Spark Plug question
 

Thanks in advance for reading this novel-sized post.. I did the search, and didn't find anything, so hopefully I didn't miss it.

I have an '08 RX that I purchased last year with 25K miles.. Currently has 35K miles. When I bought it, there were some tiny backfires upon startup with a cold motor. They'd go away as the car warmed up and leaned out. Performance and fuel consumption were normal. I pulled the ignition coils to have a look and saw small white burn spots on the back. I put the car back together in anticipation of bringing it to the dealer to have the coils checked and possibly replaced, but first I installed new NGK Laser iridium spark plugs. After I replaced the spark plugs, the symptoms went away.. Engine purred from startup to shutdown. That was last May..

Since I live here in the Northeast, I haven't driven the car much over the winter. I've put 10K miles on the car since I bought it. I started the car up last month and noticed I had the tiny (And I mean tiny.. But I'm an anal pain in the a$$, so I couldn't stand to hear it.) backfires on startup again. I pulled the spark plugs to look at them. They looked pretty good, I think the trailing plugs looked a bit carbonized. I put in a fresh set of NGK plugs and once again, engine purred like a kitten when started cool (50-ish degrees F).

I went in and asked my Mazda service advisor if he suspected the ignition coils, since the car had 35K miles and we know they can go bad. I explained that I had just replaced the plugs after only 10K miles. SA asked where I had gotten the plugs.. I told him I got them at my local Auto Zone, and he told me I should by the plugs from the dealer's parts counter, and that even though the plugs may be the "proper" plugs, the Mazda plugs may be a different heat range.. He gave me the speech about "The same brand of spark plugs may come in different heat ranges" yadda yadda.. I suggested that yes, perhaps normally, but for the rotary engine, NGK doesn't make plugs in different ranges, and I suspected them to be the same exact factory plugs. I asked if Mazda could possibly deny coverage of the extended rotor housing warranty if I used the plugs I got from AZ.. He said possibly, if they would be found contributing to failure. (Don't see how that's possible) I did NOT want to argue with the guy who could potentially be providing warranty assistance to me.. I want to keep him happy.

So, anyway, I went to the parts counter and asked the guy to give me the Mazda part numbers for the plugs, and he printed them out for me, and told me they are $167 a set. I took the numbers home and cross-checked them on google with the Mazda part #'s and found them to be the exact same numbers! I bought a set of NGK's part #'s RE9B-T and RE7C-L for about $20 each with a 30% off coupon, I was set for about $70 vs the dealer's $167. (Mazmart also sells this same set for about $75)
Service advisor says "We don't usually have a big markup on things like spark plugs" but it appears to me they're selling them for double what the rest of the world is. No big shock there, I just want to make sure I'm not missing something?
The NGK laser Irridiums I'm getting at A-Zone ARE the same, yes? I am keeping all receipts for parts I replace myself to prove maintenance should I ever need the rotor housing warranty, so I'm trying to be sure I'll be covered. (Dealer says if I brought it in for them to check out, they replace the plugs with their set for $200.)

Along the same lines, are most of you guys replacing spark plugs at those kind of intervals? (10K) That's one year of driving this car for me. The car is still a blast to drive, and I certainly don't mind spending $70 on plugs once a year to keep the car healthy.

Thanks again for reading this long post!

Yes, they are all the same. Please don't ask the service advisor for advice again, it a lesson in futility and they can do all sorts of inane stuff that can screw with warranty coverage.


If you are on the factory coils at 35k, change those too. Wires have the same advice. No point in changing the plugs if the coils aren't sending a good spark.

Weak coils will cause carbon fouling of the plugs faster.
...which will...
Carbon fouled plugs will increase the resistance of the spark, increasing the amount of heat in the coils, accelerating coil failure.
...which will...
(repeat)

Wire degredation will step in and cause the problems in both directions at an even faster rate.

It's been dyno proven that the OEM ignition at 30,000 miles is already down ~30whp over brand new ignition. Feel free to replace plugs early, but don't neglect the coils or wires when they are needed.

See the new owner's thread, ignition section, for a list of options for where to get coils from, and the advantages/disadvantages to each, along with the rough cost.

Airtraffic, what part of the Northeast? Im in PA.

Willard1,

I'm in PA as well.. Allentown/Bethlehem area.

40-45 minutes from me. If you want, you can buy your coils and wires and bring them up to Stroudsburg, I'll do the install for you, show you how easy it is. (it's unclear if you already know how easy it is)

Sunbury/Selinsgrove area here

service advisors usually don't even know where their ass is let alone that you can buy OEM parts outside their parts department. most of them have very little knowledge about cars, which is another contributor to why i left the field(previously being a mechanic should be a requirement, but obviously it isn't as most advisors could have been used car salesmen).

Thanks RIWWP! I think I can handle the coils/wires, but I won't hesitate to look you up if it turns out I need help!

Yeah.. I get scared when it turns out that I know more about my cars than a dealer's service advisor! That happens frequently.. This guy isn't too bad, and probably knows more about the -8 than most Mazda SA's, but he's definitely wrong about this topic!

Sunbury.. I think you're about 2 hours west of me?

yea roughly two hours.