When an R107 cranks without starting — or dies under load — the fuel pump is on the short list. These cars push fuel from the rear of the vehicle through a filter to the injection system. Some late cars use dual pumps. This guide covers location mindset, symptoms, tests, and replacement habits that avoid repeat failures.
Related: fuel pump relay, K-Jetronic, won’t start.
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Where the pump lives
Expect the pump (or pumps) near the fuel tank at the rear — often accessible from below or through trunk/side covers depending on year. Exact brackets and splash shields differ across 450 / 380 / 560. Do not rip into the first round can you see without confirming you have the fuel pump and not a related accumulator or filter housing.
560SL note: some setups use dual pumps. If you replace only one of a tired pair, the survivor may fail soon after. Inspect both.
Symptoms of a weak or dead pump
- No brief prime whir at key-on
- Long crank, especially hot
- Screaming / groaning pump tone that used to be a quiet whir
- High-speed stumble as demand rises and pressure falls
- Intermittent no-start that “comes back” after a cool-down
Pressure and volume tests
Confirm power and ground at the pump connector first (relay, fuse, OVP). Then measure:
- System pressure with a CIS-capable gauge
- Volume delivery if your procedure calls for a timed fill into a safe container
A clogged filter can mimic a dying pump. Replace the filter when you replace a pump that may have shed debris. A proper CIS / K-Jetronic fuel pressure tester pays for itself the first time you avoid guessing.
Replacement outline
- Relieve fuel pressure safely; no sparks; fire extinguisher nearby
- Support lines; clamp carefully without crushing ancient hose
- Swap pump and sealing washers/hoses that are cracked
- Route wiring and hoses away from heat and sharp edges
- Key-on prime, check for leaks, then start and recheck pressure
Cost mindset
OEM-quality pumps cost more than no-name units for a reason. Budget filter, hoses, and clamps in the same job. Labor is higher if fasteners are rusted under the car.
Related parts to inspect while you are there
- Fuel accumulator (hot-start rest pressure)
- Tank screen / debris
- Pump relay contacts
- Ground straps at the rear
Details that finish this topic
Location
Exact locations vary by year and market. Use a diagram for your chassis, photograph connectors before removal, and do not rely on a single forum photo from a different model year.
single vs dual (560SL)
On an R107 this means verifying the condition on your year/engine before ordering parts. When in doubt, match the data card and engine number, then confirm with a specialist who works on 107s regularly.
noise
pressure/volume test
Work methodically: reproduce the symptom, check power/fuses/relays, measure before replacing, then retest the same conditions (cold vs hot, loaded vs idle) after the repair.
filter
replacement steps
cost
Treat any dollar figure as a starting band that moves with city labor rates and how seized fasteners are. Get a written estimate for your VIN before you commit — especially for welding, tops, and injection work.
