Misfire codes and check engine light

Hi all. Looking for a little insight. I have a 02 C2 with 50k. Last night driving home, as I pulled in the driveway, my gas light went on. This morning I started it and a solid check engine light illuminated. I put fresh gas and went to a local auto parts store to read the codes with a generic reader. Three codes. P0300 P0304 P0306. Based on the previous threads in the forum, looks like there are multiple misfires on cylinders 4 and 6. Could low/bad gas cause a misfire? Drove it to work after refueling and the car ran as smooth as ever. I seem to recall on my boxster that the check engine light will reset itself after so many cycles. Is there any truth to that? Thanks in advance for you help.

Yes, the CEL will reset after a number of cycles and no more codes reported. Yes, bad gas will cause a misfire. Run some Techron in your fuel. Recommendation, buy a cheap OBDII reader, read the codes and erase them, then see if they come back.

Thanks for the super quick advice. I appreciate it.

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My info is that the DME will not register misfires if the fuel tank is low on fuel. I think this may be tied to the low fuel level warning light. Bad gas could cause misfires. It would have to be pretty bad. (My only experience with "bad" gasoline came about when a family member's Infiniti sat one year while she was in the hospital. I drove the car once or twice (briefly) after she was admitted to the hospital and the engine started ok, idled ok, ran ok. But about 6 months after she first entered the hospital she gave me permission to use the car to drive our elderly parents around as I found them a place to live. The engine felt kind of sluggish, but being unfamiliar with the car and it being no Porsche I assumed this was the way it always felt. I drove the car enough though that the fuel tank level got low. I filled the tank with Shell V-Power (the engine required premium) and after the fill up the engine was transformed and remarkably too. It really impressed upon me how stale ("bad" I guess) gasoline could affect engine performance.) Recently, a couple of times in the last month my 03 Turbo has misfired upon cold startup. I suspect it is a combination of old coils (they are original and have almost 125K miles on them) and the unusually humid air that has been around off and on for a while now. What I do is check the error codes. (I have a device that connects to the OBD port and maintains a cell phone connection that sends this data to a web site that I can access from the comfort of my easy chair or desk chair so I do not have to go out and connect an OBD2 code reader (though I have one).) I write them down and then erase the codes. As long as the engine doesn't misfire after engine start say under hard acceleration or even just tootling around town, and the engine doesn't exhibit any other untoward behavior, I don't worry much. 'course, I have it on my list of things to do is to have the coils (and probably the plugs if they are due or near being due for replacement) changed probably next week some time. Oh, and yes the CEL will be extinguished after so many warm up cycles provided the error condition does not reappear in that time.

My thought was that perhaps some debris that may have been on the bottom of the tank was sucked in to the fuel system. Just a WAG since the tank was low. I'm going to erase the codes and monitor it. Again, thanks for the input.

Like Macster, when my coils have been on their way out, higher humidity has exacerbated the problem. If it's raining, snowing, or humid, and you get the CEL's, you should start budgeting for replacements.

what are we talking as far as replacement cost?

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Agree, I believe extra humid conditions here in SoCal kicked off similar codes to OP last month. I cleared codes and similar ones popped up with next drive, though not exactly the same cylinders. Replaced original 60k coils/plugs and all is well. Cost was $350 for parts (Pelican). Not a fun job to do yourself (Pelican DIY). I would consider paying my trusted indy next time. Took me about 5.5 hours (ex Indy, IMSA mech.!). If I were to do it again, it would take about 3 hours. Best, Paul

good to know. Thank you.

Was doing a little poking around and found this YouTube vi on replacing plugs and coils. http://www.youtube.com/watch?v=AvcvscL1uZI

Coils are ~$40 each if you shop around. If it was bad gas, all the cylinders would have been ranfomly misfiring. Could be also the spark plug gaps are getting big.

Coils and plugs solved the misfiring issues for me. It can definitely be intermittent. The job was a pain (maybe because I stubbornly would not take out the exhaust).

