A Mercedes-Benz GLA 260 arrived at the workshop with two worrying symptoms: the engine was difficult to start, and it would stall as soon as the transmission was placed into gear.
These symptoms can sometimes be blamed on the transmission, engine computer, ignition system, or fuel injectors. However, the first step should always be to check whether the engine is receiving the correct fuel pressure during starting and under load.
In this case, computer diagnostics quickly pointed toward a fuel supply problem.
The vehicle was a 2014 Mercedes-Benz GLA 260, with approximately 224,454 kilometres on the odometer. To protect the customer’s privacy, only the last three digits of the vehicle identification number are shown: 064.
Vehicle Information
- Vehicle: Mercedes-Benz GLA 260
- Model year: 2014
- Mileage: 224,454 km
- VIN: Ending in 064
- Main complaint: Hard starting and stalling when selecting gear
The vehicle could eventually start, but the engine did not run properly. When the driver selected a gear, the additional load caused the engine to lose power and stall.
That detail was especially important. An engine may appear to run at idle even when fuel pressure is already too low. Once the transmission is engaged, the engine needs more torque. If the fuel system cannot supply enough fuel, the engine may shut down.
Fault Codes Found During Diagnosis
The vehicle was connected to a diagnostic scanner, which recorded the following fault codes:
- P008700 – Fuel pressure in the system is too low
- P008A00 – Fuel pressure in the low-pressure fuel circuit is too low
Both codes pointed toward the fuel supply side of the vehicle rather than the high-pressure fuel system or ignition system.
It is important not to replace parts based only on a fault code. A code tells the technician which system has detected an abnormal condition, but it does not always identify the failed component.
For that reason, the next step was to check the live data from the fuel system while starting the engine.
Fuel Pressure Data During Starting
When the engine was cranked, the fuel module data was monitored through the diagnostic scanner.
The highest fuel pressure recorded during starting was only:
1.2 bar
This reading was far below the pressure required for the fuel system to operate correctly.
A low-pressure fuel system must deliver a stable fuel supply to the engine. If the pressure drops too far, the engine may:
- Take a long time to start
- Start and immediately stop
- Misfire under load
- Stall when selecting gear
- Lose power during acceleration
- Store low-fuel-pressure fault codes
- Require several attempts before starting
The 1.2-bar reading confirmed that the problem was not simply a software issue or an incorrect fault-code interpretation. The fuel system was genuinely unable to build sufficient low-side pressure.
Checking the Low-Pressure Fuel Line
The next inspection focused on the low-pressure fuel supply pipe at the engine end.
The actual pressure in the low-pressure inlet line was also very low. This helped rule out several other possible causes, including a problem limited to the engine-side pressure sensor or a communication issue between the fuel control module and the engine control unit.
At this stage, the diagnostic evidence was consistent:
- The engine was hard to start.
- It stalled when placed into gear.
- The vehicle stored low-fuel-pressure fault codes.
- The fuel module data showed only 1.2 bar during starting.
- The low-pressure fuel line also showed insufficient pressure.
The most likely cause was a damaged or worn fuel pump assembly.
Why a Weak Fuel Pump Can Cause Stalling
The fuel pump is responsible for moving fuel from the tank toward the engine. As a pump wears out, it may still produce enough pressure to let the engine start occasionally. However, its performance may collapse when the engine needs more fuel.
This explains why some vehicles appear to run normally for a short period but stall when:
- The transmission is placed into drive
- The air conditioning is switched on
- The accelerator is pressed
- The vehicle climbs a hill
- The engine is started after sitting overnight
- The fuel level is low
A fuel pump can also become noisy before it fails completely. Some drivers hear a loud humming or whining sound from the rear of the vehicle. Other pumps fail without making any obvious noise.
In this GLA 260, the measured fuel pressure was the deciding factor. Rather than replacing unrelated components, the diagnosis followed the pressure readings and led to the fuel pump assembly.
Fuel Tank Removal Required
On this Mercedes-Benz GLA 260, the fuel pump assembly is located inside the fuel tank. Access is not available through a simple service cover from inside the cabin.
The fuel tank must be lowered and removed before the fuel pump assembly can be replaced.
This makes the repair more involved than a typical external fuel filter or fuel-line replacement. Removing the tank requires careful preparation because the technician must work around:
- Fuel vapour
- Fuel lines
- Electrical connectors
- Exhaust components
- Heat shields
- Tank straps
- Underbody panels
- Vehicle lifting points
Safety should always come first during fuel-system repairs. The battery should be disconnected where appropriate, the work area should be well ventilated, and all ignition sources must be kept away from the vehicle.
