W201/W124 cranks for a long time when warm? Check fuel accumulator A0004760521

The engine is warm, you stop briefly to refuel or shop – and when you drive on, the starter motor turns noticeably longer than usual. When cold in the morning, the car usually starts without any problem. Many owners of a W201, W124 or R129 then look first at the ignition or the battery. A common cause, however, sits underneath the car: the fuel accumulator. There is also a second question we receive regularly – for these model series there are, with A0004760521 and 0004760621 two versions. This guide shows how to check the accumulator and find the right number.

The accumulator is located near the fuel pump and filter. It dampens the pressure surges of the pump and, after the engine is shut off, keeps the fuel in the lines and injection system under pressure. If this residual pressure drops, fuel vapor can form, especially with a hot engine – the pump then has to refill the system at start-up.

Symptoms

Symptom	Likely cause
Long cranking after a short stop with a warm engine, no problem when cold	Accumulator does not hold residual pressure, vapor formation in the hot system
Long cranking after several hours of standing, normal running afterwards	Residual pressure loss – accumulator, pump check valve or leaking injectors
Gasoline smell under the car, wet vent hole on the accumulator	Diaphragm in the accumulator is permeable
Loud humming of the fuel pump, pulsation in the lines	Accumulator no longer dampens the pressure surges, check the pump too
Hot-start problems with uneven idle right after starting	More likely the system pressure regulator – see W124/W201 hot-start problems: check KE-Jetronic fuel pressure regulator
Gasoline on the vacuum hose in the engine bay	Not the accumulator – see Fuel pressure regulator faulty

Causes

In the accumulator, a rubber diaphragm separates the fuel chamber from a preloaded spring. This diaphragm ages regardless of mileage: after decades it has hardened, fine cracks form at the clamping edge, and the accumulator loses its ability to hold pressure. If the diaphragm becomes completely leaky, fuel enters the spring chamber and escapes at the vent hole.

Long periods of standing, ethanol-containing fuel and corrosion on the housing from splash water accelerate wear. On cars still running on their first accumulator, the part therefore belongs high on the list when there are starting problems – but the defect is proven by measuring, not by age.

Testing it yourself

1. Note the starting pattern. Keep a record for a few days of whether the cranking occurs after a short stop with a warm engine, after a long period of standing, or in both cases. This narrows down the cause best, and at no cost.
2. Read off the part number. Lift the car safely, clean the accumulator near the pump and filter, and read the number: A0004760521 or 0004760621. This tells you which spare part you need.
3. Visual and smell check. Check again after a period of standing: gasoline at the vent hole of the accumulator directly proves a defective diaphragm. Check the fittings and hoses at the same time.
4. Rule out other sources of pressure loss. A leaking cold start valve, dripping injectors or a defective pump check valve also cause the residual pressure to drop. Otherwise you will replace parts that are not at fault.
5. Measure the residual pressure. Using a pressure tester for mechanical injection, check the system pressure with the pump running, then switch off the engine and watch the pressure curve over 20 to 30 minutes. Take the specified values from the repair manual for your model. If the pressure drops quickly and the drop stops as soon as the line to the accumulator is clamped off, the accumulator is leaking.

Solution

A leaking accumulator is replaced completely. We have both versions in stock as tested used genuine Bosch parts: A0004760521 for 89,99 € and 0004760621 for 79,99 €. Order the number that is on your old part. If it is not legible, we check the assignment using the VIN. When installing, use new sealing rings and check all fittings after the first pressure build-up. You can find more parts under Fuel accumulators and Fuel system & injection.

Compatible vehicles

The table reflects the fitment documented in the product data and is not exhaustive. Different versions were installed within the model series – the number on your old part or the check by VIN is decisive. The product data for both additionally list the engine codes 116.020, 116.024, 116.025, 116.028, 116.029, 116.032, 116.033, 116.036 and 116.120.

Model series	A0004760521	0004760621	Category in the shop
W201 (190)	✓	✓	W201
W124 (E-Class)	✓ (per product title)	✓	W124
W126 (S-Class)	✓	✓	W126
W107 / R107 (SL, SLC)	✓	✓	W107 / R107
R129 (SL)	✓	✓	Fuel system
W123	✓	–	W123
W116 (S-Class)	✓	–	W116

You will find the basics on the component in the guide Fuel accumulator defective? Poor cold start after standing for a long time. For K-Jetronic cars with version A0004760421, read W116/W126 cranks a lot at start? Check the K-Jetronic fuel accumulator.

FAQ

How can I tell whether my fuel accumulator is A0004760521 or 0004760621?

The number is stamped or printed on the housing of the installed accumulator, sometimes additionally with the Bosch logo. If it is no longer legible due to corrosion, tell us your VIN and model year – we check the assignment free of charge.

Are A0004760521 and 0004760621 interchangeable with each other?

According to the product data, both accumulators are listed for the W201, W124, W126, W107 and R129 model series. This does not imply general interchangeability, however: different versions were installed within the model series. The number on your old part or the check by VIN is binding.

Why does my Mercedes start well when cold but poorly when warm?

If the fuel accumulator does not hold residual pressure after the engine is shut off, vapor can form in the hot fuel system. At the next start the pump has to refill the system first, and the engine cranks for a long time. This is less noticeable when cold. The pressure regulator and the pump check valve are also possible causes, though – so measure the residual pressure before replacing anything.

Can a fuel accumulator be repaired?

No. The accumulator is a sealed component with a diaphragm and spring and is replaced completely. We supply tested used genuine Bosch parts that have been checked for proper function.

What does a fuel accumulator for the W201 or W124 cost?

The fuel accumulator A0004760521 costs 89,99 € from us, the version 0004760621 79,99 €. Both are tested used genuine Bosch parts; shipping within Germany is free.

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 positive eBay reviews confirm this. Not sure which fuel accumulator belongs in your car, or whether the symptoms really point to the residual pressure? Call us at +49 179 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.