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.

[Diseasel300]

The same fuel system tuneup policies you're used to on the 61x apply to the 60x engines. The valve adjustment is hydraulic, so no adjustment required. Idle is electronically controlled by the EDS computer. There's a solenoid on the back of the IP that controls the idle performance. The idle speed should be around 650RPM or so, if it's lower than that, the EDS probably is not functioning properly and you're running on the mechanical idle stop around 500-550RPM.

[JHZR2]

The car did idle around 500 it seems. I assume there’s info on here to diagnose and it’s not big $$ to factor into an offer?

[Diseasel300]

If the OVP relay is bad it'll cause a failure of the EDS system. Look for an Antilock idiot light and a dead tach, they're your giveaway that the OVP is toast.

[JHZR2]

Originally Posted by Diseasel300

If the OVP relay is bad it'll cause a failure of the EDS system. Look for an Antilock idiot light and a dead tach, they're your giveaway that the OVP is toast.

Nope, watched the dummy lights and the tach especially close. 603 tach I assume is different than the pre-85 617 tach that Im used to... Surely more complex. So I was worried it would not be functional. I was also worried that ABS may be disabled. The light comes up when glowing, and shuts off appropriately. Brakes worked splendidly.

So Im thinking both are in a good place and operational. Is there a standard way to test the OVP relay and the EDS system? Is the 603/350 engine complex enough to offer check engine lights or similar diagnostics?

[Diseasel300]

The EDS system does put out blink codes from the plug by the battery. The test system is similar to the gas cars of the era and can use the same blink code reader. The later 603's system is above my pay grade for troubleshooting. The earlier cars had a trim wheel by the brake booster to set the idle speed. If the 350 has one, try setting it to a higher number like 5 or 6 and it should boost the idle speed. It's always possible that the solenoid or wiring is bad too. If all else fails, you can set the idle speed manually with the adjustment bolt on the back of the IP.

The EDS system controls mainly EGR functions and tach signal to the devices that use it. Later cars used the EDS system to control boost via a vacuum operated wastegate and the ARV on the turbo if present.