Preparing the Vehicle
Before removing the fuel tank, the vehicle should be safely raised on a suitable lift. The technician should inspect the underside of the vehicle and identify the tank position, fuel lines, wiring, exhaust routing, and tank mounting points.
The tank should be as empty as possible before removal. A nearly full fuel tank is considerably heavier and more difficult to handle safely.
It is also helpful to label connectors and lines before disconnecting them. This reduces the chance of incorrect installation later and makes the repair process more efficient.
The underbody should be photographed before dismantling begins. A few clear reference photos can help confirm the correct position of brackets, heat shields, and routing clips during reassembly.
Removing the Fuel Tank
The general tank-removal process includes the following stages:
- Raise and secure the vehicle on a suitable lift.
- Disconnect the battery according to workshop safety procedures.
- Reduce fuel-system pressure where required.
- Remove protective underbody panels.
- Support the fuel tank with a transmission jack or approved support.
- Disconnect the fuel filler pipe and breather lines.
- Disconnect the fuel supply lines.
- Disconnect the electrical connectors for the fuel pump and fuel-level sensors.
- Remove heat shields or exhaust components that obstruct access.
- Remove the tank straps and retaining hardware.
- Lower the tank carefully.
- Remove the defective fuel pump assembly.
- Install the replacement assembly with a new sealing ring where required.
The tank must be lowered slowly. Pulling on a fuel line or electrical connector can cause additional damage and create another repair problem.
Once the tank is removed, the underside of the vehicle becomes visible. This is a good opportunity to check for corrosion, damaged wiring, leaking fuel lines, loose clips, and signs of previous repair work.
Preparing the Replacement Pump Assembly
Before installing the replacement part, compare it carefully with the original assembly.
Check the following:
- Electrical connector position
- Fuel-line connections
- Fuel-level sender arrangement
- Mounting points
- Sealing ring size
- Pump housing shape
- Fuel filter position
- Overall assembly height
A replacement pump that looks similar may not be correct for the vehicle. Using the wrong assembly can create fuel-pressure problems, incorrect fuel-gauge readings, or electrical faults.
The sealing ring should also be inspected carefully. A damaged or incorrectly seated seal can cause fuel vapour leaks or allow fuel to escape from the tank area.
Fuel Pump Fuses
During the repair, it is useful to identify the fuel-system fuse locations.
On this vehicle, the fuse box is located beneath the front area on the passenger side. The fuel module fuses are identified as:
- Fuse 63
- Fuse 92
These fuse locations should be checked during electrical diagnosis. A blown fuse, poor connection, damaged wiring, or a faulty fuel-module power supply can produce symptoms that look similar to a failed pump.
However, in this case, the low fuel pressure was confirmed through live data and physical pressure checks. The evidence supported replacement of the fuel pump assembly rather than a fuse-only repair.
Fuse numbers and layouts can vary by model specification and market, so technicians should confirm the vehicle’s correct fuse chart before testing or replacing any fuse.
Final Checks After Installation
After installing the new fuel pump assembly and reinstalling the fuel tank, the fuel system should be checked carefully before the vehicle is driven.
Important checks include:
- Confirming that all fuel lines are securely connected
- Checking that electrical connectors are locked
- Inspecting the tank seal
- Checking for fuel leaks
- Confirming correct tank strap installation
- Verifying that the fuel gauge operates correctly
- Clearing stored fault codes
- Monitoring fuel pressure during starting
- Checking pressure at idle
- Testing the vehicle under load
- Confirming that the engine does not stall when selecting gear
The new fuel pressure should remain stable. A quick pressure improvement during starting is not enough; the system must also maintain suitable pressure while the engine is running and the vehicle is placed under load.
A road test should include repeated starts, gear engagement, low-speed driving, acceleration, and a check for warning lights.
Diagnosis Summary
This Mercedes-Benz GLA 260 was suffering from hard starting and stalling when selecting gear because the low-pressure fuel system could not provide enough fuel.
The diagnostic process was:
- Retrieve fault codes
- Monitor fuel module live data
- Measure fuel pressure during starting
- Check pressure at the engine-side low-pressure line
- Confirm that pressure was too low
- Inspect the fuel pump circuit and fuse locations
- Remove the fuel tank
- Replace the fuel pump assembly
- Recheck pressure and vehicle operation
The fault codes P008700 and P008A00, combined with a maximum starting pressure of only 1.2 bar, provided strong evidence of a fuel supply failure.