Hi guys,
1st time posting here. I am 2nd owner of a 2004 Discovery II, 150,000 miles. It ran like a dream up till about a month ago. The check engine light came on and stayed on for the first time in the couple years I've owned it. I'm in California, so I thought I should just have it checked right away before I have to worry about passing Smog in a few weeks time.

Unfortunately, I haven't been able to get rid of the problem. The 300 series of error codes have been impossible to overcome. I had a mechanic
-replace the O2 sensors front and back
-replace the sparks plugs, coil and wires.
-put in new injectors
-check the compression
-check for any leaks
Everything seems to check out fine. It does not overheat, etc. But there is a persistent rough idling and misfiring problem from cylinder 3 (codes).
Tried all the obvious repairs as mentioned above. After each one, the check engine light went away for a few seconds and then came right back. Now, it's flashing almost non-stop. I'm afraid of damaging something. Any ideas of what else I could do?
Thanks!

 

Where in CA? Was the shop a land rover shop?

 

L.A., San Fernando Valley. It's an Imports shop but not specifically Land Rover.
I forgot to mention that going downhill, the check engine light stops flashing and reverts to solid (I put it in neutral going downhill just out of curiosity, and it stopped flashing after about 10 seconds - light kept on though). Also doesn't flash if mainly idling in one place. Not sure what that means, if anything at all.

Oh! I also noticed that it's now rough when I put the gear in reverse. Could be connected to the same misfiring problem, but I'm not sure. Maybe one of you guys will read this and connect the dots!

 

 

every time the check engine light flashes, you're probably destroying your catalytic converters a little more. they really don't like misfires.
seems like the purge valve and maf weren't replaced, although they wouldn't cause a misfire on a specific cylinder.
since you can identify the problem cylinder, try changing that spark plug or the plug lead on that cylinder. also make sure that ngk plugs were installed and not bosch plugs.

 

Did you look inside the cylinders when you changed the plugs? Look at the end of the old number 3 plug? Coolant in the combustion chamber will cause this. New plugs usually cure it for a few days if this is the case. If this is the case you have bigger problems.

 

It could be a vacuum leak, could be the MAF, or possibly even O2 sensors. The engine diagnostics are pretty tolerant and will not always show the exact problem. You could also have them check fuel pressure, who knows.

You could also be getting coolant or oil into the combustion chamber from either a bad HG or from worn out valve stem seals in the heads. If it is not electrical or the O2's then something is messing with the air fuel mixture. That said, you'd want to make sure that all or your sensors are acting properly.

 

Thank you guys for your suggestions. I'll point these out to my mechanic (who is scratching his head at this point - not a LR specialist, but a good guy).
He in fact had noticed that a new spark plug had oil on the tip just after a couple days of being installed. Also, he used all Bosch parts. He is a big fan. Are Bosch parts that bad for a LR?
I would go to a LR specialist, but I'm afraid they'll want to go back to square one and charge me an arm and a leg on labor.

 

If the shop can confirm that your coils, plugs, and injectors on that cylinder are all working properly, then you have an internal issue. Burnt valve, cracked block, etc.

 

What about coils and wires? easy to swap left to right and see if it makes a difference.