Hello all, this is my first post on the fourms. I am starting this fourm as a way of documenting my journey and help keep my sanity during this process of tracking down misfire. 2 weeks ago I bought a clean 03 with 96,000 miles. Drove it 2 hours home and it purred like a kitten, next day I took a 15min drive in it and then parked it as I waited for a new front driveshaft, ultraguage, and plates. A week later the ultraguage came in, I plugged it in, and I took it for a drive, codes p0300 and p0301, came on. Now I am diagnosing the issue and will update the fourm with each step.

 

multiple misfires detected and misfire on cylinder 1 (driver side front).

cheapest option do a tuneup and check for vacuum leaks with a smoke test.

new plugs
new wires
new coil packs

coil packs are pretty unlikely to need replaced, but it's so much easier to just replace them all at once as, it's a few hours of work to remove the intake and replace them all.

wouldn't hurt to boroscope the cylinders with the plugs out to see their condition, but I'd start there with a basic tuneup.

 

Yes, I have noticed the oil is overfilled by the previous owner, this can cause misfire too, I will probably change the oil and plugs today, if that doesn't fix it. I will then do a pressure test, I don't assume a blown gasket or seals, the vehicle never overheat, and yes I know the gages are unreliable. But it drove great on the way home and on test drives ultragauge shows normal temperatures. It also revs high for 30secs on startup but I hear this is normal.

 

Most common misfire cause is a bad plug wire followed closely by a bad/failing coil.

 

Alrighty, changed the oil to a 15w40, and put in a Purolator boss filter, will have to do a longer filter upgrade later. Anyways while going to change the plugs found this...... might be the issue?

 

Probably not. Misfire on one cylinder likely not related to something that affects all of them.

 

My 2000 Disco with 186k started running a little rough last week, Code 306 misfire. The original Bosch coil went bad. Replaced it tonight. It is taking me about six hours of hard work to change those coils.

 

Welp, last night I pulled cylinder 1 plug looked black, as pictured below pulled the next plug healthy, and an different brand. Wanted to replace all the plugs last night, but SAI would have made it a hell of a job. So I just replaced one spark plug like the PO obviously did before me, and will see how long till it fouls. This will be a temporary fix since it got rid of my misfire. I guess next step is plugs and wires. Looks like newish 7mm wires are on there now, but maybe they have gone bad.

 

Might be a reason that plug continues to foul...

Take a look inside the cylinder with a cheap USB phone camera (boroscope) and look for signs of coolant.

 

i just discovered this same problem. Had an 1171 code.