603.97 Tune Up?

[JHZR2]

Hello,

Happened upon a very nice, very low mileage (just over 110k, documented)1991 350SD at a very attractive price. Like decent enough that I’d price in the replacement of a 3.0L with the 3.5 head, and still be at a fair value. All the things that would be major issues - cracked dash, r-12 ac, etc. work and are perfect. No rust, never used jack points, records.

I’m sure there will be some things needed. Standard steering play Im used to with w123 cars, windshield seals as a matter of standard process given that they’re a known issue in the w126. Id do those things regardless.

But I’m a 616/617 guy. Yes, I helped with a 96 E300d that went 250k, but that was mainly shop repaired.

So if I want to get the 603 to a good starting point, what would be necessary? What would constitute a tune up of a 603?

The reason why I ask is that there’s one belt, no valve adjustment, so...???

Other reason why I ask is because it starts beautifully, idles great, drives smooth and strong. Can’t tell much about smoke/smell, but that’s probably a good thing. Engine is clean and seems leak free.

But driving slow, around town on a test drive, when slowing rapidly to a stop, the idle bogs. Kind of like on a gas car with a slow idle air control valve. Im not familiar with the idle control on a 603, though I’d guess it’s semi-electronic.

Is idle behavior going to be improved/baselines by fuel system updates (hoses, filters, etc? Or is this more electronic?

I don’t think this car has any rod bending related attributes yet. I listened to the idle on YouTube of a bad one. Again, price is worthy of the chance for a car this young in mileage. But if the idle behavior is somehow linked, I’d want to be aware now.