Identifying a bad coil
 

On my way home the other day, I started getting some power loss sporadically. It eventually became consistent and the CEL came on. I took it to Autozone (my scanner is "borrowed") and it pulled a P0300 "Random multiple misfire" code.

I used to have an older Audi A4 and we had coils go bad, but plugging in our ODBII sensor would tell us exactly which cylinder it was that was misfiring, pop out the coil, in goes a new one, and ready to rock.

I took it to a shop to have them diagnose the issue, and they basically called me when it was done saying they replaced the coil and the only real way to know which coil it was was to go through each one and swap the coil. I would think their advanced scanning equipment would tell them right away what cylinder was misfiring. In the end, it cost me $366 which seem super high to replace a coil that costs $115 and takes 10 minutes to swap.

Is this Nissan completely different in that the internal computer can't identify which coil is misfiring, and provide that information to a scanner? They told me the Nissan computer doesn't provide that info like the Audi computer does. I wanna say this is bogus and they hosed me, but I don't know for sure.

Would a quality scanner have told me which cylinder it was that was misfiring?

I don't think that's super high because most shops charge around $90/hr. and like you said, they have to swap each coil to determine which has the random misfire. So around 2.5hrs. to do all that swapping around isn't too bad. As far as those ''mechanics'' saying that Nissan doesn't have the sophisticated diagnostics computer that Audi does, is wrong. Nissan goes one step further, in telling you that you're having a ''random'' misfire(meaning not consistent on that given cylinder), by throwing the P0300 code to give you a 'heads-up' that one is about to fail. A ''consistent'' misfire on a cylinder will be designated by the codes P0301 - for cylinder 1, P0302 - for cylinder 2, P0303 - for cylinder 3, P0304 - for cylinder 4, P0305 - for cylinder 5, and P0306 - for cylinder 6.

Well they only swapped one coil and the car is running fine now (at this point ive had two coils replaced, so if it happens again im just going to replace the remaining 4). If the other cylinders are misfiring too, shouldn't it throw a code for each cylinder that is misfiring? If it was truly random, wouldn't it be a separate issue than the coil?

Sorry, just rereading your post, you mean the "random" misfire is still specific to a certain cylinder but because it's not consistently misfiring on that cylinder the computer doesn't put out a cylinder-specific code? That seems stupid, cuz then diagnostics are way more time consuming...should I have kept driving it until it was consistently misfiring to get the cylinder-specific code?

They weren't all misfiring. One was misfiring randomly, not consistently. If it was consistent, it would've thrown the P030X code(with "X" designating the cylinder number that the misfire occurred in).


I can see why you would initially think that this logic is stupid, but if one coil is randomly misfiring it would be illogical for it to throw a P0301, 2, 3, 4, 5, or 6 code over an inconsistent misfire within that cylinder. Instead, it throws the P0300(random misfire) code to isolate the problem to within that realm; leaving it up to US to troubleshoot further, or until the misfire is consistent within a given cylinder.

When I say stupid, i really just mean unhelpful. It should still indicate the cylinder to direct the troubleshooting, and in this case would have saved me $250 hence the frustration, cuz i can definitely replace the coil myself, and probably could have isolated the cylinder if i had more time but i was pressed to get it fixed.

The nomenclature should allow for a separate set of codes to indicate intermittent misfire specific to a cylinder.

On another note, if the cylinder misfire is intermittent as it was, can I keep driving the car? It's my understanding that unburnt fuel sent through the exhaust is bad for the cat, but if the misfire is not consistent I don't know how seriously it would damage the cat.

I agree. It would be helpful to identify a random misfire on a cylinder as P0301R(with "R" representing "random"), but it doesn't.


As far as driving like that, I wouldn't recommend it. Along with contaminating the oil with gas and running raw fuel through the cat, it will also present a fire hazard within the cat itself from all the unburned fuel going through it. If you absolutely must drive it, disconnect that injector so that no fuel is fed into that cylinder.. And that's basically just to get it to where you need to get it to perform the repair. Nothing more.

Makes sense. Can you disconnect the injector without taking off the intake manifold? I know to replace them you have to remove the intake manifold upper and lower. Just for future reference...

No. You're going to have to remove the plenums, unfortunately.

If the car doesn't specify which cylinder is misfiring (which it won't until a certain threshold is met,) you CAN identify the cylinder in about 10 minutes doing a relative compression test with a scope and a pair of low amp test probes. I use an old Sun re-branded 2-channel Snap-On scope I've had for many years to perform this test. One channel signals to cyl 1, the second channel signals to power. Based on the ignition sequence, you'll see exactly which cylinder is misfiring over time (particularly if your scope is set up to record glitches.) It's more than a $250 investment to get a halfway decent automotive scope, but for issues like yours, it'll make your life a lot easier. If the garage you took your car to was any good, they would have used this method to figure out which cylinder or cylinders were misfiring.

Here's a how-to:

While i definitely appreciate the value of a good diagnostic tool, at this point i only have 4 coils left to replace so when the next one goes ill just replace all 4 for $300 myself.

Yeah, I actually miss the old days of the distributer caps, rotors, ignition coils and ignition wires. It was a lot easier to work on those. Throw in points and a condenser, along with an old analog Sears dwell/angle/tach monitor and I'm in heaven :)