PO171 and misfiring on 2003 XG 350
 

I have spent a lot of time and money trying to fix the PO171 code and misfiring on my 2003 Hyundai XG350. Lately I replaced the MAP and MAF sensors, but the misfiring is still there and is now accompanied by a new sharp tapping sound from the rear side. I have checked for vacuum leaks and replaced fuel pump and filter, fuel pressure regulator, PCV VALVE, cracked hoses, air filter, and two ignition coils. Scanner shows new MAF is working well. Appreciate any help.

Since you have a scanner available to use , will it tell you which cyls are misfiring P030X would help to isolate and provide some direction.

The 171 code is indicative of a lean condition and any data stream info would help also.

Can you provide this info?

Thanks for your reply! Yes, I will hook up my little scanner and write down what live data it provides.

Data stream 03 Hyundai
 

My little scanner showed the following in data stream with engine running. I don't know a whole lot about this yet, so I'm just including the ones that I'm guessing are most important. I haven't seen a PO30X on it, so I don't know exactly where the misfiring comes from. Tapping sound on firewall side of engine got quieter as engine warmed up. Engine starts up quick. Revs well. But there is the uneven putting at exhaust pipe instead of a steady flow. Sorry if I have included too much data. May be repetitious. I took pictures as engine got warmer. Thanks very much for any help you can give.
ECT 25 degrees C. STFT1 25%. LTFT 1 0%. STFT2 -3.9%. LTFT2 0%. RPM 1066/min.
IAT 19 deg C. MAF 9.59 g/s. O2S11 0.0V. STFT11 25.0%. O2S12 0.1V. STFT12 25.0%.
02S21 0.42V. STFT21 -3.1%. O2S22 0.78V. STFT22 -3.1%. Fuel Sys1 OL. Fuel Sys2 CL. ECT 82 deg. C.
With engine more fully warmed up: STFT1 4.7%. LTFT1 0.0%. STFT2 -5.5%. LTFT2 0.0%. RPM 726/min. IAT 20 deg C. MAF 5.49 g/s (goes up as rpm increases). O2S11 0.040V. STFT11 4.7%. 02S12 0.040V. STFT12 4.7%. O2S21 0.62V. STFT21 -7.0%. 02S22 0.80V. STFT22 -7.0%. STFT1 4.7%. LTFT1 0.0. STFT2 -8.6%. LTFT2 0.0%. RPM 718/min. MAF 0.7 lb/min. 02S11 0.04V. STFT11 4.7%. O2S12 0.060V. STFT12 4.7%. O2S21 0.10V. STFT21 -8.6%. 02S22 0.76V. STFT22 -9.4%. STFT11 4.7%. LTFT11 0.0%. STFT2 -10.9%. LTFT2 0.0%.

I guess we're kinda spoiled because most times the info is listed in a column, but anyway, I would suspect a vacuum leak or extra air getting into the #2 bank from the STFT reading which is attempting to compensate for a lean mixture.

Let us know what you find, thanks.

OK. Thanks for your help. I'm going to scan again looking at fuel trims and O2 sensors. I have checked for vac leaks but maybe the valve on the canister is stuck open.

We should determine what the tapping is all about.
If there is a valve problem that needs to be checked out. quick compression test should provide an answer for that.

Hello, again! Thanks for your input.
That tapping noise that appeared after I replaced the MAP and MAF sensors earlier has gone away. The engine always starts right up. The last two times when I started it cold there was very little to no misfiring for a short time. There was some smell of gas at the tail pipe. Then, as the engine warmed up, there was a lot of misfiring again. No black smoke.
This seems to me to possibly be the effect of a bad coil failing when warm and causing misfires on its two cylinders. Two of the coils are brand new. The third one is about 4 years old. All are Korean made Densos.
When I drove it, the engine stumbled and did not run well. I guess I'll drive it a little more and see if a code pops up. Here is some of the data I collected today with my small scanner with the engine running and both fuel systems at Closed Loop:
Idle RPM 703
MAF at idle 5.3 g/s
MAF at 2000 9.8 g/s
O2 SENSORS (idle, readings fluctuating)
1 1 0.74 V
1 2 0.62V
2 1 0.36V
2 2 0.84V
FUEL TRIM (at idle)
STFT 1 6.3%
LTFT 1 19.5%
STFT 2 - 5.5%
LTFT 2 -14.8%

Thanks again for your help. There is a lot to learn.

Did some more testing and found that the coil on the #6 plug was putting out weak or no spark. So I assumed I needed a new coil to replace that one even though it is only 4 and a half years old. But then I got the idea that I should see how one of my new coils would act in that position. So I put a new coil that I had bought earlier into that #6 position and found that then it also produced a weak spark. The older coil that I had switched with it was putting out a good spark. So the trouble seems not to be with the coil, and I'm guessing that there may be a problem in the part of the harness that goes to #6. Now I have ordered a new harness for the front side of the engine and one more new coil. Hopefully this will finally get rid of the misfires. :o

Put in the new harness and put a new coil in that position. Still misfires when it warms up. When it's cold it runs good for a minute or so. Guess I will try a new misfire sensor, though I replaced it about 5 years ago. Maybe the bad coil cause it to fail. Hopefully I won't need to replace the crank sensor. Not so easy. Suggestions?