I have been wrestling with a #2 and #4 misfire code, and "random misfire" code on my 2002 Sonata 2.7L V-6 engine. I ran a diagnostic scan and found that the B1S1 oxygen sensor was reading high all the time. I replaced the sensor with a new one but got the same result. I checked the harness for proper voltage inputs, all tested according to spec. To test the new O2 sensor, I put it into B2S1 and it worked fine, voltage varied from .1 - .9V. The B2S1 sensor, now installed on B1S1 reads high V all the time, just like the old one did.

I replaced all the plug wires and spark plugs. While doing so I did a compression check, 175# plus or minus 5# on all six cylinders. When I disconnect #2 or #4 spark plug wire there is no change in engine rpm. I also tried disconnecting #6 for comparison and there was a definite change in rpm. I went back and rechecked compression on #2 and #4. I left both spark plugs out in case the head gasket was blown between the two cylinders. Both read 155#, a little lower than earlier, but I theorized that the cylinders were awash in fuel.

I tested the fuel injector harnesses with a "noid light" and they both flashed like they should. I can smell fuel on the plugs and when I insert my borescope into the hole, the top of the piston appears to be wet. The previous owner told me he installed new injectors. I read on this site that the coils can short internally, so I purchased a new coil pack - same result. When I pull a plug wire boot, sparks flash to the valve cover indicating that the wire is not broken, I also tried running the car with the plug installed in the wire to make sure it was "sparking" - it was. I realize that each coil fires two plugs so I also removed the alternate pairs #1 and #5 and the engine rpm drops like it should

Scratching my head at this point, I thought maybe the camshaft lobes were worn so I removed the valve cover. Camshaft is in terrific shape and valves were being moved up and down just like they should. No metal or missing followers or broken pieces anywhere.

I have worked on cars all my life and consider myself pretty knowledgeable but this has me baffled. I have the three things an engine should have, fuel, spark, and compression, yet these two cylinders are not firing. I am going to the salvage yard tomorrow and plan to grab a couple injectors just for testing purposes. Also the harness side plug for the coil pack has disintegrated, so I am going to grab one of those too, even though I have tried pulling and wiggling the connector and it doesn't seem to change anything. Besides, since each coil fires two cylinders at a time, if the plug was not making contact, I should have 4 dead cylinders, not just two.

Also I considered the PCM not controlling the ground circuits for the coils, but again if I had two non-functioning coils, I should have 4 dead cylinders since each coil fires both plug pairs at the same time. What am I missing?

WHEN I figure this out, I will definitely share what I learned.

 

Re: 02 Sonata #2 and #4 misfire - I forgot to mention - I also checked for an intake manifold leak by spraying carb cleaner all long the intake manifold gasket where it meets up with the head. No change. Could there be another vacuum leak somewhere that would affect #2 and #4 only?

 

Did you figure out what was keeping the voltage high on the B1S1 O2 sensor?
It wasn't a bad O2 sensor. So it had to be the wiring??

The coils work on what I believe is known as a waste spark system
It will throw spark to a set of cylinders. One will be ready for ignition and the other will be in a different stage of the four stroke cycle.

I was going to say I had a cylinder #3 misfire on my Elantra and that was due to a bad in-take manifold gasket.
But, you covered that. Please keep updating. Would like to know the fix. Thanks

 

Is there any oil in the spark plug wells of either #2 or #4?

 

I did not see any oil on the old plugs or on the new ones I have installed - I have driven the car several times so there has been sufficient run time for oil to get in the cylinders. The other thing I thought of was water getting into the cylinders via the intake gasket? First, I am going to swap out the injectors I got at the salvage yard and see if that makes any difference, then I might run the engine with #2 or #4 plug removed and see if anything spits out of the spark plug hole. Another possibility I thought of is bad o-rings on the injectors allowing extra air to enter, will check those out when I remove the injectors.

 

I was thinking about the possibility of a valve cover gasket leaking oil to the outside of the spark plug. That oil would be a electrical conductor causing a short preventing spark formation.

 

I have to ask the following questions,
About when did the problem appear?
The person before you bought the car, did some work correct?
Do you know if he/she had a reason to get involved in injector replacement and if that was the reason they sold the vehicle?

Now that opens a whole new set of questions,
What if they put the plug wires in the wrong place?
What if they switched the injector plugs accidentally?
From what info you provided , I would ask if the plugs to those 2 cyls are wet with fuel because of an overfueling condition?
Is it producing an unusual amount of black smoke from the exhaust, which if it is , you could be looking at a converter for the exhaust syst once the misfires are corrected.
Your replies should help shed some light on the problem and you have some knowledgeable folks trying to assist and I'm sure just like me are anxious to know just what is causing this.

AND "WE AIN'T DONE WITH THE QUESTIONS YET"

Can you please either send me a crude diagram of where the plug wires are placed or tell us here in word description how they are connected ?

 

#2 and #4 appear wet with fuel when I remove them. The previous owner was trying to correct a stalling at stop sign issue. He told me he replaced the fuel injectors, MAF sensor, TPS, and IAC (idle air control). The old TPS and IAF came were in the car when I bought it. I drove it to work several times and I never had the stalling issue and it seemed to run okay so I went ahead and fixed some of the other issues, missing trim pieces, new tires, etc. Then I detailed the car and power washed the engine. This is about the time it started to act up. I started it one morning and it would not idle and when it did, it idled rough as if several cylinders were misfiring. This is what led me to think the coils may have become damaged inside due to the power washing (I should have covered the electricals in plastic, but I failed to do so).

The injector harness is still intact and the plug-ins line up with each injector so I don't think they could be mixed up. I have verified and checked the plug wire positions and they are correct 3-6, 5-2, 1-4. (Could also be 6-3, 2-5, and 4-1, since both plugs fire at the same time - I have several different pair combinations but all in this same format).
I have not seen any black smoke but the Bank 1 O2 sensor reads high all the time indicating a rich condition.

I also removed the exhaust system to check for a plugged converter, but when I shined a light down the exhaust pipe, I could see it through the converter grid so It doesn't appear to be plugged. I also ran the car briefly (so as not to disturb the neighbors) with the exhaust off and looked for any junk coming out of the small converters attached to the exhaust manifolds, Again everything appeared to be okay.

When I did the first compression test, water came out of cyl #2 (I had all the plugs removed) but at the time I thought it was water left in the spark plug tube from the power washing. The second time I did the compression check, I did not notice any water coming out. Also when I ran the car with #2 and then #4 plugs removed to see if the spark plugs were working, I did not see any water spewing out of the cylinder either, but I plan to recheck this just to make sure there isn't coolant leaking into the cylinders.

I won't have any time to work on the car until next week, but in the meantime, any suggestions would be greatly appreciated.

 

Did you ever replace the coil pack? Coil packs can short out internally.

 

Power washed the engine??
(hand over face ... )
The news just keeps getting better and better ... lol