QuestionOkay, I swapped the coils and cleared the ecm code. Drove it today and now it's setting a P0171, lean bank 1. Should I move onto looking into the airflow into the engine or still assume the misfire is originating from the ignition system?
Thanks,
chad
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I checked the resistance on both coils with a multi-meter. Recently I cleared the computer codes and today the engine light came back on so I'm going to swap the coils and see if I can get the misfiring to follow the potentially bad coil. Is there another way to check them other than that?
Thanks again,
chad
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The text above is a follow-up to ...
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I'm selling a friends 2000 Sonata 4cyl. with 87,000 miles. Recently he replaced the alternator and hes not the most mechanically inclined person. After he finished installing the alternator the ecm set a code for misfire on number four cylinder. Driving symptoms include: when the engine is cold, good start but will die after idling for about thirty seconds. Restart, rev in park, just fine, but put it in drive and it stumbles on acceleration. After it completely warms up though, the car is near perfect and you cannot reproduce the problem. The alternator seems unrelated but since I don't have the shop book and my friend doesn't really remember exactly what he removed to get to the alternator, I figured maybe he unplugged or reset something incorrectly, i.e. the throttle cable and this is the cause of the problem.
I've replaced the spark plugs and the coils checked out fined ( I haven't checked the plug wires for arcing but will today). I've also read all of the threads about misfires but if it were a true misfire why would it disappear when the car warms up. So is it a misfire or a badly adjusted throttle cable, if thats even possible?
Thanks for any help,
chad
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This definitely sounds like an actual misfire. You mention the appropriate trouble code, stalling, and a stumbling on acceleration.
Since the engine is only misfiring on one cylinder, you should concentrate on things that could affect that cylinder only. That would make a poorly adjusted throttle cable an impossible cause. Continue proceeding with normal misfire diagnosis, starting with the secondary ignition components, which you've already begun. If you can, try to do your diganosis while the vehicle is cold so you can hopefully diagnose while the problem is occurring.
Out of curiosity, how did you check the coils. Was this a resistance check or did you swap them or do some other sort of performance check?
I cannot think of any reason why improperly replacing the alternator would cause a misfire, so I doubt that's related to the cause.
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If the code keeps coming back, swapping is the best way. I've never once received a bad resistance reading on a Hyundai coil, but I've seen many had problems. The failure mechanism seems to only occur under high voltage operation.
AnswerCheck your fuel pressure and check for an intake manifold gasket leak on the cylinder that was misfiring. Low fuel pressure will cause a misfire as well as making it necessary for the computer to hold the injectors open longer than normal, making it think the engine is running lean. Hence, the P0171. Similarly, the intake leak will produce unmetered air in the engine on one cylinder, hence a misfire and a P0171.
Your fuel pressure should be 38 PSI with the vacuum hose connected to the regulator and 47-50 PSI with the hose off. The pump should be capable of producing about 90 PSI. You can test this by pinching off the return hose and observing the fuel pressure. Any readings significantly below the above indicate you'll likely need a new fuel pump.
I think it's also likely that the P0171 is unrelated to the misfire, however. Fuel pump issues tend to get worse the longer the vehicle runs. Check the PCV and purge (up to the purge control valve) hoses especially to be sure they're not leaking. If you don't have any leaks, chances are good that the P0171 culprit is the air flow sensor. But I wouldn't jump on this wagon right away. You'll want to diagnose the misfire first. The oxygen sensors cannot work properly while the engine is misfiring, and could cause a false lean or rich code to set.
