W126 V8/W124 300 E-24 long cranking when warm? Fuel accumulator 0438170055

The engine is warm, you park the car for ten minutes – and when you drive off again, the starter motor cranks noticeably longer than usual. When cold in the morning, on the other hand, it usually starts without any fuss. Owners of a 420 SE, 500 SL, 560 SEC or 300 E-24 then often look first at the ignition, battery or starter motor. A common cause, however, sits on the underbody: the fuel accumulator. On the V8 and 24-valve models with KE-Jetronic there is a second hurdle – a larger accumulator is fitted here than on the two-valve engines, and the numbers are regularly mixed up. This guide shows how to check the accumulator and how to order the right one with confidence.

The accumulator sits in the fuel supply line at the rear of the underbody, near the fuel pump and filter. After shut-off it keeps the pressure in the system up for a while and also dampens the noise of the electric fuel pump. If the residual pressure drops, fuel vapor easily forms in the hot system – the pump then has to build up pressure again at the next start.

Symptoms

Symptom	Likely cause
Long cranking after a short stop with a warm engine, no problem when cold	Accumulator does not hold the residual pressure, vapor formation in the hot system
Engine starts poorly after standing for a long time, then runs normally	Loss of residual pressure – accumulator, pump check valve or leaking injectors
Fuel at the small port at the bottom of the accumulator, smell of fuel under the vehicle	Diaphragm in the accumulator leaking, fuel gets into the spring chamber
Fuel pump hums noticeably loudly	Accumulator no longer dampens – but check the pump itself as well
Hot-start problems with rough idle right after starting	More likely the system pressure regulator – see Check KE-Jetronic fuel pressure regulator
Idle fluctuates with a warm engine, starting itself is fine	Not the accumulator – see Check & replace idle speed actuator

Causes

Inside the fuel accumulator, a rubber diaphragm separates the fuel chamber from a preloaded spring. This diaphragm ages regardless of mileage: after three decades it has hardened, fine cracks form at the clamping edge, and the accumulator can no longer hold the pressure after shut-off. If the diaphragm becomes completely leaky, fuel runs into the spring chamber and escapes at the small port at the bottom.

Long periods of storage over the winter, fuel containing ethanol and corrosion from road spray on the underbody accelerate wear. Especially on V8 vehicles that are only driven during the season, the accumulator is therefore a typical candidate. The defect is confirmed by measuring, though, not by age.

Testing it yourself

1. Note the starting pattern. Keep track over a few days of whether the long cranking occurs after a short stop with a warm engine, after standing for a long time, or in both cases. It costs nothing and already narrows down the cause quite well.
2. Read off the part number. Lift the vehicle safely, clean the accumulator near the pump and filter and read off the number. For the V8 and 24-valve engines it must read 0 438 170 055 or A 000 476 09 21 / A 000 476 10 21. If you find 0 438 170 035 or 017, a different version is fitted – see the 'not to be confused with' note below.
3. Visual and smell check. After a period of standing, look at the small port at the bottom of the accumulator. Fuel at this spot directly proves a leaking diaphragm. Check the banjo bolt and copper sealing rings at the inlet 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 will also let the residual pressure drop. Otherwise you end up replacing the wrong part.
5. Measure the residual pressure. Use a pressure tester for the KE-Jetronic to check the system pressure with the pump running, then switch off the engine and watch the pressure curve over 20 to 30 minutes. You will find the specified values in the repair manuals 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 the cause.

Solution

A leaking accumulator is replaced as a complete unit – repairing the sealed unit is not intended. We have the fuel accumulator Bosch 0438170055 / Mercedes A0004760921 in stock as a used genuine Bosch part, tested and inspected, for 119,00 € per pc. According to the part numbers, the A0004760921 replaces the earlier A0004761021. When installing, use new copper sealing rings on the banjo bolt and check all fittings after the first pressure build-up. You will find more accumulators and injection parts under Fuel accumulators and Fuel system & injection.

