I have a misfire that comes and goes. Starts as the truck is accelerating at around 2000 rpm and in 3rd gear. SES light begins to blink and the engine stumbles. SES flashing about 10 times, then remains on and throws a "P0304" code. If I back off the accelerator, smooths out. Only if I try to accelerate while in a higher gear does it stumble, SES flashes and throws another P0304. Pulled the #4 plug, all looks fine. Have done the coolant test for both pressure and exhaust gases by my Indy mechanic, all good. The odd thing is this situation goes away by itself for maybe 2 months then it came back.

I'm about 10K into a total rebuild, everything replaced except block, crank, pistons and coil packs. Same Indy shop did the work with parts I sourced from Atlantic British, Lucky 8 and Rovers North. Truck has been amazing until about 6 months ago, when this off and on miss on #4 started.

Could it be an odd or unusual electrical issue? Indy shop is a small shop that specializes in British vehicles as well as normal "boring" everyday cars as well, so they understand the oddities of LR. Planning to take it back in on Thursday, 7/27, and have another go at it.

Thoughts or directions would be most appreciated by my mechanic and me.

Thanks!!

 

Swap known good fuel injector, say from #2, with your #4. See if the miss moves to cyl #2. Free test.

Swap known good spark plug, say from #2, with your #4. See if the miss moves to cyl #2. Free test.

Swap left coil pack with right coil pack.

See if miss moves.

Put a fuel pressure gauge onto your fuel rail. See if there is a stumble above 2000 RPMs (could be your fuel pressure regulator or even your fuel pump). Free test (i.e. not buying new parts just to make an educated guess at a test).

Check your air filter and the full airway intake path (possible obstructions...or the opposite, possible air leak behind the MAF).

Information from the above should tell you if you need to clean or replace any fuel injectors, fuel pressure regulator, fuel pump, spark plugs/wires/coil packs.

If your engine temp is fine after all of the above and you still need ideas, then consider seafoaming your valves/rockers.

 

Just make it easier for you. Get a complete tune up and go from there. I dont know how many miles you have but as you have mentioned the coils has not been replaced and your are having ignition problem right now. Cheers

 

Replaced all plugs/wires when the problem arose the last time, coil packs have about 100K since the last replacement (truck has 250K, 10K after rebuild). Can coil packs work intermittently and/or work or slightly fail as in the case of this misfire? All has been great after replacement of MAF sensor on 6/1. Miss came back w/ a vengeance on 7/19. Now throws P0304, P0300, P0305. Have not cleared the codes since the beginning of this round. Using and onboard Scan Gage II. Thanks for the continuing help!!