Grdenko, ever since you started to mess with the EGR, your entire vacuum system has been seriously screwed up. The same vacuum system supplies the EGR, ARV, transmission, and climate control, but not the door locks in your model. I think all these troubles explain why that line you found plugged was like that in the first place.

The vacuum problem would not explain the lights, and it may or may not explain the hissing.

In the 1987 300E that I once owned, the sight glass was behind the driver's side headlight. Just follow the A/C lines until you find the receiver/drier (the cannister looking thing). It will be there.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References