Hello, friends!

Longtime lurker, first-time poster. I bought a 2004 D2 SE7 from a friend in November 2019 with 114k on the clock, with previous ownership history in the southeast US (Georgia, Kentucky, Alabama) until April 2019. It's my second Rover during the last few years, so I'm fairly well-versed on the typical Rover danger points. Service history on this Disco is a big unknown, unfortunately, and it sat barely being driven from April 2019 – October 2019. When the weather turned cold(er) shortly after I bought the Disco, the coolant leak at the throttle body heater plate got worse, coolant dripped onto my Cyl. 3 spark plug, and the misfire game began. FWIW, I replaced the throttle body heater plate and associated gaskets. No more coolant leaks, and no other obvious coolant loss.

I bought an UltraGauge immediately after I bought the Disco. At first, I got P0303 + P0300 misfire codes, flashing CEL, and an obvious judder at idle. Made sense, considering coolant had been dripping directly onto Plug #3 for a few days. I immediately took it in to my trusty indy mechanic for plugs (NGK Iridium), wires (Magnecor 8mm), and injectors (remanufactured from eBay), largely because I had no idea if any of those items had previously been addressed. After buttoning it all back up, I then had P0300 and P0308 on the UG, very rich mixture coming out the back, and a full scan tool revealed misfires on all of Bank 2 (cylinders 2, 4, 6, 8).

At this point, spark and fuel are all good, confirmed after tearing into the intake manifold again, testing each injector, plug, and wire, and swapping plugs and wires. Both upstream O2 sensors have also been replaced with brand new units. When replacing the O2 sensors, my mechanic discovered a bad ground, and that the Bank 2 upstream O2 sensor had the wires above the harness twisted together with shrink tubing around them, and were thus not plugged into the ECU correctly. AHA, we thought, that MUST be what's causing the Bank 2 misfire issue. He corrected the wiring to the ECU from the O2 sensor and confirmed voltage at the ECU, but I am still getting the P0302, 0304, 0306, and 0308 codes. Cylinder #8 is, by far, throwing the most consistent codes. #s 2, 4, and 6 are less consistent. We've swapped the O2 sensors, and we're still getting misfires on all Bank 2 cylinders, which leads me to believe the issue is not with the O2 sensor. Compression is within spec, as well. Idle is fine, and there is an occasional stutter when driving, but not constant.

I should also add that when both upstream O2 sensors are unplugged, the misfire goes away, but then I get O2 codes (obviously).

I've read way too many pages on this forum in the last couple days regarding misfires, and I can't seem to find anything obvious that we haven't tried yet. Is it possible that my Bank 2 cat is fried? Is it possible that the SAI system has a vacuum leak somewhere on that bank, but not the other? Do I need to go back to non-Iridium plugs? Is it really possible that my two brand new O2 sensors are fouled already?

Sorry for the long post, but I wanted to provide as much info as possible. At this point, I'm at a loss, and my mechanic is, too. Any help is appreciated, and please feel free to ask questions. Thanks in advance.

- BL

 

I would swap out all O2 sensors to start. These are typically considered consumables. Also replace the coils and do a smoke test to check for vacuum leaks. This would only be a couple hundred bucks if you do the work yourself and it would rule a lot of things out.

Also if the truck has been sitting for awhile have you swapped out with fresh gas?

 

We've done the extremely flammable liquids test for vacuum leaks (brake cleaner and LP) around the intake manifold, with no obvious RPM fluctuation there. Would a vacuum leak have the misfires concentrated to one bank like they are, or would it be more random across all cylinders?

Yep, it was filled up with a fresh tank of gas before all of these problems began.

I did ask my guy about swapping the downstream O2s, as well, because those haven't been done yet. Should I start with the downstream, and if I'm still having problems, replace the upstream O2s again?

Thanks!

 

My approach would be to replace the remaining two O2 sensors. If that does not resolve it, I believe a failing coil could cause this type of issue on one side.

 

Gotcha.

If the coil failed/is failing, that would create a noticeable spark issue, right? Spark is perfect on all cylinders on that bank, tested with an inline spark tester. Also, as I understand it, one side of the coil pack controls Cylinders 8/2/5/3, and the other side controls 1/7/6/4. It would seem strange that only the sections of the coil pack controlling Bank 2 cylinders (i.e. 2/4/6/8 from "both sides" of the coil pack) would fail, but the Bank 1 cylinders would not be effected.

Not trying to be argumentative, just want to make sure I'm looking at the right areas. Cheers!

BL

 

Based on what you have posted I suspect you a bad 02 sensor on bank 2. Here is my reasoning:

1. Misfires go away when you unplug 02s - when that is done the ECU reverts to speed density mode and uses rpm and tps only to determine fuel. Might be slightly rich but will run reasonably well.
2. When 02 sensors fail, they typically fail open - 0 volts. That tells the ecu the bank is full lean, so it compensates by going full rich. Giving you full rich misfires. The downstream 02 can see this and causes the CEL to flash indicating the cat is seeing to much fuel to burn off.
3. Oxygen sensor is the only thing common to all cylinders on a bank - the coils are split.

Easiest way to confirm this is to swap left and right 02s, you should see the problem jump banks. alternatively you can get a elm327 bluetooth scanner and a the torque app and look at your 02 voltage GRAPHS!! - not digital voltage readings. The 02s should cycle between 1 and 0 volts. On a cold start they should be near 1 and the start cycling after about 2 minutes of running. My guess is yours starts at 0 and never moves.

Let me know if you have questions.

 

That is a great response. I do trust a guy who's owned a dozen Discos.

That all makes sense to me, and agreed on the coils. My current upstream O2 sensors on both banks are brand new, and I believe the misfire codes remain at Bank 2, even when the Bank 1 and Bank 2 upstream O2 sensors are swapped. I'd need to confirm that with my mechanic, though. Assuming my upstream O2 sensors are working correctly, could a failed/faulty downstream O2 sensor on Bank 2 cause my Bank 2 misfire issues?

I have two new O2 sensors and eight Champion 322s ready to go in, so I guess I'll find out soon enough.

 

Downstream 02's do not affect engine running at all, they are only there for monitoring emissions. The bank 2 upstream wiring could be your issue or the sensor itself. The easiest way to test wiring vs sensor is to swap left and right.

 

Got it. Thanks very much for your help. I'll update here this week, once I know more.

 

Alright, folks, apologies for the delay. All four O2s have been replaced, and all new plugs have been installed. Still getting those same 2-4-6-8 misfire codes. I should note that I'm not getting any O2 sensor codes, and swapping sensors side-to-side makes no difference.

Mechanic is leaning toward a PCM issue next. I've done a bit of research as to PCM issues/replacement, and it seems that replacing the PCM can be a minefield in terms of a new PCM not cooperating with existing hardware. Otherwise, if there is a different logical next step, I am always open to ideas from you all. Thanks again for the help.

- BL