Changed spark plugs, now misfiring when warmed up

N

noxor

New member
99 3.4L 4x4 automatic here. 145k miles.

I changed my spark plugs from the single ground to the double ground NGKs.

Next day I drove an hour and it was fine. I started it up to go home and it was idling rough. When I got back home I had error codes for cylinders 3 and 6.

I have replaced the spark plug wiring harness and that seemed to work until I was an hour down the highway and it came back (it was bad enough to cause check engine light to flash and I have AAA so i got towed home).

What should be my next steps?
 
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Since it was running OK for an hour I would double check and make sure something didn't shake loose starting with cylinders 3 and 6.
 
Did you buy the plugs from a reputable seller? I recently ran into a similar issue where I bought plugs (denso) off ebay for my sequoia and they were counterfeit. Immediately #8 started misfiring and I replaced just that plug. All was well for about 15k until I got multiple misfire code and did more researching and discovered mine were counterfeit. If your vehicle was running fine before you put the new plugs in and the wiring is routed correctly, I would look at the plugs being defective. Good luck Mike M
 
Tucson Mike said:
Did you buy the plugs from a reputable seller? I recently ran into a similar issue where I bought plugs (denso) off ebay for my sequoia and they were counterfeit. Immediately #8 started misfiring and I replaced just that plug. All was well for about 15k until I got multiple misfire code and did more researching and discovered mine were counterfeit. If your vehicle was running fine before you put the new plugs in and the wiring is routed correctly, I would look at the plugs being defective. Good luck Mike M
Click to expand...

Yep, my money is on the counterfeit plugs. Take a picture of them for some lol
 
I would put two old plugs into Ctls 3 and 6. Note those two are on the same coil, so one bum plug can cause both to misfire.
 
19963.4lsr5 said:
I guess the real question is where did he buy them from.


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Click to expand...
Yes. I am suspecting bad parts as well. I added a youtube video on NGK plugs. Everyone should watch it as the counterfeit plugs are extremely close to the real plugs.

They were even bought on Amazon so not from a clean source. Its amazing to the extent china goes to. No reason for counterfeit parts. :(

Sent from my SM-J337V using Tapatalk
 
Tucson Mike said:
Did you buy the plugs from a reputable seller? I recently ran into a similar issue where I bought plugs (denso) off ebay for my sequoia and they were counterfeit. Immediately #8 started misfiring and I replaced just that plug. All was well for about 15k until I got multiple misfire code and did more researching and discovered mine were counterfeit. If your vehicle was running fine before you put the new plugs in and the wiring is routed correctly, I would look at the plugs being defective. Good luck Mike M
Click to expand...


Thanks Mike and everyone else! I bought the plugs from rock auto and they match the pics online (made in japan etc).

I pulled them all, gapped them to .044 (they were .040), and installed to 13 ft/lb per spec. I had originally hand tightened them and then added some with a wrench before but after using a torque wrench I definitely didnt apply as much pressure earlier. I am thinking it was simply loose plugs.

I ran it for a while tonight, and will take it to work this week, but so far so good now!
 
Well I spoke too soon. An hour into a drive the problem came back.

Put in new spark plugs I got from autozone and the problem persists.

Am checking coil packs out and then will move onto distributors etc.
 
Sounds like a bad coil to me. I would swap the coil pack from one cylinder to another and see if the misfire moves around. Pretty quick swap. If it's stays on the same 2 cylinders, then you may have an issue with the wire to the coil pack.

FYI - there is no Distributor on these engines.
 
Once upon a time I changed the spark plugs in my 1999 Mercury Cougar. Ran pretty all right except it would hesitate under significant acceleration. Finally, after a buttload of troubleshooting, I traced it down to a cracked spark plug...... could be your issue...
 
Do you have the ability to test the plug wires and coils with a meter as I commented earlier on this thread? That may help in narrowing the issues down quickly. :-)
 
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