Tech Tips

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Fuel System

Fuel Injection Problems

BOSCH PUMPS MATCH OEM REQUIREMENTS
The heart of every electronic fuel injection system is its electric fuel pump.  Usually located inside or near the fuel tank, the pump's job is twofold:  to push fuel from the fuel 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, otherwise 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 fuel requirements of the engine.  Certain Audi models with Bosch Motronic, for example, 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.

NO-START CONDITION
 A diesel engine that cranks normally but won't start regardless of the outside temperature either has low compression or a fuel delivery problem.  If compression is okay, check the fuel gauge to make sure you have fuel in the tank.  Then check the fuel filters and lines for obstructions.

If a distributor-type injection pump isn't pushing fuel through the lines to the injectors, it may have a faulty solenoid.  Listen for a "click" at the pump when the ignition switch is turned on.  No click means the solenoid and/or pump needs to be replaced.  If it clicks but there's no fuel coming through the injector lines (and the filter and lines are not obstructed), the pump is probably bad and needs to be serviced or replaced.  Any Bosch authorized diesel service shop can check the pump's performance.

INJECTOR PROBLEMS 
Diesel injectors can suffer from the same kinds of ailments as gasoline injectors including varnish deposits, clogging, wear and leakage.  Bosch injectors are designed to minimize such problems and are made of heat-treated high alloy chrome steel to resist wear, and have needle-to-body gap tolerances that are plus or minus 0.5 microns for maximum fuel delivery efficiency.

Because diesel injectors operate at much higher pressure than gasoline injectors, their opening pressure can drop over time.  Up to 300 psi is considered acceptable, but more than 300 psi means the injectors should be replaced or reset back to their original operating specs.  You'll need a pop tester to check the opening pressure of the injectors if you suspect this kind of problem.

You may find that one or more injectors need nozzle tips.  Replace them with new Bosch nozzle tips, not "reconditioned" nozzles.  Reconditioned nozzles may cost less, but often do not perform or last like new nozzles.  Regrinding a nozzle alters the needle-to-seat position and changes the spray pattern.  As the spray holes wear, spray pattern and fuel atomization are affected as well.  This, in turn, can have an adverse effect on fuel economy, noise and emissions.  Used nozzle tips also lose their temper from long-term exposures to high operating temperatures - which means they won't hold up over the miles like a new injector.  

Dirty injectors will lean out the air/fuel mixture, causing a loss of power, rough idle and, sometimes, white smoke in the exhaust.  Leaky injectors will richen the air/fuel mixture and cause black smoke.  

Black smoke is usually a signal that there's too much fuel, not enough air or injector pump timing is off.  One of the most common causes of this condition is an air inlet restriction.  The cause may be a dirty air filter, a collapsed intake hose or even an exhaust restriction.

White smoke usually occurs when there is not enough heat to burn the fuel.  The unburned fuel particles go out the tailpipe an typically produce a rich fuel smell.  It's not unusual to see white smoke in the exhaust during cold weather until the engine warms up.

Bad glow plugs or a faulty glow plug control module can cause white smoke on engine start up.

If white smoke is still visible after the engine has warmed up, the engine may have one or more bad injectors, retarded injection timing or a worn injection pump.  Low compression can also be a source of white smoke, as can air in the fuel system.  

STALLING PROBLEMS
If a diesel stalls when decelerating, check the idle speed.  If low, it could prevent the pump governor from recovering quickly enough during deceleration to prevent the engine from stalling.

Water in the fuel also can cause stalling by making the metering valve or plungers inside the pump stick.  The pump will need to be cleaned or replaced.

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