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Thread: P0303 and failed compression test

  1. #1
    Registered Member

    CX7 2007 Sports

    Join Date
    Nov 2008
    Location
    Whittier
    Posts
    10

    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
    10
    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 02:34 AM.

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