on my SD and I believe your car too the bolt is is threaded into the firewall side of the intake manifold. If you look at the back (firewall) of the intake manifold you will see a plastic tube running around to a bolt head there. That is the banjo fitting. The bolt is quite long and is I believe 12mm. You unscrew this bolt and then remove it saving the washers that are on either side of the fitting that the plastic tube connects to.
This bolt is hollow and allows for the injection pump to sense boost from the turbo and give the engine more fuel top burn.

Clean the bolt out and I usually take a piece of wire and clean the hole to eliminate any blockage that may be building up inside the manifold. Screw the bolt back in and do NOT overtighten it as with a hollow bolt it can snap off.

Make sure that the plastic line coming off the fitting is not blocked with crud either.

if this does not change things then it might be an alda (altitude compensator on the injection pump) related problem. These are delicate instruments though and can be easily broken so you might want to do a search on this forum and read up on it before attemtping anything there.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique