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Old 07-28-2013, 01:41 AM
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Location: Julian, NC
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728K-mile 1984 300SD with engine trouble - go figure

Hi all,

I'm new to this forum. Great information on here!

I probably know the answer before I even ask the question (do an engine swap), but I thought I'd ask anyway. I have a 1984 300SD with the original engine, which has never been overhauled. Highway car all its life, and still looks awesome. I had a valve job done (broken valve guide was causing all kinds of problems) at 703K miles. Ran great for about 20K, and it started acting up again. Right now, I have a really rough (and blue-smokey) idle. Once you get it running down the road, it still runs like a champ. But at idle, it's smoking, running rough, just really carrying on. The exhaust smells to me like raw fuel. I have a perfect spare engine. Today, I switched injectors. No noticeable difference (I was hoping it was just a sticky injector).

The main thing that concerned me when I changed out the injectors was that there was engine oil on the end of both the front two injectors. A little bit on the rear injector, and the other two, 3 & 4, were pretty much dry. Would that be a sign of rings letting oil past? Like I said, I'm no mechanic. Just an ignorant guy asking more knowledgeable folks!

I've been driving these old MB diesels for many years, but I'm no mechanic. I can do parts swaps, etc., not a complete idiot, but not an expert either. Just wondering if anyone has any ideas on this. I want to keep this old engine alive as long as possible. That was the goal when I bought it. Is it time to give up the ghost and accept the fact that she needs an overhaul (or engine swap in my case), or do you think there's a chance to keep her going?

Any input will be greatly appreciated. Thank you!

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Old 07-28-2013, 02:05 AM
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Welcome to the forum ...

I would start with: 1) a valve adjustment and 2) a compression test. Even with the high mileage, no sense giving up on it before getting some data. A valve adjustment can make a pretty big difference if the clearances were way off. And compression numbers will tell you a lot.
Any trouble starting? Does it burn oil?
1987 300D, arctic white/palomino--314,000 miles
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2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
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Old 07-28-2013, 02:12 AM
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RE: Welcome to the forum ...

Thanks much. Like I said, it's been about 25K miles since the valve job. The man who did that work is very respected amongst owners of these older MB cars in this area. He did a compression test then, and said that although the compression was a little low on all five cylinders, some worse than others, it wasn't as low as he'd have expected, given the mileage. He seemed to think the old engine still had a good deal of life left in it at the time. The valve adjustment (naturally) was done at the same time, but I haven't had it done since. I guess it's certainly due. I'm going to give him a call this week, and see if I can get it done. Sure can't hurt....
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Old 07-28-2013, 04:06 AM
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If you're going to see the mechanic anyway I'd ask him to do have a look at the problem.

He might agree to do a leak down test to get a better idea of where to problem lies.
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Old 07-28-2013, 05:04 AM
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That's some massively high mileage, impressive, how many miles have you put on the car? I'm no expert either, but if the valves were resealed, it could be now putting more pressure on the rings.
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Old 07-28-2013, 07:22 AM
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As you know (or maybe don't)

you are way overdue for a valve adjustment. Such adjustments are scheduled for 15k miles on your engine. I would do that first, then look for improvement.

If no improvement, a leak-down test would be in order for the reasons given above.

Please keep us up to speed. Most on this forum are interested in the OM617 aging process.
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Old 07-28-2013, 08:57 AM
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750KMILZ, you aren't by any chance the guy they did a news profile on, are you? Some guy in NC, bought an SD with 580k on it, now over 700k. Amazing to think that there may be SEVERAL of these out there!
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Old 07-31-2013, 12:11 PM
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The oil found on the injectors could be blow by, take the hose off the valve cover or remove the oil cap at idle, is there lots of pressure? If so bingo, could be rings, could be valves, could be timing but yes internal NOT injection pump issues. For example the oil cap should stay put once unthreaded, not fly off from pressure. If there is not a ton of blowby look at the injection system, maybe chain timing, start of injection timing, lack of pressure. A good pump recalibration costs money but really can refresh performance on an older engine, it was night and day on two of our cars (that had everything else checked out).
I would figure it out before swapping engines, then you know what parts are good in the future even if you decide to swap in the end. Compression does not tell much of the story, sure it can tell you when it is worn out (a drive will usually tell you that, or hard to start!) but little else. The timing and pressure of the injection pump is probably a key here.
Possibly the cam timing could be off since the head work too...thinking aloud maybe the keyway is drifting (seen it on VWs often after work). Chain could be too loose and skipped time, of course at this age and mileage it could be anything!
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Old 07-31-2013, 12:13 PM
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Realistically an engine with 728k miles on it is extremely due for a full overhaul in every way....its probably not worth putting a lot of work/$$ into that engine unless you want to fully rebuild.
-diesel is not just a fuel, its a way of life-
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Old 07-31-2013, 08:31 PM
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Originally Posted by AwfulMechanic View Post
That's some massively high mileage, impressive, how many miles have you put on the car? I'm no expert either, but if the valves were resealed, it could be now putting more pressure on the rings.
My Qs well as my shared initial suspicion surrounding the symptoms of probably cause as described.

I would want a redo on the compression check, with readings provided in writing to refer to.

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Old 07-31-2013, 10:27 PM
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Time to find another engine bro.

Put that one on the stand and tear it down and let us know what you find. Were the same valves put back in it during the guide job? I'd look for worn out rings or bad piston.

I have also heard of this happening where you go through the top end and the bottom end dies shortly thereafter.
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Old 07-31-2013, 11:37 PM
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I think I can speak with some knowledge on high mileage 617's.
it is hardly worth the effort to rebuild. Every moving part will be worn. Best replace the motor. You can pull the old motor down for fun if you wish.
What is the trans like? At that mileage it is probably beyond being rebuilt.
Diff is probably very loose.
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Old 08-01-2013, 12:35 AM
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Originally Posted by engatwork View Post
I have also heard of this happening where you go through the top end and the bottom end dies shortly thereafter.
It happened to my friends 240d, he was really upset because it developed a bad head gasket shortly after he bought it....had a local merc guy redo the head and gasket.....but the bottom end couldn't handle the increased compression so a bearing went bad...

If you have the cash buy a rebuilt engine, will promise it will last the rest of your life and its cheaper then a new car....if your on a budget find a good used lower milage one...

Raises a question, can one still buy a new engine through the dealer?
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Old 08-01-2013, 01:11 AM
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I would adjust the valves and check compression before condemning the engine. How bad is it smoking? How much oil does it use? I'm guessing there's a little life left in it, but after over 700,000 miles, I wouldn't expect much.
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Old 08-01-2013, 10:20 AM
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Raises a question, can one still buy a new engine through the dealer?[/QUOTE]

I asked my local dealer this for a bit of fun...

Australian prices, $19,500 for an engine and $7,500 for a transmission

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