Miska, my "new" 1987 300D is giving me nagging issue and I feel fried trying to figure it out these 4 days. When I first bought it, it had power and torque. But it seems to have lost its power now. It seems to struggle getting at 70 mph and above.

I have driven it about 400 miles before this issue.
Recent work done since I've bought it:
1) BioKleen to clean the nastiness out of the system.
2) Fuel Filters were replaced after BioKleen did it's magic.
3) I rebuilt the injectors.
4) Replaced all the fuel hoses.
5) I have fixed EVERY vacuum line. I made sure everything was spot-on.
6) Disconnected the ALDA temporarily for a short test drive. Not a difference.
7) Injection Pump timing is correct.
8) Fuel strainer has been replaced.
9) No rusty leaky lines. The body has little rust, but the undercarriage is clean.

Ideas are welcome as I am baffled.

The car otherwise is a joy and starts in -fahrenheit without assistance. It has had a transmission overhaul and head refresh and re-ringed.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

Check your overboost protection circuit.

From the nipple on the manifold, the sample of manifold pressure goes through solenoid Y30 on the inner firewall next to the brake booster. Then back from Y30 to the ALDA.

The solenoid is actuated by the pressure switch on the manifold adjacent to the nipple. The power comes from the fuse, through Y30 coil, to the switch on the manifold, to ground.

There are a couple failure modes here:

- nipple, hose, solenoid valve plugged or disconnected - manifold pressure never makes it to the ALDA

- pressure switch stuck closed (i.e. overpressure) - Y30 energizes, dumps manifold pressure to atmosphere and diverts it from the ALDA so the ALDA gets no pressure signal

- Y30 stuck in energized position even though no voltage on it - same effect as above

I would start with testing the Y30 control circuit. Y30 is a 3 way valve, with 3 ports.

Port facing down is the in or common (hose from the manifold)
Port facing forward is the out when de-energized (hose to the ALDA)
Port facing up is the out when energized (has a vent cap on it )

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech