I have an '82 w123 (240d) with a '79 617 N/A, 4 speed manual.

The engine became harder to start over time and eventually sputtered out one day on idle at a stop sign. Theres a long saga in another another thread you can find through my username if you are curious.

Since then I have sourced a good used IP and swapped it, despite relatively low and uneven compression numbers. (220 240 190 240 250). I was able to get the engine to start with the new IP after 30-40 seconds of cranking, but the car smokes badly and has a misfire, even after warming up. And it didn't want to restart even when hot.

So given those compression numbers and that once able to start it runs poorly, I have the impression the engine needs rebuilding, probably more than just a valve job. (Am I wrong and missing something here?) So I was looking at pulling the engine.

The options I am looking at are:
1. Find another used motor to drop in. What are my options? Are there better alternatives to the 617 for the w123? I would love better mpg...

2. Teardown, head rebuild, re-sleeve cylinders and machine block, new piston rings and hope that I can reuse the pistons if they spec out. What else should get replaced on a high milage engine when this happens?

Would appreciate any wisdom or input. I am on a budget but am also curious to learn. I have previously rebuilt one other engine (a vw 1.6 diesel) so I have some grasp of the concepts at hand, but its a very different engine.

Do a valve adjustment before you throw in the towel. If the valves are too tight or have zero lash, you'll lose compression. If the engine is just tired and worn out, you'd be time and money ahead to source a used and running engine and just swap it in. Rebuilding a MB diesel does not fit well with the word "budget".

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Do a valve adjustment and a leakdown test. The results of a leakdown will help tell where the compression is being lost by where the air escapes, then you will know if the rings are ok and just the valves are leaking.

If the rings leak you could try a solvent soak on them, run it hard for an extended period and re-do the test.

Diagnosing is particularly important when funds are limited, it saves doing unnecessary repairs.

Good luck!!!

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