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2000 Honda Civic GX Misfire - P0304 P0302 P0300 P1399 P0420

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  • 2000 Honda Civic GX Misfire - P0304 P0302 P0300 P1399 P0420

    I have a civic with 280K miles and get the following codes:

    P1399 - Misfire in any cylinders
    P0300 - Random misfire
    P0302 - #2 misfire
    P0304 - #4 misfire
    P0420 - Cat below threshold

    I have a good computer scanner that has live data and freeze shots with codes if that helps.

    The head gasket was replaced 120K miles ago. Recently replaced was plugs, wires, cap rotor, distributor, fuel pressure regulator, front O2 sensor, Cat. When the car was given to me it was misfiring in all cylinders and was giving P0300-P0304. I replaced a plug that went bad, and drove it. It ran great for the first 20 miles in the early morning. It started right up, idled fine, and ran normal. In the evening though on the 20 miles back I started noticing periodic hesitations (misfires). It seemed to be at lower RPMs while cruising and not so much under acceleration. Im going to do a leak down test on it this weekend. I was told a mechanic did a chemical block test and found hydrocarbons in the coolant. Could head gasket be bad again?
    Attached Files

  • #2
    Hydrocarbons in the coolant could also be from an internal leak in the high pressure cng regulator, but likely is in the engine. I would not try to rebuild, just look for a used engine if you have a leak and want to save the car. At this point I would look at parting it out.. I believe that car has 2 cats , one on the manifold and the second under the car.

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    • #3
      I've had my '99 GX for over eight years now. I'm up to 336K miles. The biggest problems I have had over the years with misfires has always been the plug wires arcing inside the metal tubes; especially at lower RPMs. I've haven't had a problem with the head gasket yet; maybe because I've never had anything fail that would cause the engine to overheat.

      Even though you think that your spark plug wires "aren't that old", replace them anyway and see if it fixes the problem. I've pulled a good bit of my hair out trying to figure out these misfiring problems, only to find in the end that it was just bad plug wires that I thought "weren't that old".
      Last edited by hypermilinggx; 2 weeks ago.

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      • #4
        Originally posted by Lakewood90712 View Post
        Hydrocarbons in the coolant could also be from an internal leak in the high pressure cng regulator, but likely is in the engine. I would not try to rebuild, just look for a used engine if you have a leak and want to save the car. At this point I would look at parting it out.. I believe that car has 2 cats , one on the manifold and the second under the car.
        It looks like the "fuel pressure regulator" was replaced back in April before he gave me the car. I already have a junkyard engine to swap, but the guy I got the car from had spent so much money on engine parts, I began wondering if it would be better to try and fix this instead, then sell the junkyard engine. Would hydrocarbons in the coolant cause misfiring?

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        • #5
          "Would hydrocarbons in the coolant cause misfiring?" No , but on a cng fuel vehicle , is possible reason for it to be in coolant.

          Head gasket leaks can be tricky to find if small. I had a Toyota that would leak exhaust gas into the coolant only while warming up. I would see a temperature spike during warm up only. Not when cold, and not when at normal temp. In hindsight, I should have installed a pressure sender and gauge on the cooling system while trying to find the problem.
          Last edited by Lakewood90712; 2 weeks ago. Reason: typo

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          • #6
            So I noticed that it doesnt misfire, other than when under load at low RPM. I feel like it might be a fueling issue somewhere, as it is consistent at the RPM and load, and all cylinders seem to misfire and throw code. The high pressure fuel regulator was replaced, which might explain the hydrocarbons in the coolant, but seems like headgasket wouldn't constantly misfire all 4 cylinders, yet run fine otherwise at other RPM's and loads.

            Im going to check out the cap and rotor next, but wires and plugs looks good. Any other suggestions? Is there a fuel filter that could be replaced that might cause these symptoms?

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            • #7
              There is only one fuel filter on that car and changing it won't fix that problem. I had the same problem with my '99 GX - low RPM misfiring under load and I changed my spark plug wires that weren't that old and "looked good", and it fixed the problem.

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              • #8
                Originally posted by hypermilinggx View Post
                There is only one fuel filter on that car and changing it won't fix that problem. I had the same problem with my '99 GX - low RPM misfiring under load and I changed my spark plug wires that weren't that old and "looked good", and it fixed the problem.
                Did you have codes misfiring all four with the random misfire?

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                • #9
                  The fuel filter is on the high pressure side, between the manual shutoff and high pressure regulator. Takes an inexpensive Parker element. 15 minute job. The dealer will have it at about 3x the cost it can be obtained online. I also doubt it is the cause. Be absolutely sure the line and housing is de pressurized if you decide to check it. You can check the element and reuse if good. I have inspected, but never had to change one on 2 Honda GX over 8 years. There are a few youtube videos on the filters.

                  ps someone posted having to replace the distributor base-plate ( an electronic part) to fix a similar problem. I dont recall when the thread was posted, but is was many years ago.

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                  • #10
                    Originally posted by Bhollehday View Post

                    Did you have codes misfiring all four with the random misfire?
                    No codes. That's what was so frustrating about it. How can you have misfires and the CKF sensor not pick it up? The problem was most noticeable with the A/C on. Replacing the spark plug wires that "looked good & weren't that old" fixed the problem. Of course, you can have plug wires that are arcing so badly inside the tubes, that it WILL throw misfire codes. I had that in the beginning with the factory Honda wires. I now use the blue NGK wires.
                    Last edited by hypermilinggx; 1 week ago.

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                    • #11
                      Originally posted by hypermilinggx View Post

                      No codes. That's what was so frustrating about it. How can you have misfires and the CKF sensor not pick it up? The problem was most noticeable with the A/C on. Replacing the spark plug wires that "looked good & weren't that old" fixed the problem. Of course, you can have plug wires that are arcing so badly inside the tubes, that it WILL throw misfire codes. I had that in the beginning with the factory Honda wires. I now use the blue NGK wires.
                      Put some bluedevil in it and replaced the #2 injector and no more misfire codes.....for now.

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