Modern auto spark plug installation thoughts...NGC

[brider]

On a modern car with a 100k spark-plug-change interval (platinum plugs, .044 gap spec), have any of you just installed them without checking gaps, or do you still go old-school and set each plug's gap?

Been having a mis-firing problem on my wife's old ('04) minivan for over 30k miles now, but now that my kid is almost it's sole driver, I'm hearing it from him every day, and I can't ignore it for another 30k.

I was waaaaay over the 100k spark-plug-change interval when it started to mis-fire, and I assumed it was because of the old (140k) plugs. It was random, disappeared at higher engine revs, etc. Check of the codes said "random misfire" and that was it. I let it go so long because of the 6+ hr labor quotes from the shop to get at the rear bank of the 3.5 V6 transverse engine. So one afternoon I did it myself.

I went back in with OEM Platinum plugs, and made an attempt on each one to gap them to the .044 spec. I was kind of anal, and probably worked a few of the electrodes more than I should've to get then to the gnat's _ss, but nothing broke or cracked, and I got them back in OK.

It began to misfire again almost immediately, and after some internet searching, I found consensus that most people are installing these modern plugs right out of the box with NO gapping check. Goes against my intuition, but wondering if maybe that caused my problem to NOT be cured, and helped it get so much worse now?

Mis-fire never went away, but changed characteristics slightly, and now 10k miles after that plug change it purrs like a kitten on initial startup and usually until you stop somewhere, but then on warm start-up, it will misfire continuously.

I am tempted now to go back in with new plugs and NOT gap them, and replace the coil-on-plug (COP) units while I'm at it, which I DIDN'T do the first time.

Thoughts? Could a decent shop diagnose something above and beyond a "random misfire" code? I'm afraid I'll pay an hour's labor just to have them plug an OBD reader in and give me the same, sad diagnosis.

[bigbikerrick]

It sounds to me like one of your coils may be bad, and after it heats up, causes the misfire. Does the trouble code isolate it to a specific cylinder, ?
Rick.

[RinkRat II]

 This^^^^.  It should have thrown a code for misfire and needs to be checked through your OBD port. Repeated misfire can ruin a cat converter and expenses mount up real quick. Most auto parts stores can check the code for you no charge. You can also check the coils with an ohmmeter. 
    Always gap your plugs, who knows who dropped that box or not.
      Paul B

[brider]

It sounds to me like one of your coils may be bad, and after it heats up, causes the misfire. Does the trouble code isolate it to a specific cylinder, ?
Rick.

I checked the OBD before swapping out the plugs and the code (can't remember the #) was "random misfire", no cyl # identified.

[LowRyter]

you might change the plug wires.

I am not sure if the OBD II scan will pinpoint the misfiring cylinder but it might be worth a check.

[fotoguzzi]

I needed some new coils on my Mazda tribute.. each one was like $78 at NAPA.. I went to ebay or Amazon and got a set of 6 for just over $50 IIRC.
they worked fine but might have been china knock offs. just saying if you shop around you can find better prices.

how old are the plug wires?

[Chuck in Indiana]

The two questions about plug wires above are leading you in the right direction, I think. At that mileage, they are past due.

[Bisbonian]

You said it was Coil on Plug, correct? So no plug wires on the car?
You probably got the P0300 codes which is normally for the random misfire. This can be hard to diagnose on your own.

When you pulled the plugs, was there a good amount of oil on the tips of the coil boots? If so then it is possible that you have a leaking valve cover gasket which is shorting out the coils in the wells.

Early Miatas were good for this as each coil ran 2 plugs. If someone washed their engine and got water down the spark plug wells the wires would short and give the P0300 code. Fix was a new coil (and sometimes a new Converter).

[jbell]

My car was suffering similar symptoms to yours after about 125K.  Changed plugs and coils.  I seem to remember advice from somewhere against gapping the plugs due to the fine nature of the center electrode.  Since the back three cylinders require a good amount of work to get to, I bought name brand coils through ebay.  Read several reports of people having to go back and re-do the cheap coils after about 10K miles.  Hope yours works out.

[Mike Tashjian]

Every high mileage plug I have bought is only adjustable by the ground electrode.  Do not mess with the center electrode.  Most new plugs are pre gapped and should be gently checked if you think they got bumped in shipping.  Dielectric grease is your friend when installing coil over plug boots.  Mike

[pat80flh]

A decent scan tool, (not just a code reader), should be able to show the misfiring cylinder in the data logs.  There are also universal(obd} codes and manufacturer specific codes,  a different scan tool may give you something beyond a "random misfire". 

   It may be worth your time to bring it to the dealer, tell them exactly what you want, pay the hour to have it diagnosed.

   Or you could just replace the 3 COP units in the back, the easy to get to ones never fail. I don't generally check gap on plugs with anything but my eyes.  But if you were gentle gapping them they should be ok. 

[brider]

A decent scan tool, (not just a code reader), should be able to show the misfiring cylinder in the data logs.  There are also universal(obd} codes and manufacturer specific codes,  a different scan tool may give you something beyond a "random misfire". 

This intrigues me. What type of scan tool might be able to do this? Now that I have 3 modern cars, I might just buy the tool and be done with it. Any suggestions on a tool brand/model? I assume they can link up with a laptop to download data?

Regarding a leaking valve cover: This engine was/is as clean as a whistle at 170k miles when I changed the plugs, and the coil rubbers were clean and dry. While gapping the plugs I was careful not to touch the center wire except with the gap measuring tool, but I was nervous about too much fiddling with the ground electrode.

I fear replacing the plugs with new ones, un-gapped, straight out of the box, may throw another unknown into the equation, but I recall the plugs I pulled out had larger than .044 gaps, and it ran better then than it does now.

Based on this discussion, I'm going to look into a scan tool, but may go ahead and replace the plugs again with new coils this time.

Thanks for the input, guys, it's good to have a group willing to offer advice so quickly.

[mechanicsavant]

I'm wondering about a week ground connection . It's random,it's not cyl. specific, it's hi milage it goes away @ higher speed (coil  loads are high @ low rpm  acceleration) . A possible cold solder joint in harness? Corroded ngine to chassis gnd. Poor body repair. The possibilitys are "almost" endless .good hunting!

[Aaron D.]

A bad injector on my car acted a lot like an old, bad spark plug.

[Toecutter]

Not gonna lie... I set all my plugs to the gap suggested by the plug manufacturer, not the bike manufacturer. I check the specs on the plug, and double check the gap before install... but always use NGK's specs.