Important – not to be confused with: The two-valve engines M102 and M103 (e.g. 300 E, 300 SL with 12 valves) need the smaller accumulator Bosch 0 438 170 035 (A0004760521 / 0004760621). The most common mix-up: 300 SL needs 035, 300 SL-24 needs 055 – likewise 300 E with 035, 300 E-24 with 055. K-Jetronic vehicles such as the W126 380 SE/SEL/SEC, R107 380/450 SL, as well as the R107 500 SL up to 02/1984 and W126 500 up to 08/1984, have the accumulator 0 438 170 017 (A0004760421).

Compatible vehicles

The table shows the fitment documented in the product data. The number on your old part or a check by VIN is what counts.

Model series	Models per product data	Category in the shop
W126 / C126 (S-Class, 1985–1991)	420 SE, SEL, SEC (126.034 / .035 / .046); 500 SE, SEL, SEC (126.036 / .037 / .044) from 09/1985; 560 SE, SEL, SEC (126.038 / .039 / .045)	W126
R107 (SL, 1985–1989)	420 SL (107.047) and 500 SL (107.046) 01/1986–08/1989; 560 SL (107.048) 09/1985–08/1989	W107 / R107
W124 (E-Class, 24-valve, 1988–1993)	300 E-24 (124.031), 300 CE-24 (124.061), 300 TE-24 (124.091), AMG 300 E / CE / TE 3.4	W124
R129 (SL, 1989–1993)	300 SL-24 (129.061) 03/1989–06/1993; 500 SL (129.066) 03/1989–09/1992	Fuel system
W463 / W461 (G-Class, 1989–2001)	200 GE, 230 GE, 300 GE (W463) from 09/1989; G 230 and G 300 up to 1995; 500 GE / G 500 03/1993–09/2001; 230 GE (W461) 09/1989–08/1992	W463 (G-Class)

Does not fit according to the product data: Mercedes with four-cylinder M102 and six-cylinder M103 (two-valve), W126 380 SE / SEL / SEC (K-Jetronic) and R129 300 SL with 12-valve engine. For the two-valve engines, read W201/W124 cranks for a long time when warm? Check fuel accumulator A0004760521, for K-Jetronic vehicles W116/W126 cranks a lot at start? Check the K-Jetronic fuel accumulator.

FAQ

Which fuel accumulator fits the W124 300 E-24?

According to the product data, the Bosch accumulator 0438170055 with the Mercedes number A0004760921. It is listed for the 300 E-24, 300 CE-24 and 300 TE-24 as well as the AMG 3.4 models. The 300 E with the two-valve engine M103, on the other hand, needs the smaller accumulator 0 438 170 035.

What is the difference between fuel accumulator 0438170055 and 0438170035?

The 055 is the larger version for the KE-Jetronic of the V8 and 24-valve engines. The 035 belongs to the two-valve engines M102 and M103. The two are not interchangeable – the number on your old part or a check by VIN is what counts.

Does A0004760921 replace the number A0004761021?

Yes. In the product's part numbers, A 000 476 10 21 is listed as the superseded number for A 000 476 09 21; both correspond to the Bosch accumulator 0 438 170 055.

Why does my W126 V8 start well when cold, but poorly when warm?

If the fuel accumulator does not hold the residual pressure after shut-off, vapor can form in the hot fuel system. At the next start the pump first has to build up pressure again, and the engine cranks for a long time. The pressure regulator, injectors and the pump's check valve are also possible causes – so measure the residual pressure before replacing anything.

How much does the fuel accumulator 0438170055 cost?

At our shop, the fuel accumulator 0438170055 / A0004760921 costs 119,00 € per pc as a tested, used genuine Bosch part.

KFZTeile-Thot is a specialist business for tested and refurbished Mercedes parts with our own test benches – every part is tested before shipping. Over 1,000 positive eBay reviews confirm this. Not sure whether your vehicle needs the 055 or the smaller 035? 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.