Hello all, I've hit the problem-solving wall hard in trying to diagnose the cause of my 2002 Discovery's (117k miles) misfire and am in dire need of advice on where next to look. The misfire started a month ago. Prior to that, I replaced both head gaskets (#6 cylinder had a blow-out) and installed new fuel injectors - that was two years ago and the engine ran fine afterwards. Using the "HawkEye Diagnostic" reader from Bearmach, the initial trouble codes that were displayed when the misfires started to occur were: 300(fault 62), 302(fault 51), 1300(fault 75), and 302(fault 64). I zeroed in on cylinder #2 and removed the spark plug which was clean...none the less, I cleaned the electrodes with some emory cloth and confirmed the gap was correct. The engine was cold at this point and when I started it, it ran properly for awhile....and then the misfire returned. I noted an interesting anomaly with the HawkEye now...the OBD codes indicated that the misfire was occurring in cylinder #2 (302) however, the live data feature of the HawkEye indicated that cylinder 8 was misfiring - the live data read-out of the other cylinders (including #2) showed no misfires. In addition to the previously noted codes, now code 174 (fault 35) was displayed. At this point, I replaced all the ignition wires and put in two new spark plugs at cylinders 2 and 8. Upon start-up, the engine ran perfectly and I did notice an increase in the engine's performance regarding pick-up/acceleration. Then, the same pattern of misfire returned - the engine initially runs properly and then after having driven for about 30 minutes, the misfire suddenly occurs; or having driven to a closer destination with the engine performing properly and then shutting it down to go to a store, upon starting-up, the misfire occurs again upon start-up with the same codes pointing to cylinder #2; and now with the additional code of 447 (fault 96); and with the same anomaly with the live data read from the HawkEye indicating cylinder 8 misfiring. I checked the spark plugs for both cylinders #2 and #8 and confirmed that they were indeed sparking by putting an old spark plug in the cylinder head, firing-up the engine, and holding the new spark plug (#2 and then #8) that was connected to the ignition wire against a grounding spot. I did a compression check previously when I was troubleshooting another problem and interestingly, cylinders and 2 and 8 had the best compression of all the cylinders - 170psi and 156psi respectively. I would be grateful for any ideas that you may have of what might be causing this misfire problem...thanks.

 

I would recommend you stop throwing money at it and start diagnosing. You have an advantage that the fault is isolated to those two cylinders. 2 and 8 are on the same ignition coil, so I am suspecting that but before jumping to conclusions go through the methodical troubleshooting starting with simplest first. Swap known good for suspected bad. Start with the plugs as they are easiest to change, swap 2 and 4 and see if the fault code moves. If not, then swap #2 wire with #1. If the code does not move, swap coils left and right. Report back after that.

 

Hello Extinct, thank you for your recommendation...and I concur with your logic. I didn't mention earlier that I did swap spark plugs between cylinders 4 and 2 and with cylinders 6 and 8 and noted the same misfire behavior. Since the plugs were relatively old, I decided to invest in 2 new ones to see if the newer (and possibly better conductive spark plugs) would make a difference - they didn't. I didn't know that the ignition coils were interchangeable - I'll give that a try...thanks.