Thanks. I was planning in doing the plugs with the 60k service. I picked up a code reader and cleared the codes. Been driving it all afternoon and no codes yet. Thank you all for your input. I truly appreciate it.

Don't worry, chances are it will come back. Cars rarely ever fix themselves.

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There is probably some debris on the floor of the tank. When my Boxster's fuel pump failed at around 200K miles (I think it was around then, I am forgetting and too lazy to dig out the repair invoice) at my request the tech checked the condition of the inside of the tank -- lucky for me the fuel level was quite low when the pump quit so the bottom of the tank was very visible once the pump was removed -- to determine if there was any need to completely drain (via a siphon) the tank to remove any water or debris and he reported no signs of water and very little debris. Not enough to worry about. There was some debris stuck in the fuel filter, which in my 02 Boxster is the fine plastic mesh that is part of the pump housing. This housing attaches to the floor of the tank so the mesh portion is at the lowest point it can be. The amount of debris was not scary and whatever finer debris made it past the mesh apparently hasn't affected the engine any, as the injectors are original and now with over 282K miles on them. The injectors have I believe their own filter consisting of a very fine metal mesh/screen that is designed to trap particles that would block one of the holes in the injector tip. Anything smaller would be injected into the chamber and eventually exit out the exhaust. Bottom line is the misfires are from something else, most likely coils. It is rare plugs cause misfires but you want to change these when due either from miles or time.

Thanks for the input. I think you're right. Will put this on the to-do list.

Good morning guys, this is a lot of GOOD info and a great video, I don’t mean to hack this tread but I got a CEL misfire cyl #2 I unplugged the batt and 30 min later reconnected it and the cel was erased, but this morning when I started the car it idle for about 2 min the car sputter lightly for a sec and the cell came back. any advice?

It is what it is....bad coil or bad spark plug #2 cylinder. Replace.

What DBJoe996 said. Without knowing the car's history was anything done to the car recently? I'm thinking that if the plugs/coils were done recently this misfire could be due to an bad connection between the coil and plug or between the coil and the engine harness. If the plugs/coils have not been done recently and are in fact close to due to be changed based on time or miles just change them all. If the coils are original and have some miles/age on them you might want to consider changing all of the coils, too.

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The only thing I did to my car was put a K&N filter and did the airbox mod.

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Well, that's important. While the MAF is properly protected from dirt (and oil is not a worry with a dry type filter) and can operate for years and hundreds of thousand of miles with no problems, unfortunately this doesn't always apply when the stock air filter is replaced with an aftermarket air filter that requires oil to bolster its filtering. Aftermarket air filters that require oil to filter can result in the MAF getting fouled. And I do not know what air box mod you refer to but the stock air box is designed to ensure the air flow into the intake hose between the air box and the intake manifold is linear so the MAF which sits between the air box and the intake manifold receives linear air flow. Turbulent air flow at the MAF can cause all sorts of hard to diagnose symptoms. Thus modifying the stock air box is done at the risk of upsetting the very critical air flow from the air box past/through the MAF and into the intake manifold. My advice would be to carefully remove and clean the MAF. While I normally do not believe MAF cleaning is necessary, and I certainly do not clean the MAF in either of my cars, in this case I think it called for. Then to eliminate air flow as a cause I would advise you to return to the stock air filter and air box configuration. If the misfire goes away then you can consider how much if any of the air filter/mods you want to reapply. If the misfire doesn't go away come back for further comments/counsel from the forum member.s

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The K&N filter is dry, Should I replace it with a OEM one? As far as the Air box mod, I went a head and bought a smooth silicone intake pipe that replaced the OEM accordion intake hose. The I took out the hemholz box and capped it. According to the Code scanner it was a Cyl 2 misfire. I reset the Cel and it hasn't come back yet.

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Agreed with Macster about potential side effects on intake mods. If the misfire code is always cyl 2 and cyl 2 only though, it's more likely it's the coil/plug on that cylinder.

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I installed a scan gauge. Now it will be easier to monitor the cel.

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Let us know what you find.

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+1 I'm curious as well. Coils/plugs were just replaced on my car due to a flashing CEL & running rough. Last week, it threw a flashing CEL again while idling at a stoplight on the way to lunch. The light did not come back on when the car was started up after lunch and has stayed off ever since. The weird part is that there were no stored or pending codes when I hooked it up to my scanner.

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An air filter designed to be oiled that is not properly oiled, in this case has no oil, is not going to do as good a job of filtering the air. The best air filter for these engines is the stock air filter. A number of cars have obtained a high number of miles with stock filter and are still running just fine. If the air filter was not any good these engines would not have reached the miles they did. Additionally there are a number of UOA results posted and I have yet to see one with high silicon numbers, silicon being a sign dirt is making past the filter into the engine. Whether you want to replace the aftermarket filter with the stock one is up to you. What you can do, what it appears you are going to do, is just drive the car and see if the misfire code comes back. If it doesn't... think about if you keep the aftermarket air filter in service properly treating it with oil so the filter delivers its optimum filtering and protects the engine. If the misfire comes back, I would advise you check the plug's installation -- is it possibly loose? -- and the coil's installation -- is it securely connectted to the plug and it is securely and properly connected to the engine wiring harness? to make sure the misfire doesn't have a simple explanation. If you can find no apparent explanation for the misfire, swapping coils is a technique to determine if the new coil is possibly defective. (The other day I had my Boxster's plugs changed. The coils were found to be ok. The tech however tossed in along with the old plugs a new coil that was bad. It seems these new coils have some electronics in the coil that can increase some the likelihood the coil can suffer from premature failure as these electronics are a weak spot.) If the misfire follows the coil well, there you are. If it doesn't then the plug has to be suspect. In the case the plug is suspect, I would not swap plugs. I treat plugs as an install once item. I would just replace the suspected bad plug with a new plug. If the misfire remains then it could be the air box -- the #2 cylinder just being the most sensitive cylinder (granted a bit of a stretch) -- but it could be something else. Going back to the stock air filter and air box would be preferable to diving into an engine teardown. If the misfire remains after all the above them come back for further discussion.

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That is not weird just the way things work. In many cases if the CEL triggering condition goes away the CEL is turned off and the associated code is erased. In some cases while the code is no longer active, and there is no pending code, there is a permanent code. Permanent in that the code can only be erased if the DME fins the triggering condition absent over a number of engine starts/warm up cycles.

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Thanks for chiming in. I was a bit surprised to find no stored codes. For troubleshooting purposes, it would be nice to know which cylinder(s) were misfiring. The scanner is in the car, so I'll be ready next time. This, of course, guarantees that it won't happen again.

Ok so far the Cel hasnt come back, And of course now that I have install a "scan gauge"

Mine did random misfires every few days this pring after I used Seafoam to winterize the car. I replaced all the plugs and still got the codes until I have flushed a tank or 2 of fresh gas through it. Reset the codes and keep an eye on it to see if they are the same codes every time, same codes every time points to plugs/coils, swap a "bad" coil over and see if the code changes to diagnose the coils. Random changing misfire codes points to bad fuel.

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Since you just had plugs/coils done and the misfires occurred after this I suspect a coil is not properly installed. Techs tell me that often they find a coil not connected to the wiring harness connector. This can cause misfires at that coil. Thus I believe the misfire will return. When it does my advice would be to when the engine is cold again carefully check that the coil is properly connected to the plug and the coil is properly connected to the wiring harness. Or take the car with the misfire complaint back to the shop that did the plugs/coils and have this addressed there. The coil could be loose, or just bad. The plug could be bad too. But an improperly installed coil is more common that a bad new one or a bad new plug.

The coil plugs can be tricky to reconect. They have a plastic sleeve that slides around and gives a false sense that they are fully seated. You need to make sure that you hear a click and then to pull on the wire to see if it stays put.