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Smart Fortwo 451 CDI Oil Change — OM660 0.8L Diesel

Easy 60-75 min $40-80Smart Fortwo 451 CDI

Service interval: Every 6,000 mi or 12 months — whichever comes first · Short-trip / city-only CDIs — shorten to 4,000-5,000 mi

Tools you'll need

  • Jack and jack stands (or solid ramps)
  • Wheel chocks
  • Drain pan (5L+)
  • Cap-style oil filter wrench for the CDI cartridge housing
  • Socket set with the right hex/Torx for the diesel drain plug
  • Torque wrench
  • New crush washer for the drain plug
  • New filter housing O-rings (in the CDI filter kit)
  • Funnel
  • Nitrile gloves and shop rags

Fluids & specs

FluidSpecCapacity
Engine oil — 451 CDI (OM660 0.8L diesel) 5W-30 low-SAPS, ACEA C3, MB 229.51 approval (MB 229.31 acceptable on older pre-DPF cars) ~3.7L with filter

What this is + why it matters

The 451 CDI is the OM660 — Mercedes' 0.8L three-cylinder turbodiesel. It's the most common 451 variant in Europe and very rare in North America. The oil change job is the same shape as any other Smart oil change (warm, drain, filter, refill, check), but the diesel has its own specs and its own ways to get this wrong. The parent oil-change page covers the 451 petrol and the 453 in the same pass — this page is the deeper take on the CDI specifically because there's one rule that gets owners into expensive trouble: low-SAPS oil only.

DPF-equipped CDIs (most of them) run a diesel particulate filter that catches soot and burns it off in regen cycles. Full-SAPS oil — which means most cheap quick-lube 5W-30 — pushes sulphated ash, phosphorus, and sulphur into the exhaust. That ash builds up in the DPF and clogs it, and a clogged DPF on a CDI is a four-figure problem. ACEA C3 with MB 229.51 approval is the spec that keeps the DPF clean. Get this right and the OM660 will run a long time. Get it wrong and you'll be replacing a DPF before the timing belt is due.

The other thing that's different from the petrol cars: timing belt history matters more than oil history on a CDI. Owners who think "I'll just stay on top of oil" are missing the bigger ticking item. See Timing Belt CDI — that's the job that decides whether your engine is still alive in 30,000 miles, and it's not optional.

What you'll need

The tools and fluid are listed above. A few diesel-specific notes:

  • The bottle has to say MB 229.51 (or 229.31 on older pre-DPF cars). If it doesn't say that approval, it doesn't matter what else is on the label. "European spec 5W-30" is not enough. "Diesel-rated 5W-30" is not enough. The exact MB approval number is what you're looking for.
  • Don't reach for the M132 petrol filter element by reflex. The CDI filter housing is different and the element inside is diesel-specific. Bring up the CDI part number, not the petrol one. Filter kits should come with new housing O-rings — use them all, every time.
  • A 5L jug is the right size. You need 3.7L; the rest is for a top-off later in the year.
  • Get a fresh crush washer. Reusing them is how you end up with a slow drip on the floor.

