Smart Fortwo Coolant Flush — 450, 451, 453
Service interval: Every 4-5 years per workshop manual · Always after any cooling system repair (radiator, hoses, water pump, thermostat)
Tools you'll need
- Drain pan (5L+)
- Socket set for the lower radiator hose clamp or block drain
- Distilled water for mixing (if using concentrate)
- Funnel with a long neck
- Old towels for inevitable drips
- Bleed screw tool (flathead screwdriver works on most)
- Pressure tester (optional but useful for confirming no leaks)
Fluids & specs
| Fluid | Spec | Capacity |
|---|---|---|
| Coolant — 450 / 451 (petrol and CDI) | Mercedes 325.0 or 326.0 (blue / G05-equivalent), MB-Approval 325.x | ~4-5L total system (varies by variant) |
| Coolant — 453 | Renault Type D (Glaceol RX, blue/green), pre-mixed 50/50 acceptable | ~4-5L total system |
What this is and why it matters
Coolant doesn't wear out the way oil does, but its corrosion inhibitors and pH buffers do. Old coolant turns mildly acidic, eats away at aluminium passages and the water pump impeller, and stops protecting the gaskets the way fresh fluid does. On a Smart, where the engine sits behind you and the cooling system runs long lines front-to-back, an internal corrosion problem is expensive to chase. A flush every four to five years is cheap insurance.
The other reason to take this seriously is fluid spec. Smart cars use specific coolant chemistries — Mercedes blue (G05-style) on the 450 and 451, Renault Type D on the 453. Mixing these with green ethylene glycol or pink Dex-Cool gels the system. That's a head-gasket-and-radiator-out repair to undo. Get the spec right.
What you'll need
Listed above. A few clarifications:
- Mercedes 325.0 or 326.0 is the spec called out for the 450 and 451. Buy a coolant labelled MB-Approval 325.x (or G05/HOAT-compatible from a brand that explicitly lists Mercedes approval). It is sometimes sold as "blue Mercedes coolant" or under the Zerex G05 name. Check the back label for the approval number.
- Renault Type D is the spec for the 453 (the engine and platform are Renault). Often sold as Glaceol RX Type D. Don't substitute Mercedes spec on the 453 even though the colour can look similar.
- Distilled water only when mixing from concentrate. Tap water introduces minerals that scale the system.
Step by step
- Engine fully cold. Cracking a hot cooling system at any point is a hospital trip. Park overnight or wait at least three hours after a drive.
- Lift and secure the car. Jack stands on the rated points; chock the wheels. You'll need access from underneath.
- Open the expansion tank cap to break the vacuum. Don't leave it loose, but cracking it speeds the drain.
- Place the drain pan under the lower radiator hose or block drain. Smart cooling lines run rear-engine to front-radiator, so there's a fair amount of fluid spread across the car. The fastest drain on most variants is the lower radiator hose at the front of the car.
- Crack the lower hose clamp and ease the hose off. Coolant comes out fast — be ready. Let it drain fully. If your variant has a separate engine block drain plug, use that as well to get the coolant out of the rear.
- Optional: flush with distilled water. Reconnect the lower hose, fill the system with distilled water, run the engine to operating temperature with the heater on full, drain again. Repeat once. This pulls out residue from old coolant. Skip if the old fluid came out clean and the right colour.
- Reconnect the lower radiator hose with a fresh clamp tightening. Confirm the block drain (if used) is closed and torqued.
- Refill with the correct coolant mix. Pre-mixed 50/50 goes in straight; concentrate gets mixed 50/50 with distilled water before pouring. Fill slowly through the expansion tank with the funnel; air will burp out as you go.
- Bleed the system. This is the gotcha. The Smart cooling loop has high points front and rear that trap air. With the cap off and the engine cold, start the car, set the heater to full hot and the blower on low. Watch the expansion tank — bubbles will come up as the thermostat opens. Top off as the level drops. Squeezing the upper radiator hose by hand helps move air. Let the engine reach full operating temperature and the cooling fan cycle on at least once. Then carefully cap the expansion tank.
- Test drive and recheck cold. Drive the car for 15-20 minutes, let it cool fully overnight, recheck the expansion tank level the next morning. Top off to the cold mark if needed. Air pockets often work themselves out over the first day or two of driving.
Common gotchas
- Coolant chemistry must match the car. Mercedes blue on 450/451, Renault Type D on 453. Mixing with the wrong type — green ethylene glycol, pink Dex-Cool, or even mixing two different "blue" coolants from different OEM specs — can gel or precipitate. Worst-case it kills the water pump and clogs the heater core.
- Bleeding is where most flushes go wrong. Skip the bleed and you get an air pocket at the thermostat that causes intermittent overheating, rough warm-up, and sometimes a P0117 or P0128 code from a sensor reading air instead of coolant. There's a workshop-manual bleed sequence specific to each variant — follow it.
- The 451 has a coolant pump that doesn't love air. Running the system dry, even briefly during the bleed, can damage the pump seal. Keep coolant in the expansion tank during the bleed; don't run the engine with the tank empty.
- Watch the expansion tank cap pressure rating. A worn cap that doesn't hold pressure causes the system to boil at lower temperatures. If your cap is more than five years old, replace it while you're in there — it's a cheap part.
- Overheating after a flush usually means trapped air, not a bad thermostat. Re-bleed before you condemn other parts.
- Used coolant is toxic to pets and wildlife. It tastes sweet. Catch every drop in the pan and dispose of it at an auto-parts store recycling station.
When to skip DIY
Coolant flushes are doable at home, but the bleed step is what separates a clean job from a recurring overheating problem. If you don't have a way to keep the front of the car level (or slightly nose-up) for the bleed, or if your variant's bleed procedure isn't clear from the workshop manual, this is a job worth taking to a Smart-experienced shop. Expect $150-250 for a proper flush and bleed at a competent indie. After any major cooling system repair — water pump, thermostat, head gasket — a vacuum-fill machine at a shop is faster and more reliable than a gravity bleed at home.
Parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| 5L Mercedes blue coolant (concentrate) | $25-40 | Search Google |
| 5L Renault Type D coolant | $25-40 | Search Google |
| Distilled water (for mixing concentrate) | $2-5 | 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.
How-to videos
Related fault codes
- P0117 Coolant Temperature Sensor Low on Smart Fortwo 451 / 453
- P0128 Thermostat Stuck Open on Smart Fortwo 451 / 453