On paper they look like twins. Both are 3.2-litre inline-six common-rail diesels. Both came out of Stuttgart within three years of each other. Both made roughly 200 horsepower in big, comfortable Mercedes saloons. So why does one score 68/100 and the other 90/100?
This is a case worth opening, because the answer explains something larger about buying older diesels: seniority within a family matters more than the family name.
Quick answer: The OM648 (90/100, BEST) is one of the most durable diesel sixes Mercedes ever made. The OM613 (68/100) is its rougher predecessor, held back by a single but costly weakness — injectors. If you find a well-kept OM648, buy it; treat the OM613 as a project that needs an injector budget.
The two suspects
OM613 — 68/100 (ACCEPTABLE)
The OM613 was the first-generation 3.2 CDI six (1999–2005, ~197 hp). Mechanically it's a tough, old-school Mercedes lump — but it carries one expensive habit that drags its score down:
| Problem | Severity | Repair cost |
|---|---|---|
| Injector failure (expensive on an I6) | High | €800–2,500 |
On a six-cylinder, injectors are never a cheap fix — there are more of them, and access is tighter. That single line item is the difference between "rugged classic" and "money pit," and it's why the OM613 lands in merely acceptable territory.
OM648 — 90/100 (BEST)
The OM648 (2002–2006, ~204 hp) is the same basic recipe, refined. The injector troubles that defined the OM613 are gone; what remains are the ordinary, predictable maintenance items of any older diesel:
| Problem | Severity | Repair cost |
|---|---|---|
| EGR valve fouling (city use) | Moderate | €300–700 |
| Glow plugs + injector seals | Minor | €150–400 |
No high-severity faults at all. This is what a 90/100 looks like — an engine whose worst day costs you a few hundred euros, not a few thousand.
Found in: Mercedes C-Class, E-Class (E 320 CDI), S-Class.
The evidence, side by side
| OM613 | OM648 | |
|---|---|---|
| Displacement | 3.2L I6 | 3.2L I6 |
| Years | 1999–2005 | 2002–2006 |
| Score | 68/100 | 90/100 |
| Verdict | ACCEPTABLE | BEST |
| Worst fault | Injectors (€800–2,500) | EGR (€300–700) |
The whole 22-point gap comes down to one component class. Eliminate the injector failures and the OM613 would be a very different engine — which is precisely what the OM648 is.
The verdict
The OM648 wins decisively. It's the better-sorted version of the same idea: a big, unstressed, mechanically honest Mercedes diesel six that, looked after, will outlast the car around it.
The OM613 isn't a bad engine — it's a good engine with one known, expensive weakness. If you buy one, buy it cheap and budget for injectors up front. If you can find the OM648 instead, pay the small premium and skip the gamble.
For the modern equivalent of this "which generation?" question, see how the four-cylinder diesels evolved in our OM651 vs OM654 comparison.
Scores are calculated from documented issues, repair costs, and failure patterns. Reviewed by the EngineScope editorial team. Methodology →