Spark plug Bosch or AC DELCO

the spark plug is less than 10k km old (bosch) so I initially did not change it. Decided to change it anyway to original mercedes (turns out made my NGK) bam emission passed, LTFT now hovers at 5-8%.

I've changed my spark plugs three times, always with the Bosch Iridium, never had any problems. Pre-gapped, takes me about 30 minutes from start to finish. Lasts me about 90,000 miles.

The new ignition switch is in. The spark plug wires are Bosch copper and now have the Bosch super 3 ground electrode plugs installed. So with a fresh ignition switch that eliminated the odd electrical changes we were seeing in the dash lights, now stronger cranking and it does run better.

Over the years I’ve found that the factory recommended spark plugs typically work best. There are exceptions, but my ‘95 5.7 recently got some plain Jane ACDelco plugs and it is running great.

Bosch W6DPO. They’ve always been expensive, but now stupidly so. I’m not paying $250 for plugs, either. The DENSO IW20TT you referenced is a perfect replacement because it’s the exact same heat range as the W6DPO.

The new plug is AC Delco 41-156. Cannot tell any difference in my Gen 2 after replacing plugs. It ran fine before and did afterwards as well.

I'm still not entirely convinced that it is weak or under ignition versus over fueling. Either way it just seems like it's having a hard time burning the fuel that it is getting. I still can't help but wonder about the Bosch Platinum 2s. I have had good luck with them before my Mark 2 GTI and the car did run well with them for a good 10 days before all of this started.

I personally refuse to put anything Bosch made in my cars. All were genuine Bosch parts from my local parts place that has never done me wrong.

First it was their wipers, paid good money for them, squeeled worse than the worn old ones. Figured it's a fluke.

Then I had a set of spark plugs, they were gapped correctly, but the car ran like absolute garbage and I couldn't figure it out until I put the probably 20 year old Valeos back in and it ran fine again.

Then I had a crankshaft sensor that was bad out of the box, and that was the last straw for me. I don't care if it's coincidence or a bad batch, or if they've improved it since, I'm done with any Bosch parts unless it's NOS from the early 2000s or earlier.

My 2000 with California emissions (regular cat and then two pre-Cats at the exhaust headers) would only run properly with the oem spec NGKs. Threw Bosch in their once and it ran terribly.

Ever since Bosch moved spark plug production to Russia, the quality has been sketchy.