plug/coil recs for 4.4

[Tugela]

2008 LR3 4.4 is stumbling intermittently at idle, GAP tool reads a code for cylinder 5 misfire. Symptoms are similar to the misfire I had last year on cylinder 3, which a new ignition coil fixed. Coils and plugs were all replaced by previous owner about 20,000 miles ago. With that context...

• What's a reasonable lifespan for spark plugs on this engine? I haven't inspected them yet, but plan to. 20K seems low, but maybe within the range of what to expect. EDIT: AB service interval for plugs on this engine is 90K miles.
• Is there a consensus on best replacement plugs? Atlantic British offers this set by Champion which I'll order barring any better suggestions.
• Replacement coils have a wide range of prices, from about $10 each from vendors on Amazon to $70 each from AB, with a few options in between. I've had good luck with NGK/NTK oxygen sensors on my D1 so that's a known brand, theirs are about $42 each. Any experience here with aftermarket options (good or bad) or should I suck it up and order OE? I have no problem spending more if it makes a difference, but I'm equally happy spending less if quality is comparable.

I'm leaning towards replacing the whole set. Having two failures within a year suggests that more may be on the way.

 

[discostew]

It’s possible that you have an ign misfire, but not likely. A spark plug will misfire at idle and light load conditions. Ign coil and any other secondary ign problem would probably present under load.
You’re probably looking at restricted injectors. The easy fix is to run a couple cans of 44K thru it. Not both cans in the same tank but maybe one can with 3/4 tank of fuel.
Usually I flush the injectors with another BG product. But that’s just because I need immediate results to pass go and get my money. If it’s my personal car I use the 44K like I described.

 

[Tugela]

@discostew Interesting, but not surprising. My trusted local indie shop suggested the issue was injectors. They road tested it with a can of Sea Foam in the tank and said it improved, but the issue hasn't really gone away. I usually add a bottle of BG44k to the tank for 100 miles of driving before I change the oil (5,000 mile intervals). I was hoping it would be an easy fix like plugs or coils, but given their age I am (reluctantly) inclined to agree with your assessment of the injectors. Is there a way to test for that definitively so I'm not throwing parts at this? I'm not seeing anything obvious on the GAP.

 

[discostew]

You could smoke it for a vacuum leak near that injector. Each cylinder has its own gasket so it could be one of those.
Fuel trim would usually be lean if the injectors are restricted.

 

[Tugela]

Resolution: it was the ignition coil. Replaced and once again the engine is running so smoothly I have to check the key position to know if it's on.