P0305 Misfire

[98TxTJ]

Noticed that I had a little rough idle at stoplights last night, then this morning the Check Engine light came on just as I pulled into the work parking lot.

I had the OBD reader with me, so hooked it up and it showed a P0305 misfire (cylinder 5). The code wouldn't clear.

Took the afternoon off to figure it out. First thing I did was pull the number 5 plug and found this:

I've never seen a plug gap close up like that before. These went in about a year ago, they're Autolite APP-985 plugs. I'm wondering if I gapped them wrong and the gap was too tight.

I wire brushed it, filed the nipple off the ground node, re-gapped and put it back in. Then I pulled the rest and cleaned them up too. None of the others were close to this, but were starting to build a nipple on the ground. Filed it off before re-gapping.

Cleared the code and it hasn't come back, so easy fix this time.

 

[JKP]

Never seen that before.

 

[98TxTJ]

Never seen that before.

A friend says he's seen it on his turbo cars. He says it's carbon buildup from fuel impurities.

He suggested I need cooler plugs since I rip up and down the interstate twice a day. I'm just going to stick with these and see what happens now.

 

[RMETeeJay]

Noticed that I had a little rough idle at stoplights last night, then this morning the Check Engine light came on just as I pulled into the work parking lot.

I had the OBD reader with me, so hooked it up and it showed a P0305 misfire (cylinder 5). The code wouldn't clear.

Took the afternoon off to figure it out. First thing I did was pull the number 5 plug and found this:
View attachment 253989

View attachment 253990

I've never seen a plug gap close up like that before. These went in about a year ago, they're Autolite APP-985 plugs. I'm wondering if I gapped them wrong and the gap was too tight.

I wire brushed it, filed the nipple off the ground node, re-gapped and put it back in. Then I pulled the rest and cleaned them up too. None of the others were close to this, but were starting to build a nipple on the ground. Filed it off before re-gapping.

Cleared the code and it hasn't come back, so easy fix this time.

Looks like ash deposits from this chart. How many miles? Was the oil changed regularly?

ASH DEPOSITS
Recommendation: Caused by the use of leaded fuel, fuel additives, and/or oil additives. Check for worn piston rings and/or valve guides. Misfiring may occur due to the deposits on the electrodes.

 

[Rubicon88]

I have a repair...

First thing we'd do is put in ZFR5N NGK plugs...it's what the factory recommends...

Start there...end there should be good to go!

 

[98TxTJ]

Looks like ash deposits from this chart. How many miles? Was the oil changed regularly?

About a year old — 10k to 12k miles. Oil changed regularly by the shop that installed the engine in 2018. I take it there for changes since the engine is under warranty until August.

 

[JKP]

Nothing magical about ZFR5N NGKs.

 

[Rubicon88]

Nothing magical about ZFR5N NGKs.

You're right...and that's why they're perfect for these engines. They're very simple and work great with this very simple platform.

I fix this shit all day every day...
Moreover some posters here have had issues using other brands that went away switching to the OEM plug.
I've never encountered a problem with the OEM recommended ZFR5N. Not ever once!

If the OP took the Jeep to the dealer the first thing they'd do is put six fresh new ZFR5N plugs in and reevaluate.
Any time a problem arises it's good practice to start at ground zero with parts pertaining to a problem. In this case...a misfire. The ignition system is heavily involved in firing of said engine so upon learning the pluf being used isn't the OEM plug...
they'll get the Jeep to gound zero.
Ground Zero being the recommended part per the mfg.
It's also only $3.50 per plug that plays very nice with these engines.

That said the funny thing to me is thinking magic could be obtained from a spark plug on one of these engines. That's rich!

 

[98TxTJ]

I have a repair...

First thing we'd do is put in ZFR5N NGK plugs...it's what the factory recommends...

Start there...end there should be good to go!

Cleaning and gapping these seems to have worked just fine. It's running great. If I have another issue with the gap bridging, I'll change them out.

 

[Rubicon88]

Cleaning and gapping these seems to have worked just fine. It's running great. If I have another issue with the gap bridging, I'll change them out.

It's your rig...but those particular plugs you shouldn't even be gapping. They're pre gapped from the factory and honestly I can't remember the last set of plugs we've got in for a customer that had an out of spec gap for the application....no matter the plug tip/electrode.

 

[Jerry Bransford]

Nothing magical about ZFR5N NGKs.

Exactly. Despite a misleading inference, the engine was not designed around NGK spark plugs.

 

[JKP]

The actual factory recommended plugs for a '98 are... Champion RC12LYC.

 

[up on the mountain.]

Nothing wrong with the stock ngk plugs. have been using them for six years. no misfires.

 

[98TxTJ]

This wasn't supposed to become a debate on which which plug is "right" for my engine, but it is an internet automotive forum so that is obligatory, I guess.

I picked the plug based on recommendations in this thread https://wranglertjforum.com/threads/what-are-the-best-spark-plugs-for-my-jeep-wrangler-tj-4-0.4196/

If the problem were the plug brand and model, I would assume that there would be a lot of postings on misfires related to that plug since Jerry is pretty well respected here and his advice has probably been taken by many.

I, personally, have not seen a gap bridge like that and I was curious as to possible cause. I don't think it is the plug itself. So the first reason I posted was to display a potential novelty and perhaps discover an uncommon cause.

Second, part of why I posted is that in this case a P0305 misfire was actually and directly related to the plug itself and did not lead to a goose chase of testing or replacing injectors, plug wires, distributor cap and rotor, COP rails, crank position sensors, O2 sensors, and looking for vacuum leaks as everyone suggests their pet solution to the problem.

As far as gapping, I did check, and perhaps adjust, the initial gap in the plugs as there are two required gaps depending on year of TJ. Not sure how I would have been assured of a proper gap from the factory in such as case. After I discovered this problem, I didn't feel it was necessary to run out and buy a new set of plugs when I could open up the gap, file the ground electrode (not the tip!) back flat, and then re-gap to spec.

And finally, to say it one more time because the point seems to have been missed by some: It worked. No codes. Engine runs smoothly. Butt dyno says it has more power. If I get more misfires, I'll address it then.

 

[Jerry Bransford]

Nothing wrong with the stock ngk plugs. have been using them for six years. no misfires.

Absolutely, there is nothing wrong with using NGK plugs at all as they are a good quality spark plug. As there is nothing wrong with using other good quality brands of spark plugs. Some get that, some don't.