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Hello and thank you for your question,The code P0303 indicates a engine misfire on the #3 cylinder and the P0316 means the computer is seeing a engine misfire in the first 1,000 revolutions . Has the coil, spark plug and injector been checked on the #3 cylinder?
Okay since you are sure the new coil is okay and you have good spark out of it to the new spark plug then I would next run a compression test on the #8 cylinder to be sure its okay and if the compression checks okay then I would swap the #7 injector with the #8 and see if the misfire moves. If you are sure there are no vacuum leaks you could very well have a bad injector and that could also cause the P0174 to set. Check test thing for me and post back and we can work from there.
Okay the code P0174 can also set due to the PCM trying to lean the system down, before you swap the injectors make sure to check the compression on that cylinder to be sure its with in specs. One other common cause for this code to set can be caused by a leaking PCV supply line so be sure to check it as well.
If its leaking it can cause the cylinder Misfire and #8 is the closest one to the source so yes be sure to check it before you do the other steps.
No problem, I'm here if you need me, start first with checking the PCV supply line from the valve to the intake where ti connects, if you find its leaking this will likely resolve the problem and save you from having to to the other steps.
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