Misfires can also be caused by coolant leaking into a cylinder.

 

Good point, we have 6 causes. Coolant in a cylinder will do it. Slightly more complex diagnostic path. Eliminate the first 5 and if we are only left with #6 we can walk you through how to confirm that.

 

But if it runs well at first, and then suddenly gets crummy, consistently, that's not plugs, wires, or coils, right? That's the ECU switching up the mixture, that's based on a number of sensors, but primarily o2 sensors and air meter. But then a bad ECU is potentially part of that...

 

Thanks for the suggestions, guys. I'll respond to Extinct's list first, although, I think my photos below provide the most valuable info:

1 - 3): Plugs, wires, and coils are new, have been tested, and are all working fine
4) Compression on cylinders 4, 6, and 8 is within normal range, as I said yesterday, but we did not test the others yesterday. There is also no sign of exhaust gasses returning into the overflow coolant bottle.
5) We ran new numbers with the scanner today, and I've attached photos of the readings below (let me know if they don't show up). Fuel trim 1 - Part Load was -6.1, Fuel trim 2 - Part Load was -4.6. I have a video of the Engine Stability, which was all over the map on cylinders 2, 3, 4, 5, 6, and 7, ranging between 0.00 - 8.00. Cylinders 1 and 8 were solidly 0.00 (can't figure out how to upload that...).

This scan today was done with the donor ECU, which we believe is working fine. We also swapped MAFs with the donor Disco and ran the scanner again, and the numbers did not change. We drove it for about 5 miles today (at low speed) with the donor ECU, just to confirm which codes we got before we hooked up the scan tool. We got P0300 and P0303 on the UG, then brought it back to the garage and ran the scan.

Thank you all again for your continued advice. Hope you are all having a nice Memorial Day!

 

Just going summarize, runs fine for a few minutes then stumbles runs rough one side only.

So 2 questions :

1. Does it stay stay smooth at idle or does it get rough
2. Do things get worse mostly under load

No coolant gases does not exclude a head gasket, but black plugs are generally over fueled and rich, you replaced the injectors it is possible you got a bad set as it is only on 1 side.

You could pull the upper intake and fuel rail and test the injectors and swap sides.

 

Sorry, that scanner does not tell me much. You need an elm327 off of Amazon and the free torque app for android and graph the oxygen sensor voltages so we can see the cycling. Good to know you did 1-5, but just to confirm it is now isolated to #3 cylinder? have you swapped wires, plugs, and coils off of that cylinder to a known good one? What does the plug look like in that cyl? 1 bad injector could cause a misfire - leaking or plugged either way, but the color of the spark plug would tell the tale. You can swap the injector to another cylinder to confirm that is the problem.

 

bad knock sensor? or harness to 02 sensors or knock sensor? Seems to go bad when warmed up and going into closed loop....? Cam position sensor? Using which type O2 sensor , Walker?

 

Sorry, guys, I should've been more clear yesterday and Sunday. The ONLY time it ran well at first on start-up, then started running slightly rough with the CEL on and flashing, was immediately after we swapped in the donor ECU + BCU. It started missing after a few minutes, and we got the P0300 + P0306 codes on the UG as we were test driving it at that time with the swapped ECU + BCU.

Yesterday, still with the swapped ECU + BCU, we took it for another drive for maybe 4-5 miles to fill up the tank (the fuel light had come on on Sunday). While we were driving, we got the P0300 again, and the P0303 on the UG (first misfire code on that bank in months). After we put in half a tank, we drove back and hooked up the Snap-On scanner. The scanner pulled misfire codes for cylinders 2-4-6, and now cylinder 3. We did not get a flashing CEL at all yesterday, despite the codes. My buddy needed his ECU + BCU back in his Disco, so at the end of the day yesterday, we swapped my old (original) ECU + BCU back in. The engine did not sound or feel noticeably different with my ECU + BCU hooked up, although it was barely driven.

By my layman's estimation, I would not say that this is producing a "violent" misfire sensation. There is a slight vibration/juddering at idle, but not terrible. It doesn't seem down on power when driving, although I haven't taken it on the highway. It sounds, feels, and accelerates like any other Rover I've driven, and MPGs are no worse than normal on the UG. The only slight stumble is at about 1,000 - 1,5000 RPM, but even that does not trigger the flashing CEL, and it goes away above about 1,500 RPM. If it wasn't for the CEL and the slightly-rich exhaust, I don't know that I'd even notice an issue. Under load when driving, all seems normal, i.e. no worse than at idle.

I want to examine the plugs again to see if they're fouled, now that we've driven it a bit, and swap them around from side-to-side (along with the wires). I'll also work on getting some better O2 sensor data.

 

BTW, The ultragauge has no real way of knowing MPG... It is an estimate based on you inputting fuel information and it is just doing the same math as you would do manually using the odometer and the amount of gallons you used. There is no electronic signal that tells MPG.

 

This sounds so similar to my issue, so I'm really hoping you can get yours resolved with a smoking gun so I can fix mine! I'm in the middle of doing everything you are, except I haven't considered to ecu/bcu thing yet, and it seems that didn't fix your problem anyway. Mine runs great on startup, then goes closed loop or whatever and starts flashing CEL. Codes are all for one side of the engine, all 4 cylinders misfire. Runs fine under power, just idles terrible.

The two of us can't be the only people with this issue...