1993 Mercedes-Benz 400E Fuel Pressure Test Cost

The fuel system for all types of vehicle operates by pumping fuel from the fuel tank, sending that pressurized fuel through fuel lines, regulating the pressure, and circulating unused fuel. Since the entire system must be pressurized to function properly, when a fuel system has lost pressure it will surely fail. When this occurs, it is known as a fuel delivery issue, and the object of diagnosis focuses on finding the cause of pressure loss. Many times, locating the loss in fuel pressure is the proper procedure for diagnosing the issue, so fuel pressure testing may be conducted at one of a few locations in the fuel delivery system.

Fuel system pressure is created in the fuel tank for all vehicles. The fuel pump activates when the vehicle is in use, and constantly pumps pressurized fuel from the fuel tank, through the fuel lines. This fuel will flow through a fuel filter, though many vehicles use a fuel filter inside of the tank before the fuel is pumped. Once past the fuel filter and fuel pump, the fuel will transfer through fuel lines to the engine bay, and will take one of two paths. The fuel system may have a mechanical fuel pump to create very high pressure in the fuel system, which is the case for diesel vehicles and many performance based gasoline engines. For diesel vehicles, the fuel will first run through a fuel and water separator to remove water from the fuel. Once fuel has passed the high pressure fuel pump, it will run through a fuel pressure regulator, fuel rail or fuel injector line, and finally through fuel injectors. Most gasoline vehicles do not use the high pressure fuel pump, and will simply route pressurized fuel through a fuel pressure regulator, fuel rail, and fuel injector(s). In both cases, this provides several locations to be tested for fuel pressure testing.

Loss of fuel pressure typically results in a 'no start' or 'start and stall' condition. This means the engine will crank but not start, or the engine shuts off as soon as it is started. Also, sputtering when going uphill, pressing the accelerator, or idling is common, and when fuel pressure is only high enough to run the engine at idle, pressing the accelerator will turn the engine off. There are multiple other issues possible, but a lack of power and engine performance in general are likely.

Fuel system pressure loss will typically cause a vehicle to become useless. If the vehicle starts, accelerating or keeping the engine running may be difficult or impossible. If fuel delivery is lacking, this will result in a lean running condition, which can generate enough heat to cause engine or exhaust manifold damage, especially on turbocharged vehicles. Fuel system issues should be inspected and repaired immediately, and will likely prevent operation.

Fuel system pressure tests are only conducted when an issue with the fuel delivery system is noticed or suspected. Since this is the case, there is no interval for determining when this will occur, but these tests can be avoided by keeping the fuel tank at or above 1/4 full, and changing the fuel filter on time.

When a fuel pressure test is conducted, the technician will come to an understanding of where the fuel pressure drops based on finding the last place fuel pressure exists. Once the fuel pressure drop is located and pinpointed across one component, the technician will need to identify if that component is serviceable, if it is properly actuated, and if it must be cleaned, repaired, or replaced.

Replacing fuel system components responsible for loss of pressure will require depressurization of the fuel system, catching remaining fuel from the fuel lines, and using specialty fuel line disconnect tools in order to remove components from the fuel delivery system. In some cases, the fuel tank or rear seat may need to be removed, and any fuel system components between the fuel system and engine may need to be removed and replaced. When electrical components are disconnected, it is important that the battery is disconnected, so settings in the vehicle may be lost after the repair.

We recommend conducting a fuel system test after checking the fuses and relays, but before replacing components without diagnosis. This will prevent many good parts from being changed, decrease cost, and prevent frustration.

Fuel systems can lose pressure for a variety of reasons, but if the odor of fuel is evident around the vehicle or under the hood, the likely issue is a fuel leak. Vehicles with fuel leaks should never be driven, as this poses a distinct risk of fires, especially if the leak is in the engine bay.

Fuel system pressure testing is something that a well experienced DIYer can do at home. The required tooling is normally available to rent or purchase inexpensively, and the portion of the fuel system which is pressurized is linear. This means that the fuel moves from point 'a' to point 'b' with a few stops in between. As long as safe practices are followed when working with fuel and the fuel system and the fuel system is thoroughly understood, this can be accomplished in an afternoon to save on diagnostic costs.