Timing Belt Replacement on Smart Fortwo 451 CDI (Diesel)
Service interval: Per workshop manual — confirm by VIN. Mercedes spec is typically around 90,000 mi or 6 years, but this varies by year and market — verify before you plan the job.
Tools you'll need
- Mercedes timing pin / locking tool set for the OM660 / cdi engine
- Long torque wrench (multiple ranges)
- Engine support bar (engine drops slightly during the job)
- Coolant drain pan and refill (water pump comes out)
- Workshop manual with the exact CDI procedure
Fluids & specs
| Fluid | Spec | Capacity |
|---|---|---|
| Coolant | Mercedes-spec G05 / G48 (check the workshop manual for your VIN year) | ~4-5 L total system; less if you're only refilling what spilled |
What this is and why it matters
The Smart Fortwo 451 CDI — the diesel — uses a rubber timing belt, not a chain. Belts have a service life. Chains don't, in any practical sense. So if you own a CDI 451, this is the one engine job you absolutely cannot ignore based on age and mileage alone.
The CDI is an interference engine. If the belt snaps while the engine is running, the pistons hit the open valves. That bends the valves, sometimes damages the head, and turns a $1,000 belt service into a $3,000+ head repair or a salvage decision. The belt itself almost never warns you before it goes — it just lets go.
When the belt comes off the engine for service, the tensioner, idler pulleys, and water pump all become accessible. All four wear at roughly the same rate, all four cost more in labor than parts, and a failed water pump or seized idler will eat the new belt. So the rule on a CDI timing belt is: do everything together. Belt only is false economy.
The interval Mercedes specifies is typically around 90,000 miles or 6 years, but this varies by VIN year, market, and which service bulletin applies to your car. Do not take that number as gospel — pull the actual workshop manual for your VIN before you decide the job is "early." Some EU-market 451 CDIs have a tighter interval. If the previous owner doesn't have a stamped service record showing the belt was done, assume it wasn't.
What you'll need
The CDI timing belt is not a casual DIY job. You need Mercedes-specific timing pins and crank locking tools to keep the engine timed correctly while the belt is off. Without them you will lose timing, and on an interference engine that means bent valves the moment you turn the key. A long torque wrench (the bolts have specific specs across multiple ranges), an engine support bar (the mount comes off), and a coolant drain setup are all required.
You also need the workshop manual procedure for the CDI specifically. The 451 petrol procedures don't apply — different engine, different layout, different spec. Manualslib has the workshop manual archive; pull the diesel CDI section.
Parts: a timing belt kit (belt, tensioner, idlers) runs $150-300 from a reputable supplier. Water pump is $80-200. Coolant for the refill is $25-50. So $300-600 in parts is a fair budget, with some range for OEM vs aftermarket.
Labor at an independent Mercedes / Smart specialist runs $400-800 depending on region. Dealer labor is higher. So $700-1,400 all-in is the realistic full range at a shop.
Step by step
This is a shop job for the vast majority of owners, so the high-level outline is what matters — not a step-by-step procedure that would substitute for the workshop manual.
- Drain coolant.
- Remove the engine cover, accessory belts, and any underbody panels needed for access.
- Support the engine and remove the right-side engine mount and bracket as required.
- Remove the timing belt cover.
- Rotate the engine to TDC on cylinder 1 and lock it with the Mercedes timing pins.
- Release the tensioner and remove the old belt.
- Replace tensioner, idler pulleys, water pump, and the belt as a complete set.
- Tension the new belt to spec, double-check timing marks, rotate the engine two full turns by hand, and re-verify timing alignment.
- Reassemble in reverse, refill coolant, and bleed the cooling system.
- Run the engine, confirm no leaks, no abnormal noise, and no fault codes.
The detail at every one of those steps is in the workshop manual. Don't improvise.
Common gotchas
Skipping the water pump. It's right there. Pulling the belt back off in 20,000 miles to chase a coolant leak is a brutal way to learn this lesson.
Using a non-Mercedes-spec belt or tensioner. Aftermarket kits exist but quality varies. Stick with Continental, Contitech, INA, Gates, or genuine Mercedes for the kit. Cheap eBay timing kits on a CDI is gambling.
Reassembling without rotating two full turns by hand to verify timing. This is the last sanity check before you start the engine. If you skip it and timing is off by a tooth, the first cranking attempt is the bent-valves moment. The check is free.
Trusting an unstamped belt history from a previous owner. "I think it was done" is not a service record. If you can't see the receipt and the date, it wasn't done.
Taking the interval as fixed. It varies. Pull the workshop manual for your specific VIN year and market.
When to skip DIY
Skip DIY unless you have all of: the Mercedes-specific timing tools, current workshop manual coverage of the CDI procedure, a clean working space, and prior experience timing a belt-driven interference engine. The penalty for getting this wrong on an interference engine is high enough that "I'll figure it out as I go" is not a reasonable approach.
A Smart-experienced independent or a Mercedes specialist will quote $700-1,400 for the full job done right. That is genuinely fair money for the work involved and the consequences of a mistake. The cars that get neglected on this service eventually become the cars that someone parts out. Don't let yours be one of them.
If you're shopping a CDI 451 right now, timing belt service history is the single most important thing in the records. A documented belt + water pump done in the last 50,000 miles is worth a meaningful amount over an identical car with no record. Treat it that way in the negotiation.
Parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Timing belt kit (belt + tensioner + idler pulleys) | $150-300 | Search Google |
| Water pump (replace with belt; same access) | $80-200 | Search Google |
| Coolant refill | $25-50 | Search Google |
| Independent specialist labor | $400-800 | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread.
Manual references
- Browse Smart workshop manuals on smartcarmanuals.com — model-specific reference manuals on the home page; pick your chassis code section for torque specs and detailed procedures.