I'm replacing the fuel heat exchanger, was about to remove intake manifold and after I took the plastic engine cover off this hard gunk is built up in the last injector spot, any way to remove it... i.e. brakekleen, gunk remover, gasoline? also is this an egr problem?

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Oh wow, black death on a non CDI engine.

Use the same thing people use to break down the clean death gunk on the CDI engines.

You have a leaky injector.

I think it is just blow-by that gravity takes to the #6 injector hole, use brake cleaner or bio-diesel.

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1999 Mercedes E300TD daily driver sold at 238K miles 106K miles were mine, rust worm got it :-(
2006 Mercedes CDI new daily driver! 56,000 miles May 2016 now 85,625 Apr 2018 and Apr 2019 101,000 miles Apr 2020 109,875. March 2024 135,250, Dec 2024 145,000 miles