How does the OBD2 determine P301: Misfire cylinder #1????

Jay ne Ohio · **Joined:** Sun Dec 12, 2010 5:49 pm **Posts:** 216

Does anyone out there know HOW the OBD2 determines when a cylinder is misfiring on a CRD? It must get some kind of signal from the injector, but what is it measuring? Is there anyway to see if there is the correct voltage coming out of the wire harness connector at each injector?

Background: I bought an 06 that was making a knocking sound, black smoke and throwing a code P301.

It had the original timing belt at 122k, so I figured it must have slipped a notch. I checked the timing with the camlock and flywheel tools and it was exactly correct. Since I had it apart, I installed a new belt, pulleys and tensioner. Put it back together and it is still running exactly the same.

There is no air in the fuel system. I've bled the filter several times with no air coming out of the bleeder screw. Torquescan says that the rail pressure is running 7500 at idle (750 rpm) and 20,000psi at 3000 rpm.

I've disconnected the harness at each injector and notice the engine running rougher as each is unhooked and it smooths out when rehooked.

I bought a brand new injector and installed it yesterday in the number one spot. It did not help. The P301 code came right back. I did not have a scanner to input (learn) the new injector code into the ECM.

When I unhook the MAF sensor while running, the knocking becomes worse. If I unhook it when the CRD is off, it still knocks when started. There are no tunes installed on this Jeep, so the EGR and FCV were both functional. I removed the CAC to engine hose and verified that the butterfly valve was open.

I then removed the FCV with the intention of removing and cleaning the EGR. However, I decided to install a blocking plate on the exhaust outlet of the EGR and the exhaust inlet of the FCV (I removed the pipe between the two devices). I reinstalled the FCV and the engine seemed to run a little smoother, but still had a knock and smoke.

I have inspected the hoses and there are no cracks rips or holes. Torquescan says that the boost pressure is 7.5 psi at idle and 20 psi at 3000 rpm.

I thought there may be a chance that the oil pressure could be low (Torquescan does not seem to measure the psi) which might cause the lifters to malfunction, so I changed the engine oil and filter today (the previous owner had installed Mobil1 5w30). I installed Rotella 5w40 T6.

So what am I missing?????

My current thinking is that the OBD could be correct about the misfire in cylinder #1. Possible causes: bad injector, wiring issue, ecm issue. I thought I could rule out the injector, but perhaps the new one did not work because I did not "learn" the code into the ecm. But I've heard of guys swapping ECM's without "learning" the injector codes. I do have another 06 that I may swap ECM's to see if the problem follows.

The other possibility is that there is a bad rocker or lifter in cylinder #1 which is causing the misfire. But how would the OBD2 know this???? So that brings me back to my original question: How does the OBD2 determine misfire?

Any and all thoughts are welcome!!!!!

MrMopar64 · **Posted:** Mon Aug 27, 2012 4:31 pm

The ECU looks at the segment times from the crank sensor and compares it against all the other cylinders.
If one cylinder doesn't match the expected behavior, or if one cylinder has significantly different behavior, then it flags that one as the misfiring cylinder.

Considering the age and mileage of the vehicle, it's highly probable that the rocker arms are worn and not opening all of the valves completely.

You could pin out the harness to the ECU and check for peace of mind that the wiring is correct.

The last but most highly unlikely root cause could be the ECU. I'd suggest you to swap it with your 06MY vehicle, but it won't run more than 2sec due to the immobilizer unless you have a DRBIII to change the VIN with.

geordi · **Posted:** Mon Aug 27, 2012 5:05 pm

Save your strength on swapping anything electronic, the ECU is working just fine at telling you what is happening. Check my thread "Tapping on top end" for more info on everything I have gone through on this, but I just saved you about $400 in gaskets and potentially 16 hours of labor.

It is a lifter being stuck. If the situation continues, it could get stuck to the point of holding the valve open long enough (and far enough) that the piston hits it, which will snap the rocker arm entirely.

I will be reassembling my engine starting tomorrow, but the ONLY thing wrong that I have found (dropping almost $300 in diagnostics and another $400 in parts today) is the lifters were stuck. I compressed each of them with a vise to free them up, soaked them in diesel fuel to clean the innards, and they are all now compressing easily and smoothly. If you need to replace any of yours, I have a source that flat out SMOKES any of the prices I have seen in the USA, and they are genuine VM parts. Pull your valve cover / upper intake, and have a look-see at the lifters. If you can't compress them with your fingers, they are stuck and need to be cleaned. If they are bent / collapsed or the rocker arms look bent or broken, then they have to be replaced. You don't need to replace them all together, but it might not be a bad idea.

Dennis MacGyver · **Posted:** Mon Aug 27, 2012 5:09 pm

To test for a bad injector, you could swap the # 1 & 2 and see if the code points to a miss-fire in the 2nd cylinder in place of the 1st that you are now seeing.

CATCRD · **Posted:** Mon Aug 27, 2012 5:22 pm

Jay ne Ohio wrote:

I have inspected the hoses and there are no cracks rips or holes. Torquescan says that the boost pressure is 7.5 psi at idle and 20 psi at 3000 rpm.

I'm still stuck on these numbers. Are you sure it says 7.5 psi at idle? Is that gage pressure or absolute pressure? It should be around atmospheric pressure at idle. So it should read zero or 14.7 depending. Is the 20psi number under load, or just revving in neutral?

Jay ne Ohio · **Joined:** Sun Dec 12, 2010 5:49 pm **Posts:** 216

Thanks guys!

Mr. Mopar: That is exactly the info I was looking for. Thank you for sharing your knowledge here again.

Geordi: I've been following your thread and suspected that we may be on the same path, but you are about a few days ahead of me. The previous owner took this CRD to his stealer and the tech was "absolutely" certain that it was the EGR or the fuel pump.... When I heard it run I felt it was a slipped timing belt.... it needed done anyhow... Then the misfire code led me to believe that it was the injector..... so now I have a spare injector....

Dennis: Yes I could have swapped injectors to see if it followed. That would have saved me a few dollars. But as mentioned above, I now have a spare, which I will probably need someday for one of my three CRD's.

CAT: Now that I think about it, I think that TorqueScan said 7.5 psi at 1500 rpm going up hill holding a steady speed, but I could be wrong. I did not write it down and I cannot get Torquescan to work today at ALL!!!! When I get it working again, I will recheck. I think you are correct that it should be zero at idle. The 20 psi was accelerating hard going up a hill. I just downloaded Torquescan yesterday, so I am not familiar with all the features yet.

geordi · **Posted:** Mon Aug 27, 2012 9:00 pm

20psi pulling a hill would be correct at WOT, if it gains one more psi to 21, that is when the computer will start being pissy and throwing the overboost code and hitting you with limp mode.

How does the OBD2 determine P301: Misfire cylinder #1????

Who is online