Rich fuel mixture after spark plug change

Question

I recently changed spark plugs on a 2010 Mazda 3 2L NA engine. I went to a major automotive parts website and ordered the relevant plugs that "fit" this model and year - I chose NGK Iridium IX plugs because I have heard good things about their product. The first plug I installed, I measured the gap against a gap gauge and found it was too tight (ca. 0.04"), so I opened it 0.01". Then I noticed the box says don't bend the plugs.

I figured I'd start the car with other 3 original plugs in place to see if I got a misfire warning, but the car started - albeit slightly rough - and no warning showed up. I went ahead and replaced the other 3 plugs without bending them. The car turns over beautifully now, except the exhaust smells extremely rich. Still no misfire warnings. But my eyes water when I walk around the car and I catch a whiff of the exhaust.

Otherwise, I used a dab of dialectric grease along the ceramic portion of the plug, and I used no antisieze on the threads. I am a complete beginner to automotive repair. I'm having trouble deciding if I did anything wrong and if anything needs to be amended.

Which scenario is possibly the case?

• The old plugs were at the limit of spec and running crazy lean, I am overthinking my reaction to the new plugs. They run slightly rich because the default gap is intentionally at the smallest spec to extend plug life. If anything, the plug I bent should be rebent or just replaced for even wear.
• The website fitment tool is not reliable, and aftermarket plugs are never a real "fit", so and I should have paid triple for the OEM plugs.
• The "no bend" warning on the box is a silly "CYA" from NGK and they should all be bent to the target 0.05" gap for best engine performance/fuel efficiency.
• Something else was wrong with the install.

Answer 1

I'm going to put my money on "something else" went wrong, or was already wrong and you may have just noticed it. However:

FIRST THING to check: did you securely install the electrical connectors for the coil overs? and - did you firmly press the coil over down onto the top of the spark plug?

NEXT: Did you securely tighten the spark plugs? Don't go too tight! But a loose plug will cause a vacuum leak and indeed can disrupt fuel-air mixtures.

Also: It wouldn't hurt to replace that one plug which you altered to match the other three as an initial test. Generally, I would go with the instructions provided by NGK. They supply many OEMs already and are quality plugs.

In addition to Carguy's advice - I'd like to add that leanness / richness is maintained by your cars ECU (Engine Control Unit). The mixture is monitored AFTER combustion by an oxygen sensor in your exhaust. It is unlikely that a change in spark plug "quality" will have any effect - unless, as you suspected, you generate a misfire due to no spark or just have terribly terribly worn plugs which are beyond the limits of your engines fuel mapping abilities.

That brings up another point - you may have a bad O2 sensor! This would obviously cause sub-optimal air-fuel mixtures since the data controlling the mixture would be erroneous (or non-exist resulting in the car to go to a default A/F setting). Like MTA said, buy a cheap scantool and check for codes. You can often see the codes before they trigger a check engine light this way. If you are not seeing any codes then a shop can hook up their fancier ($$$) scantool and monitor your mixture settings and O2 sensor data in real-time. But mess with the plugs a little first :)

Answer 2

Never bend the gap on those plugs. They are engineered precisely to your engine. The gap is generally smaller because the spark is brighter than the OEM plugs.

I'd upgrade to NGK Ruthenium HX, part number LTR5AHX (see here). They are worth every penny. Do not adjust the gap. Make sure the threads are clear by soaking them in WD-40 for a few hours and reaming the threads a couple of times with an old spark plug. Use a very small ring of dielectric grease around the insulator. Use a torque wrench when tightening because these plugs are very sensitive. Change the spark plug wires as well.

Change the oil and add Engine Restore to the crankcase and BG 44k to the gas tank. Drive the engine hard for a couple hundred miles. Follow-up with short, one-second blasts of BG Throttle Body & Intake Cleaner Spray through the throttle body, at high idle, for five minutes. Problems solved for good!

Answer 3

In the link below, the manufacturer goes over directions on how to set plug gap. The "no bend" depiction on the box is clarified in their instructions:

TLDW - Traditional "coin" feeler gauges widen gaps with a round opening. This isn't advised for platinum/iridium tips because the tip is likely to hit the tool, which compromises the plug. The new wire gauge tools have a hook style gapper that braces against the threads.

https://ngksparkplugs.com/en/resources/set-gap-when-installing-new-plugs#:~:text=If%20adjusting%20the%20gap%20on%20fine%20wire%20or,be%20adjusted%20by%20only%20moving%20the%20ground%20electrode.