US Market, 4.0 V8 2000 Discovery II. 15,000 miles on an engine rebuilt by the previous owner with pinned liners and 180* thermostat. Vehicle has 218,000 miles total. All scheduled maintenance up to date. It runs great at a steady 178* and produces great power and idles smooth.

Since late summer, about once a week I've been getting a SES light for a misfire on cylinder 3. Engine runs beautifully; the light is the only indication of the misfire.
I didn't see receipts for new coils during the rebuild and I have too much time on my hands so I replaced plugs (NGK), wires (Kingsborne), and coils (NGK) last week. I did an early oil change (Rotella T6 15W40) and coolant flush. Both were clean. Pulling the valve cover over Cylinder 3 showed immaculately clean rockers.

After all that, I'm still getting an occasional cylinder 3 misfire. What should I check next? Injectors? MAF? O2 Sensors?

EDIT: SES is for P0303 code, which I forgot to include in original post.

Thanks!

 

Will you be able to see if somehow that cylinder is leaking small amount of coolant and contaminating the plug?

 

I had a similar problem on my old 04. Turned out to be bad injector O rings. A small tear or leak there can cause misfires. I'm not saying this is your issue, but it's a relatively simple thing to replace the o-rings. While it's apart you could just replace the injectors as well if it hasn't been done before.

 

The old plug was clean when it was removed. Oil and coolant showed no signs of cross contamination when they were flushed this week. And, with the liners pinned I would be shocked if there's a bad head gasket, but anything is possible. Picture of the old plug included for reference.

You know I hadn't thought of the o-rings themselves. Looking back through the build receipts that came with the truck I can't find anything about new injectors or o-rings. The rings themselves are cheap and easy, I'll start with that next. Even if that's not the problem it's a good preventative measure on a 20 year old vehicle.




Spark plug from cylinder 3 that was replaced.

 

What brand of diagnostic tester did you check the fault with?
what was the actual fault code number?

 

Oops, Can't believe I didn't include that in the original post, my bad. It's a P0303 checked via Ultragauge.