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I recently brought home a 99 Camaro with the 3.8L V6, it has been sitting for about 10 years. The fuel pump went bad in it and never got fixed. I took it upon myself to replace it, which was a major pain but I got it done. I cleaned out the tank really good, poured in some Techron fuel additive followed by 5 gallons of non-ethanol. Next, I proceeded to change the fuel filter and removed the quick connects before the engine and cycled the ignition on and off until I got clean fuel. I pulled all the spark plugs, put 15ml of marvel mystery oil into the cylinder, and let it sit overnight. The next morning, I put a socket on the crank and turned it over quite a few times to get all of the excess oil out from the cylinder. I replaced the plugs, got a new battery, and she fired right up. One problem, it's missing pretty bad and threw a PO300. I put a spark plug tester on both sides of the wires and had spark on all 6. I tested the injectors and all were within 12.1-12.3 ohms. Kind of lost and not sure where to go from here, any suggestions are appreciated!
 

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2001 Chevrolet Camaro SS
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I recently brought home a 99 Camaro with the 3.8L V6, it has been sitting for about 10 years. The fuel pump went bad in it and never got fixed. I took it upon myself to replace it, which was a major pain but I got it done. I cleaned out the tank really good, poured in some Techron fuel additive followed by 5 gallons of non-ethanol. Next, I proceeded to change the fuel filter and removed the quick connects before the engine and cycled the ignition on and off until I got clean fuel. I pulled all the spark plugs, put 15ml of marvel mystery oil into the cylinder, and let it sit overnight. The next morning, I put a socket on the crank and turned it over quite a few times to get all of the excess oil out from the cylinder. I replaced the plugs, got a new battery, and she fired right up. One problem, it's missing pretty bad and threw a PO300. I put a spark plug tester on both sides of the wires and had spark on all 6. I tested the injectors and all were within 12.1-12.3 ohms. Kind of lost and not sure where to go from here, any suggestions are appreciated!
GM P0300 – Meaning, Causes, Symptoms, & Fixes - FIXD (fixdapp.com)
 

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Also check your o2 sensors. I had a misfire in 2 and did the same kinda spot checks you did (swapped the plugs, plug wires, coils, so on). I got kind of a similar story about my fuel system but after swapping those out I got the same misfire code and the code stated I was also running lean. Turns out I had a bad o2 sensor on bank 2. I swapped it out and haven't had a code since. Turns out the bad o2 sensor was not only functioning improperly, but because of it only reading the small amounts it could, it was falsely claiming I was running lean. After talking with my local performance shop, I learned that for some reason the 5th/5.5 gens have really shitty luck with o2 sensors. I'm not saying this is your case but I would check to make sure of this also. A telltale sign is a rough idle on a cold-start.
 
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