kicks in not to mention I had 80 miles to a half tank which is bad. I changed the fuel filter a couple hours ago didn't really feel that much of a difference so I'm here asking you guys to give me a hand

-I cleaned MAF
-cleaned EGR
-unplugged each COP to check for misfire
- fuel filter
-check for vacuum leaks

Help!
 

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Check fuel pressure. Engine running is 30-50 psi. Key on engine off is 25-40 psi. Next thing to check is to make sure that you dont have an issue with the timing. Also check the intake manifold for issues such as being cracked.

Id say either a weak fuel pump or timing issue.
 

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Discussion Starter #3
I was also thinking fuel pump when I key on engine off I don't hear this "hum" people say the fuel pump makes.. I'll check for the crack and snag me a pressure gauge is there a specific one?
 

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As long as it is a fuel pressure gauge you should be good to go. I think advance auto or auto zone sells a cheap one that has the fittings you will need to thread it on to the fuel rail.
 

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What codes is the computer throwing? If any, those need to be addressed as well. If there are issues with the front O2 sensors that explains a lot of your problems. If you have a massive exhaust leak before the front o2 sensors, that is a problem as well (referencing your problems with the donut seals and all that).

Post the codes and we can go from there.
 
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