RE 1987 300SDL with 720K. After driving at about 70 MPH for about 30 min, I exited the interstate and as I came to a stop my engine stopped. It was easy to restart but the idle speed surged from almost stopped to about 1000 RPM several times, maybe for 3 seconds. Then it smoothed out and all was well. This has happened three times over the last few weeks. Doesn't happen every time. Other than that the car runs well despite the incredible mileage. I have driven the last 500K so I know the IP has not been touched for that time. Injectors were replaced at about 300K, but seem OK now. The first 200K I don't know about. I am a little afraid that I may be walking the next time this happens. Any ideas?

Wait, 720K? Dude ...

My first motor blew a head gasket at 350K, this about a year and 10K after blowng out a radiator. I got a new engine and tranny (low miles, just over 100K from an SDL with serious body damage) put in because my existing tranny was hurting from long leaking seals. Expensive as hell to replace those.

But no, I don't have any good leads for you on that. I have a great mechanic in Redwood City (near Palo Alto), he's a 603-ophile, owns two 350 SD models (he found and put in the newish 603 in my car). I'll see him in a few days, I'll see if he has any ideas.

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1986 300SDL, 362K
1984 300D, 138K

720k??!? Awesome.

How many transmissions, head gaskets, and injection pumps? Injectors were only done once?

Does the car idle properly otherwise?

I suspect that wear and other age/use related issues have caused the electronic diesel idle control to stop working. Just this am I was doing some diagnosis on mine, and found that my base idle is too low. I pulled the connector off of the IP ELR unit, and the engine stalled. So the electronic system is working too hard.

I’d pull the ELR connector first, maybe cold at first then hot, and see what idle looks like…

See the FSM I posted here:

https://www.peachparts.com/shopforum/diesel-discussion/418139-upping-602-603-base-idle-due-bogging-down-deceleration.html

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

I will try unplugging the ELR and let you know what happens. Slightly off topic, but a little car history: I bought the car with 200K on a whim for $500 (five hundred) dollars. Obviously not cherry. I drove it home (50 miles) with very little brakes and no exhaust system. Later I found out it had a cracked head (#14). Feeling somewhat crazy and never having worked on a Mercedes before I put on a brand new head (#17) with new valves and lifters, new timing chain and tensioner, new vacuum pump, water pump, new flex discs, exhaust system, two axle shafts from a junk car, and brakes. Since then I have, as a precaution, replaced the timing chain and tensioner twice and the flex discs twice. Also two more water pumps, alternator, AC compressor, of course brakes, and some other stuff I can’t remember. New bosio nozzles were put in at about 300K. I have not touched the IP. The transmission leaks and needs to be fixed, but it pulls and shifts OK. Some rust has started. I plan to drive until I can see the road below me, then stop. Great car on the road and has a lot of soul. The bottom line is I bought a car for $500 put in $4000 in parts, a month of my time, and wound up with a car worth $2000.

So you have $4500 invested in achieving 500k miles, that puts you at just a hair under $0.01 per mile, pretty darn good in anybody's book.

Regarding your situation, it maybe worthwhile lubricating the "throttle" linkage and cable just to make sure nothing is sticking. Also check for any cracked vacuum lines to the blue flying saucer is you have one.

If I read your history right, you are still on the original trans, or at least 500k miles on one auto trans, which, is also amazing.

Good luck

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

Yeah, putting in $4500 and taking a car to 720k miles is incredible.

So tell me what happens when you pull the ELR plug. It’s on the back of the IP, two prong wired connection. Can’t miss it. Be gentle though. I’d be interested in what happens if you pull it when the car is hot idling, and then if you start the car with it off, what happens to the idle.

We also need to consider the chance of air entry, and/or a really weak lift pump that just loses it with a big change in flow when the pump and fuel get warm. Poor fuel flow can be a big issue of course.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

I’m with 87td. You got a car worth a lot more than $2000. The SDL is one hell of a car. More than once I’ve read of it referred to as the “golden age of Mercedes diesel.” That one model, warts and all.

Getting 500K out of a car that roomy and comfy, and getting 22 to 25 mpg is easily worth $10,000.

Very rough guess on the mpg - on mine that’s the range, city to highway.

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1986 300SDL, 362K
1984 300D, 138K

I have tried unplugging the ELR with the engine both cold and warmed up. In both cases the idle speed drops to about 480 RPM but the engine keeps running. Runs quite smoothly at the low RPM. It also starts right up with it unplugged. When it is plugged back in the engine revs up a little beyond the normal idle speed (by ear) then returns to about 590 as read on the tach. While in there I did notice some fuel weeping around the hose that goes from the fuel warmer thermostat to the lift pump. I will fix this, but surely that can’t be the cause? Or can it? Should I put in a new lift pump just in case?

I’d argue that your base idle is a bit low. Unplugged test idle speed shoukd be 570.

If the thermostat is doing something funky, including letting air in, then you could well have issues. Do you see bubbles in any of the clear tubing?

I’d be inclined to look into your base fuel pressure and flow before condemning the lift pump…. But I’d also be considering refreshing the seals in it regardless with those miles on it..

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Pop off the linkage to your cruse control.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Good idea. IME, it would cause a sticking high throttle/rpm, since it wouldn’t return properly or fast. But anything is possible. Always best to remove options.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Change fuel filter , check tank cap and breather on fuel tank and check tank screen or flow from tank although I expect filter change will sort it out.
High revs / extended period of time / fuel starvation of some kind

Thanks to everyone for all of this good advice. I have fixed the “weeping” suction hose and adjusted the idle, with the ERL unplugged, up to about 520 RPM. The original cruise control had been removed when I got the car so I put in an aftermarket (Rostra) one. I think it could only hold the speed of the engine up, not down. Unfortunately the clear plastic hoses have turned black so I can’t check for bubbles. The fuel cap seems OK and vents when I suck on it (through a clean hose). I have not changed the filters yet, but this will be done in a few days. Past experience has shown that a clogged filter affects the high speed operation and not the idle, but these need to be changed regardless. I just returned from Wisconsin (about 1200 miles round trip) with no difficulties. Maybe just the idle speed had drifted too low? If it happens again, I will return to this thread and beg for more help. If not, I will return on December 30th and let everyone know.

Looking forward to updates!

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

I vote Go ahead and check your compression , it would be very interesting to see were the numbers are at . If it’s got a couple of holes low it might be the cause , their will be shuttering in this case before a sudden shutdown . It’s all about fluids and filters , transmission fluid and filter maybe at 30k of usage , if One car gets the dominant use out of the fleet I’ll change this once a year . Only had 2 tranny’s fail out of 30 plus cars , one of those was a busted cooler line that let go . Proof that these tranny’s are built just as well as the 617 motor .