I just bought a 2004 Land Rover Discovery 2 SE7 w/ 105k miles a couple weeks ago I love it already but am having a few unfortunate issues.

Car was fine when i test drove it and for a few days after, and then i hopped on the freeway one day and it misfired like hell, couldn't get above 50mph, i could feel it misfiring badly at idle. Got the codes read: P1300, P0300, P0301, P0304, P0307, P0308.

Took it in to a reliable local shop, they said it had the wrong plugs (Platinum +4's), replaced the plugs with NGK's and changed my oil. Car ran fine / idled fine for a couple days and pulled hard etc., no CEL light.

Two days later, same symptoms, codes read the same. Took it back to the shop, they said it must be the coils, so they replaced both ignition coil packs and the wires (STI). Car ran fine and pulled harder than ever, idle was smoothest its been. Drove it around town fine for a day.

Next day, i take it on the highway and open it up and immediately get a code (only P0304 this time). Drove it for another ten minutes on the freeway and eventually got the codes: P1300, P0300, P0304, P0305, P0307, P0308.

It seems to be idling better now and not misfiring as much, but otherwise the same symptoms persist, and i am now in over $1,100 trying to troubleshoot this

any thoughts on what to do next? Does this sound fuel injector related? I told him about the misfiring issue the first time it happened, and his reply was "Well, the O-rings are known to become brittle and leak in these trucks", was this a potential "hint"?

Supposedly the HG was done 15k miles ago, and the previous owner was a rover enthusiast and seemed genuine... Should also note that the CarFax showed it failed emissions in December... not sure how that may relate?

 

Misfires can lead to the cats overheating. (Fuel burning in the wrong place)

Which in turn could lead to more misfires? I ask that as a question because it's my first guess but I'm not entirely certain.

Thats are a lot of misfires, so if you drove it for any length of time the cats would certainly have been in danger.

 

Not sure how handy you are around the engine, but what helped me diagnose a misfire was this Inline Ignition Spark Checker

Basically, you pick one of the plugs that reads as misfired, like P0301 is plug #1, front of truck, driver side. Unplug the wire from the spark plug and attach the device. If the light is strong and steady, then the coil and wire are good and things are right BEFORE the plug. If the light is weak and flashy, the plug and engine are not getting good current. In other words, it helps determine if the problem is before or after the spark plug. You can find a plug not read as misfired to compare the others.

This is just one tool to help point you in a direction. But, if the coils and wires are installed correctly and the plugs gapped correctly, chances are the problems is not there. But, you can never be too sure.

Grab the air intake housing from the filter to the throttle body, be sure it's seated securely. Look at all of the vacuum hoses to be sure they are secure. Vacuum leaks can also throw misfires.

 

Based on your description, it sounds like some connections are not fully seated, and have vibrated loose. Both coils are involved, since you have misfires that trace back to both coils. Also as Jamieb pointed out the gap on the plugs may be incorrect. I changed my plugs and forgot to gap one and it tossed a misfire code until I redid it. Here is a diagram of the coils and wires:

 

Thanks for the tips everyone, i'll see what i can find. It almost seems like a bare wire potentially shorting out? I noticed there was some wire showing in the group that is routed around the oil spout...
After doing some digging in the forums for similar issues, it seems like someone else had the same exact symptoms as im experiencing, and it ended up being the O2 sensors. Even though it doesn't seem likely or possible, is it worth the next try?

Today on my way into work, truck felt completely fine.. granted i didnt get above 40mph and onto a freeway, but felt perfect and no misfires/CEL.

 

are you only using premium gas? Mine won't run well at all on mid-grade or lower - Premium grade only. For most of our rovers, that sticker saying "Premium Fuel Only" is not a suggestion. They won't run on anything but Premium - starts knocking and bad things happen.

 

Yup, only premium 92-octane fuel being used.

 

Update: took it into the shop again, they figured it to be one of the fuel injectors (1) and replaced it. Drove it for 5 miles on the freeway just fine. STarted it back up about an hour later and proceeded to drive home.. 1 mile in to the drive CEL back on, continued to drive on the freeway for the remainder of the 2 miles home..

Pretty much the same codes: P1300, P0300, P0304, P0305, P0307, P0300(twice?), P0308.

Im at a complete loss at this point.

 

Hello, I recently had a misfire issue that took a while to figure out. It ended up being a short that was traced back to my ECU computer that was sending the wrong fuel trim signals to my 02 sensors.
Is your mechanic and LR guy? does he have the proper readers to check fuel trim levels etc. This might be a good place to check.

Stop throwing money at things that might work. I would not buy O2 sensors until you have this checked.

you'd have to be in a very unique situation for an O2 sensor to actually cause misfires.

If it is your ECU, you can buy a used one and have it reprogrammed for your truck. It's easy.

 

Hmm that sounds like it could be it, at least a short of some kind.