W116/W126 cranks a lot at start? Check the K-Jetronic fuel accumulator

The vehicle ran perfectly the day before, sits overnight – and in the morning the starter motor cranks for five or six seconds before the engine fires up. Right after that, it starts instantly. Anyone who recognizes this pattern on a W116, W123, W126, or R107 with K-Jetronic usually looks at the ignition or the cold start valve. Often, though, the cause is one step earlier: after shutdown, residual pressure is missing in the fuel system. That's exactly what the fuel accumulator is responsible for.

The accumulator is usually located under the floor, near the fuel pump and filter. During start-up it dampens the pump's pressure pulses; after the engine is switched off, it keeps fuel in the lines and fuel distributor under pressure. If this residual pressure is missing, the pump has to refill the system on the next start – costing the seconds that show up as “long cranking”.

Symptoms

Symptom	Likely cause
Engine starts only after prolonged cranking following several hours of standing, then runs normally	Fuel accumulator no longer holds residual pressure
Smell of gasoline under the vehicle, damp spot at the vent hole of the accumulator	Diaphragm in the fuel accumulator leaking, fuel passes into the spring chamber
Starting behavior gradually worsens over weeks	Weakening diaphragm; also check the fuel pump check valve
Poor start with a hot engine after a short stop, unremarkable when cold	More likely the system pressure regulator than the accumulator – see Hot start problems: check KE-Jetronic pressure regulator
Gasoline at the vacuum hose or at the leak-off line in the engine bay	Not the accumulator – see Fuel pressure damper defective: gasoline at the leak-off line
Engine runs rough only during the warm-up phase, fine once at operating temperature	Not the accumulator – see Warm-up regulator defective: symptoms and testing
Poor cold start, with a distinct smell of fuel during the start attempt	Not the accumulator – see Cold start valve: testing and replacing

Causes

Inside, a rubber diaphragm separates two chambers: fuel on one side, a preloaded spring on the other. This diaphragm is the wear part. After decades the material hardens and becomes brittle, fine cracks form at the clamping point, and the accumulator no longer holds pressure. Once the leak goes all the way through, fuel reaches the spring chamber and escapes at the vent hole – the clearest finding you can make without a measuring instrument.

Aging is accelerated by long periods of standing and by ethanol-blended fuels, which attack old elastomers more than the gasoline of the period did. On top of that, the housing corrodes because the accumulator sits under the floor, exposed to road spray. So a part still from first delivery belongs on the suspect list whenever there are starting problems.

Testing it yourself

1. Log the starting pattern. Over a few days, note how long the vehicle stood each time and how long the starter had to crank. If the engine starts immediately right after being switched off but poorly after several hours, that points to pressure loss while standing. Costs nothing and narrows things down the most.
2. Visual and smell check. Lift the vehicle securely, dry off the area around the fuel pump and filter, then check again after a period of standing. Gasoline at the accumulator's vent hole directly proves the diaphragm is defective.
3. Rule out simple leak points. Check the fittings, sealing rings and hoses at the pump, filter and accumulator. Porous rubber lines are cheap and cause the same symptoms.
4. Check other pressure consumers. A leaking cold start valve, dripping injectors, or a defective fuel pump check valve can also let the residual pressure drop – rule these out before the accumulator, or you'll replace the wrong part.
5. Measure system and residual pressure. Using a K-Jetronic pressure test gauge, check the system pressure with the pump running, then switch off the engine and observe the pressure curve over 20 to 30 minutes. Target values differ by model series and engine; take them from the workshop manual for your model. If the pressure drops to zero within minutes and the drop stops as soon as the line to the accumulator is shut off, the accumulator is leaking.

Solution

Once the accumulator is confirmed as the cause, it is replaced complete – it cannot be repaired. We supply the genuine part with the Mercedes number A0004760421 or the Bosch number 0438170017 as a tested used part: checked for leak-tightness and function, threads clean. Plan for fresh sealing rings and check the system calmly after the first pressure build-up. You'll find more parts under Fuel accumulators and Fuel system & injection.

Compatible vehicles

This overview reflects the fitment documented in the product data and is not exhaustive – different versions were fitted within the model series.

Model series	Fuel system	Category in the shop
W116 (S-Class)	K-Jetronic	W116
W123	K-Jetronic	W123
W126 (S-Class)	K-Jetronic	W126
R107 / C107 (SL, SLC)	K-Jetronic	W107 / R107

We've covered the basics of this component and further symptoms in the guide Fuel accumulator defective? Poor cold start after standing for a long time summarized.

FAQ

How do I tell a defective fuel accumulator apart from a weak fuel pump?

A weak pump shows up during operation: loss of power under load, jerking at high revs, too-low system pressure with the engine running. A defective fuel accumulator, by contrast, only disrupts starting after the vehicle has been standing – once the engine is running, everything is unremarkable. A pressure measurement reliably tells the two apart: check the system pressure with the pump running, then observe the residual pressure curve after switching off.

Can I keep driving with a defective fuel accumulator?

If only the residual pressure is missing, the vehicle technically keeps driving – with poor starting and unnecessary strain on the starter motor and battery. If, however, fuel escapes at the vent hole, the diaphragm is leaking and the part should be replaced immediately.

Does the fuel accumulator A0004760421 fit my vehicle?

According to the product data, accumulator A0004760421 / Bosch 0438170017 covers several K-Jetronic model series, including W116, W123, W126 as well as R107 and C107. Since different versions were fitted within a model series, we check the fit beforehand using the VIN (chassis number).

Does the residual pressure need to be adjusted after installation?

No, the residual pressure isn't adjustable – it results from the system pressure and the tightness of the system. After installation, the fittings are checked and the pressure curve is checked after switching off. If the pressure still doesn't hold, there is a second leak, often the fuel pump check valve or leaking injectors.

How old can a fuel accumulator get?

There's no fixed replacement interval. What matters is the condition of the rubber diaphragm, and that ages independently of mileage. On vehicles still with their original accumulator, the part is a plausible suspect after decades – but that's confirmed by the pressure measurement, not by age.

KFZTeile-Thot is a specialist business for tested and refurbished Mercedes parts with its own test benches – every part is tested before shipping. Over 1,000 eBay reviews (almost all positive) confirm this. Not sure whether the fuel accumulator fits or whether the symptoms really point to residual pressure? Call us at 0179 8991069 (Mon–Fri 8 am–6 pm) or write to us via the contact form – with the VIN (chassis number) and the number on the removed part.