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Thread: P0303 and failed compression test

  1. #1
    Registered Member

    CX7 2007 Sports

    Join Date
    Nov 2008
    Location
    Whittier
    Posts
    5

    P0303 and failed compression test

    Hi there! P0303 (misfire cylinder 3) just popped up after 121,000 miles on my 2007 CX-7 (i.e. turbo). Very unfortunate since it was running so nice lately.
    Anything different lately? Well, new gas tank 30 miles prior (doubt that is the problem), and 2 tanks ago I put a can of gumout in the gas tank to clean out the engine.

    Already changed all spark plugs and the coil for cylinder 3. P0303 remains. Did a compression test and cylinder 3 scores only half pressure readings compared to the other 3 cylinders.

    I'd love to hear your advice on what would cause this, and suggested remedies along with qualitative costs to address. Is this an engine removal job at >$2K job? Would you try something else first? Thanks much!

  2. #2
    Registered Member

    CX7 2007 Sports

    Join Date
    Nov 2008
    Location
    Whittier
    Posts
    5
    My dear P0303 friend, goodbye! Back to perfect compression, no more misfire, and CEL not coming back.

    Despite the inexistant response I figured I'd post the solution to this problem for the benefit of others that may encounter that issue. Quite frankly whether you have a P0301, P0302, P0303, or P0304, the same applies.

    #1, change your spark plug.
    #2, swap your coil with another one to see if that was the issue. Still misfiring, change the one not working
    #3, still an issue, do a compression test.
    #4 (if failing the test), put a teaspoon of oil in the cylinder (through the spark plug hole) and re-do compression test. Issue fixed? Bad news you have a worn piston ring... Not addressing this below.
    #5 (still a low compression compared to other cylinders), you either have a valve problem or your injector or injector seal is the issue. I was at 120,000 miles and can tell you with certainty that your seals are shot. There may also be another problem but I would start there. Change your injector seals. Some recommend the CP-E injector seal. They seem good. My mechanic wasn't knowledgeable about those so we went OEM (the issue will come back in 100,000 miles, if lasting that long).
    #6: are you willing to roll the dice that it was not your injector? (It takes 2 hours to get to the injector. If unsure, change the injector too. I did. If willing to take a chance, then at least swap two injectors).
    Problem solved. If not, sorry my dear friends, you have an intake or exhaust valve problem unless you have a gasket leak. Good luck.
    Last edited by speedymatt; 09-08-2017 at 01:34 AM.

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