You can research codes here

http://enginecodemaster.com

codes are from Rave book.

Put the Discovery in a dark garage.
Run it and look for sparks from the wires.
Don't laugh - this works.

 

You mention changing plugs and wires during head gasket job, so I'm going to stick with fuel issue for a tad longer. I'd run another tank of fuel through it, with an injector cleaner/additive and fill with quality fuel (Shell, Sunoco, etc).

Filled my 01 at Shell, 15.4 mpg (20 gal.)...last tank was from Marathon, 14.3 (20 gal.). Funny part was...I drove it easier this last tank, trying to improve on the 15.4 and this was my reward for buying cheaper fuel. So, in essences...initially saved .40 on fill up (.02 cheaper at the pump), but lost 20 some miles....which at $2.89 a gallon I lost $3.85, minus the .40 cents hypothetically savings...lol...and that equates to a $3.45 loss.

Anyhow, I'd give the fuel treatment another go and see what happens. Checking the knock sensor wire isn't a bad idea either, my 03's knock sensor wire was smashed behind the power steering bracket...by one of the six previous owners. Previous owners...they can be such a wonderful thing...lol.

If another tank of fuel does nothing, then I'd go to the cylinder that was giving you problems initially. I've seen the wires going to the injectors plug terminal so bent over, under the upper manifold, that the wire's coating was worn complete off and they were making contact with one another. I had to repair three sets of injector plug wiring on my 03 when assembling the engine. We were getting all types of misfire codes before the engine swap, now nothing...well, except a trans code that is driving me absolutely insane.

Or as previously mentioned...you may have to revisit the coil packs and verify condition.

Good luck,

Brian.

 

If you don't know, the knock sensor wire is coaxial... conductor down the center, surrounded by shield.... the signal is low voltage. If the cable is compromised in any way the shield will not be effective and the low voltage signal will be disturbed by the other electrical fields around the engine compartment.

I had one get pinched somehow and it started throwing codes. Replaced it with one from another wiring harness.

You can't just wire nut together a coaxial cable or otherwise make a field repair work with a damaged cable unless you happen to have the proper coaxial cable tools with the correct die and then the right connectors which aren't common for such small coax.

 

I replaced coil associated with cylinder 7 and that resolved the misfire. I also replaced both Knock sensors and that resolved the KS code. The previous owner had overheated the truck which cooked the knock Sensors. You could see that the plastic on the sensors had gotten hot. Now its running well except for the transfer case seal that seems to be leaking. Thanks for the help.

 

Were you experiencing more than one misfire? I've read on here previously that a failed coil would always misfire on its opposite firing cylinder. Did you ever check spark with an inline tester?

Just curious because I've been chasing a similar issue on cylinder 8.