Have you tested the back pressure at the upstream o2 sensor port?
What about the wires and ign coils?
I haven't tested the pressure. Is that something Advamce or AutoZone etc have for rent?
The wires all ohmed ok when I replaced the plugs. Not sure how to check the coils.
Could so many be faulty at the same time though?
Egr was also replaced with new tube.
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Here in small town East Texas I couldn't find backpressure gauge for rent. Reviews for cheaper DIY BP gauges were mixed & didnt feel like throwing $100+ on Pro Model.
Which is why the Harbor Freight Basic $25 IR Thermometer was chosen, and it showed my drivers side rear cat as very plugged & pass side as partly plugged.
The huge clue in your described condition is all of that happening at once.
Really could only be the cats.
You need to keep the front cats for engine management / smog reduction but the rears are optional.
As our MBs age and conditions deteriorate the rears gradually plug up but so slowly we hardly notice.
Your hammering it 80mph on the grade triggered that, is my guess.
There are a ton of YouTubes showing how to test Cats using IR Thermometer.
On my prior extensive posts I cover exactly how I used this test method & removed rear cats & benefits.
See here: https://www.benzworld.org/forums/w16...muffler-3.html
YouTube is linked in post # 6
This happens to the rears as they have no o2 sensors & thus no way to alert us as to their clogging up.
Check your state, (New Hampshire ?) to see if your state inspectors care if rear cats are gone, so long as Front Cats remain & are working properly.
IF your state requires rears to be in place, then replacing them with decent universal cats should clear up your issues.
My anecdotal research shows many states don't care if rear cats are gone.
Texas is that way.
California, not so much !
Cheers - David in East Texas