Fuel Pumps & Fuel Injection Repairs
The electric fuel pump is the heart of every electronic fuel injection system. Usually located inside or near the fuel tank, the fuel pump’s job is twofold:
- To push fuel from the tank to the injectors, and
- To create sufficient pressure so the injectors will deliver the correct amount of fuel under all operating conditions.
The pressure developed by the pump, as well as the volume of fuel it flows, must both meet the vehicle manufacturers’ requirements or engine performance, economy and emissions will suffer.
The amount of fuel pressure required for a given application will vary depending on the type of injection system (L-Jetronic, CIS, Motronic, etc.), the flow characteristics of the injectors and the engine’s fuel requirements.
For example, certain Mercedes models with Bosch Motronic require 55 to 61 psi (3.8 to 4.2 BAR) of static pressure measured with the engine off. By comparison, a BMW may require 43 psi (3.0 BAR) on some models or 48 psi (3.3 BAR) on others. The differences may not seem like much, but a few pounds of fuel pressure can have a significant impact on engine performance and emissions.
Why Fuel Flow & Fuel Pressure Are So Important
A fuel pump that doesn’t meet the OE minimum fuel flow or pressure requirements for your Mercedes can cause driveability and emissions problems.
A weak fuel pump or one that can’t generate enough pressure can upset the calibration of your fuel system. This may cause the engine to run lean or starve for fuel under load, causing symptoms such as hard starting (hot or cold), poor idle quality, hesitation or stumbling when accelerating and a loss of high-speed power.
Low fuel pressure can also be a cause of lean misfire at idle and under load, which causes a dramatic increase in hydrocarbon (HC) emissions. A car experiencing this kind of problem usually won’t pass an “enhanced” emissions test that measures exhaust emissions under simulated driving conditions on a dynamometer.
Low fuel pressure and/or lean misfire can also trigger the engine warning lamp on 1996 and newer cars equipped with OBD II (on-board diagnostics).
So, if the original fuel pump is weak or has failed, it’s important to make sure your replacement fuel pump meets all Mercedes OE fuel performance specifications. Bosch pumps do match Mercedes OE requirements, but many brands don’t – so beware when purchasing a new fuel pump for your Mercedes. Always remember – you get what you pay for. Is it really worth risking severe damage to your fuel injection system?
Why Fuel Pumps Fail
Electric fuel pumps run constantly, so after many years of service they can experience wear in the armature bushings, brushes and commutator. Pump vanes, rollers or gears can also wear causing a gradual loss of pressure and flow.
Accelerated wear may also occur if sediment or rust gets past the inlet filter sock. In some instances, a pump will fail because contaminants entered the pump and jammed it, causing the motor to overheat and burn out.
Your Mercedes’ fuel pump relies on fuel passing through it for lubrication and cooling. Consequently, fuel starvation can be another factor that accelerates wear and may even cause pump damage under certain operating conditions.
Mercedes Fuel Pump Diagnosis
If your fuel pump stops working (no noise, no line pressure), the first thing that should be checked is the pump’s voltage supply and electrical connections. An open relay, blown fuse or loose wire may be all that’s preventing the pump from working. Low battery voltage can also reduce the pump’s ability to generate pressure by reducing.