Where do I start to troubleshoot this? It's my son's car and I think he disabled the EGR somehow (resistor and diode I think he said). It's ran a lot of miles since then without problems. Other things, there's a high pitched squeal if I actuate the waste gate with a vacuum tester. It's not actuating when reving and I don't think when driving either. Pretty sure it's in limp mode.

https://youtube.com/shorts/zsTqgJ4zTmU?si=nPZt_vsbts521n5v

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-E300d '99 350k
-Suburban '93 220k
-TDI Jetta '03 350k
Sold
-F250 '96 7.3
-Dodge Ram 12V
-E320 '95 200k
-E320 Wagon 1994 155k
-300d Turbo '87 187k miles
-E320 1994 200k
-300d Turbo '84 245k (sold to Dan62)
-300d Turbo '84 180k
-300sd '80 300k
-7.3 Powerstroke Diesel 15P Van 500k+ miles
-190d '89 Non Turbo 2.5 5cyl 240k (my first MB)
Tom's Imports of Columbia MO Ruined the IP in changing leaky delivery valve O-Rings - Refused to stand behind his work. Mid-MO MB drivers-AVOID Tom's.