So I thought my rover was in great shape, until last week. Here's a run through from start to finish. Feel free to tell me where I messed up or did something wrong but advise me on how to fix it.

Back at the beginning of summer I had some slight misfires, nothing too rough and no codes, and it went away so I assumed it was bad gas. Shortly after that, I installed a full Magnaflow exhaust system because I talked with a few people and they loved theirs. Nothing really wrong with mine other than it was starting to get quite rusty and develop leaks here and there.

So I installed that and figured I would put all new 02 sensors in while I did so as well - why risk a new exhaust. I did that, and I loved it. I did however immediately get a persistent code of BOTH downstream 02 sensors tripping (P0141 & P0161). They would come up at the same exact time and stay on until cleared, and the stay off for the remainder of the rover being on and would pop back up about a mile into the next trip.

I figured the wasn't a big deal because it was only the downstream sensors and I thought all they did was tell you if the cats are bad? Which they obviously weren't because they were brand new. So I left it alone and had no change in fuel economy either.

If you guys saw some of my posts during August and September you know that I did a lot of work to my rover - lift, tires, winch bumper, and stuff like that. Still, no problem other than those two codes constantly on - which I thought nothing of and just figured it was due to the different exhaust.

Well about a week ago I started getting terrible misfires. Cylinder 6 Misfire in particular (P0306) and Random/Multiple Misfire (P0300). I thought it may be bad gas and ran the rover a day or two - maybe putting on 20 miles. It only became more rough. I checked the spark plugs and they were fouled badly (I have pictures I can post of each of them).

I pulled the plugs (Bosch +4) and put in all new plugs (I think the Champion ones I saw on the forums) and it ran fine for the last few days aside from a slightly rough idle. Well, now it came back and is just as bad. I haven't checked the plugs yet because the engine is still hot, but I'm assuming they are fouled.

Wires are newer STI's put on by PO about 2 years ago. Head gaskets were done with brand new heads around that time too by PO's Rovertech - as receipts and observation shows.

Any help would be appreciated.

135,000 miles by the way.

 

First, you mention bad gas several times. Although I'm sure it's happened, this is one of the least likely causes of automotive problems. Should be one of the last things you consider, IMHO, unless you have evidence that it's bad.

Given the immediate problem with the rear O2 sensors, I'm guessing they're either the wrong sensors, or they're bad/off out of the box. If you bought one of the cheapo eBay kits of 4, there's a high likelihood of either. I'd get that figured out by swapping in known-good units. (for diagnosis, I'd even swap my old used ones back in)

Since you installed those off-road mods, have you been using the vehicle differently? I assume you installed them for greater capability off-road, so have you been making more use of it in such a way?

The Champion copper plugs are good, I've probably logged 200k+ miles with them on various Rover V8's.

The first suspect is the plug wires. 2 years old isn't exactly new, and they fail way too often to rule them out. For a quick diagnosis, swap the plug 6 wire with the plug 5 wire to see if the situation changes. If it were mine, I'd be buying a new set of Kingsborne wires to rule it out.

Next suspect would be coils. You can either buy a new one and swap it in, or choose to diagnose by swapping your current coils side-to-side. It's a time vs money choice.

If cyl 6 is still misfiring, report back.

 

I ma dealing with the exact issue! i installed a magna flow on my disco 2 and had no issues. As a month so went by i had the same problems..... i brought it to a land rover specialist and he claimed it to be the cats that came with magna flow system is what is causing this. i have similar codes reading. i replacerd plugs and problem persisted. so i took the rover down to exhasut shop to ask if cats could do hat to my truck? they say no b ut i would love hear some othe ideas. im gonna try and replace the plug wires and see where that gets me.