You can drive it unplugged, if it drives fine wiithout the codes you may have a bad MAF (it will throw a MAF code of course). It takes a minute to get the idle right without the MAF, but your problem is a speed not idling so drive it a little without the MAF.

 

Alright, I swapped coils, wires, and switched the plugs around yesterday, and also took that opportunity to check that the plug wires were all seated correctly. I also went around the block without the MAF, and swapped in a known working MAF. None of that resulted in any progress. I am still getting the slight stumble at low RPMs, and consistent misfire codes on cylinders 6 and 8.

Our new theory is that the injectors on those cylinders are defective, and are dumping too much fuel (albeit, not THAT much) into the cylinders. At low RPM, the excess fuel is not being burnt fully, but it is being burnt fully (or, just enough) at higher RPM, which is why I do not feel a misfire or get a flashing SES at higher RPM. Am I way off with that, or is it a possibility? Injectors are the only part specific to those cylinders that we have not rotated out yet.

 

Given you have swapped everything else around, easy enough to move the injectors. Have you checked compression on those two cylinders? You could have the beginning of a blown hg between the two.

 

Compression was between 155 - 165 across cylinders 4-6-8 when we checked in May. I'll dig into the injectors this weekend and see what we find.

 

When you go to Wide Open Throttle (WOT) the computer goes into "open loop" mode where the O2 sensors stop reporting to the computer to keep things at stoichiometric. This kind of points to either your 02 sensors or to "leaky" injectors either putting too much fuel in or dribbling instead of spraying properly,

 

In my experience this is not true on Disco 2's, they stay in closed loop even at WOT which is very unusual. Most ECU's do send WOT engines in to PE mode where fueling is based solely off of MAF airflow or speed density maps, certainly all my GM products did that. Mapping Disco 2 02 voltage outputs on long uphill WOT climbs I have seen the 02 continue to cycle all the way up.

 

According the the manual just slowly pushing the pedal all the way will not necessarily make WOT... The manual says sharply depress the accelerator pedal all the way to the floor in one motion to achieve WOT. Obviously a computer controlled function rather just sensing whether you finally got the pedal to the floor.

Last time I mentioned this on the forum most people did not know.

Possibly you are not actually achieving WOT when you think you are... Then again maybe you know but at least this may be useful info for someone.

 

I have done quite a bit of ECU programming on GM ECU's, but obviously have not bought the LR hardware as it is crazy expensive and not much use for it. Most ECU's go in to PE mode at about 80% throttle, but no the Disco 2. I have done both gradual test as well as floor it from the jump. Luckily I live on the blue ridge so long uphill climbs are good for long WOT runs, and I frequently tow with mine so I get some good long WOT pulls. I have a wideband and was contemplating installing it to see what the AFR was in PE mode till I did a few test and saw that the sensors continue to cycle even at WOT. LR probably left a few hp on the table with that function, but the only real way around it is either reprogramming or using a programmable WB that will shift voltages based on a trigger (like TPS or manifold vacuum) to shift AFR to something like 13:1 at WOT.

 

GOOD NEWS: It would appear that we have solved my misfire issue. Happily, I can report today, for the first time in months, that I have no stumbling at any RPM, no flashing SES light, no misfire codes, and a better-running engine all around. Unfortunately, I haven't cracked some mysterious Rover misfire code. The issue seems to be bad couple of injectors in the batch of remanufactured injectors I got from eBay a few months back (on cylinders 6 and 8). At this point, then, I have eight new injectors, new plugs, new wires, relocated coils, a new intake manifold gasket, and some vacuum line repairs.

In short, don't buy the cheapest remanufactured injectors on eBay. Long story longer, I'll briefly recap how I got here, at least for the benefit of someone else who may stumble on here.

It began with a Cyl. 3 P0303 and P0300 misfire codes, which I believed were a result of coolant dripping onto the Cyl. 3 plug from the throttle body heater plate. I was fairly certain this Disco had not been very well taken care of in its previous life, and I thought some maintenance was due. So, we replaced all eight injectors (some of which turned out later to be rubbish), plugs, and wires. In all honesty, the original injectors very well could've been fine, but given that the manifold cover was off for the plug wires, I thought replacing them made sense (and again, I assumed it had never been done before). I should have suspected that cheap, remanufactured injectors were the cause of the gremlins I'd end up chasing for months. But, of course, one's brain says "They're remanufactured injectors! What could possibly be wrong with them? It MUST be something else!"

Due to the misfire codes at that point consistently being somewhere on Bank 2 (2,4,6,8), we began chasing O2 sensor issues, and re-wired the upstream sensors on both sides to the PCM/ECU. I do believe that helped a bit, but again, knowing what we know now, that probably wasn't necessary. We eventually landed back on the injectors being the issue once we'd replaced everything else, including relocating the coil packs with new wires, and combined those observations with the engine behavior I mentioned previously.

Something, something, hindsight 20/20, right? Hopefully, someone stumbling across this thread can get something useful out of it.

Thank you all again for the help and advice. Here's a picture of the ol' girl from last weekend, because this thread is too long and too boring, and at least needs a celebratory picture to close it out.

 

Glad to hear you got it sorted. Hopefully the education you got when you did not get what you paid for on the reman injectors will come in handy down the road.