Step by step

  1. Warm the engine. Five minutes of idle, or a short drive. Warm diesel oil drains faster and pulls more soot out with it. Don't run it long enough that the drain plug is dangerous to touch.
  2. Lift and secure the car. Jack stands on rated lift points or solid ramps. Always chock the wheels. Never crawl under a Smart sitting on a jack alone.
  3. Find the drain plug. On the CDI the drain sits on the diesel sump, forward of where the petrol M132 plug lives — different sump, different bolt size. If you've done a 451 petrol before, don't go reaching for the same spot. Crawl under and look first.
  4. Drain the old oil. Position the pan, crack the plug loose, then back it out by hand the last few turns. Diesel oil drains slower than petrol — the soot makes it heavier and stickier. Let it drip for at least 15 minutes, ideally longer. Most of the volume comes out in the first minute, but the last bit is where the suspended soot lives, and you want it gone.
  5. Swap the cartridge filter. The CDI filter housing is the diesel-specific one — different from the M132 petrol housing. Unscrew the cap, pull the old element, replace all the O-rings that come with the new filter kit (housing, cap, drain valve if your kit has one), drop the new element in, and torque the cap to spec — confirm the value in your workshop manual; CDI cap torque is in a similar range to the petrol M132 but verify for your year.
  6. Reinstall the drain plug with a new crush washer. Snug, then torque to spec. Don't gorilla it — diesel sumps are aluminium just like the petrol cars and they will strip.
  7. Refill with the right oil. ~3.4L in first, then check the dipstick before topping the rest. The CDI takes a touch more than the petrol — ~3.7L total with filter — but as always, overfilling a small engine is worse than running it half a quart light.
  8. Run the engine and recheck. Start it, watch the oil pressure light go out within a couple of seconds, then idle for a minute or two. Diesel oil pressure light can take a beat longer than petrol because of the heavier oil and the longer galleries — if it stays lit past five seconds, shut down. Then wait five minutes for the oil to settle and check the dipstick. Top off as needed. Look under the car for any drips at the plug or filter housing.
  9. Reset the service indicator if your variant has one, through the dash menu.

Common gotchas

  • Low-SAPS is the only thing that matters on the bottle. Generic 5W-30 from a quick-lube counter will not be low-SAPS unless it explicitly says ACEA C3 + MB 229.51. The bulk barrel oil at most chain shops is full-SAPS. If the counter clerk doesn't know what those numbers mean, they don't have what you need — bring your own jug.
  • Don't grab a petrol filter element by reflex. The CDI cartridge is a different part. Petrol M132 element will not seal correctly in the diesel housing. Order against the CDI / OM660 part number, not the M132 part number.
  • Black oil at 2,000 miles is normal on a CDI. Diesels cook oil with EGR soot, especially short-trip / city-only cars. Don't panic when the dipstick comes out looking like coal — that's diesel. Panic when it comes out gritty, smells of fuel, or has emulsified water in it.
  • Short-trip cars want shorter intervals. The 6,000 mi / 12 month rule is for normal use. If your CDI does a lot of cold starts and never gets fully up to temperature, soot loading is faster and the oil tires sooner. Drop to 4,000-5,000 mi and the engine will thank you.
  • The drain plug is in a different spot than the petrol. Already said it above, said it twice. The number of owners who try to drain the petrol-position plug on a diesel sump and find nothing there is non-trivial.
  • Don't over-torque the drain plug. Aluminium sump, steel plug. A heli-coil repair starts at $200 and starts because someone leaned on a 3/8" ratchet too hard.
  • Timing belt is the bigger ticking item. You can do oil perfectly for ten years and still lose the engine to a snapped timing belt. If the belt history is unknown, get the belt done before you worry about whether you bought the perfect oil.
  • EGR codes downstream of soot. If you've been on full-SAPS oil or stretched intervals, expect to eventually see EGR or vacuum-related codes like P2359. Catch up on oil first and see if they clear.

When to skip DIY

If you don't have a way to lift the car safely — proper jack stands, a lift, or solid ramps — outsource this. Working under any car on a single jack is how people get killed. Same applies if you can't dispose of used diesel oil locally (it's regulated separately from petrol oil in some regions), or if you're not confident reading a torque wrench. A shop oil change on a CDI runs £80-150 / $100-180 in most markets — but only if the shop actually uses MB 229.51 low-SAPS oil. A lot of quick-lube places will pour bulk full-SAPS 5W-30 into a CDI without thinking, and that's the road to a clogged DPF. If you're paying a shop, ask them which oil they're putting in and ask to see the bottle. A shop that doesn't know what MB 229.51 means is not a shop you want touching your CDI.

Parts & typical prices

PartTypical priceSearch
Diesel-specific cartridge oil filter element (CDI / OM660) $10-22 Amazon · eBay · AliExpress · Google
Drain plug crush washer $1-3 Amazon · eBay · AliExpress · Google
Filter housing O-ring kit $3-8 Amazon · eBay · AliExpress · Google
5L jug of MB 229.51 5W-30 low-SAPS $40-65 Amazon · eBay · AliExpress · Google

Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread. Marketplace links are non-affiliate.

Manual references

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