Bosch D-Jetronic Mechanical Fuel Injection

Resources

• Barnett, J. (n.d.). Technology explained: Mechanical Fuel Injection | Total 911. Retrieved April 4, 2017, from https://www.total911.com/technology-explained-mechanical-fuel-injection/
• Darby, S. M. (n.d.). Top Automotive Innovations: History of Fuel Injection. Retrieved March 20, 2017, from http://bestride.com/news/top-automotive-innovations-history-of-fuel-injection
• Dempsey, W., R., & Hunt, J. (n.d.). Porsche 911 Bosch Mechanical Fuel Injection Overview | 911 (1965-89) - 930 Turbo (1975-89) | Pelican Parts DIY Maintenance Article. Retrieved March 13, 2017, from http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm
• Dempsey, W., R. (n.d.). Porsche 911 Mechanical Fuel Injection Troubleshooting and Adjustment | 911 (1965-89) - 930 Turbo (1975-89) | Pelican Parts DIY Maintenance Article. Retrieved March 28, 2017, from http://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/30-Mfi_Info/30-Mfi_Info.htm
• 911 mechanical fuel injection set up. (n.d.). Retrieved March 22, 2017, from http://www.stuttgartperformanceengineering.com/mech_fi_sertup.html

How This System Works

• The Injectors.

• Are not electronically controlled.

• Comprised of a mesh filter, reducer block, delivery tip, and seating spring.

Common Problems

• Running too rich

• The Pistons.

• Comprised of a shaft cut with a helical groove (Metering land) on top and a cross-cut shaft.

• The thermostat is used to help lean out the fuel mixture. (pump is set to rich to help with starting and warm up.)

• One cause is the heater hose is improperly installed on the pump.

• They pump fuel into the injectors by rotating one way to enrichen mixture, the other way to lean it out.

• Shut-off solenoid not working properly.

• Controlled by a toothed rack that moves the pistons. The rack is driven by the space cam.

• Shut-off solenoid is used when engine RPM's are high and accelerator pedal isn't pressed.

• Has small return springs that help rotate the pistons.

• Without this switch, raw fuel will be dumped the cylinders, causing backfire.

• Oil seal failure in the pump

• Failure will cause the gasoline to leak out of the pump and into the motor oil.

How It Works Continued.

• Barometric Compensator.

• Consists of a brass bellows which push down on a mushroom pin and a return spring.

• Adjusts fuel mixture based on altitude.

• The gasoline will eat the bearings in the motor, prompting a rebuild.

• Solenoids

• One is used for fuel shutoff and the other is used for fuel enrichment.

• Oil Inlet Feed

• The "Ten Demandments."

• A list of diagnostic procedures.

• Or you can replace the system with Weber Carburetors, which run at $4000.

• Mostly used to try and diagnose a poor running car.

• Injectors can be cleaned and tested for $25 per injector.

• Main criteria for a rebuild: Worn space cam.

• If pump needs to be rebuilt, it costs about $1650. (By a pro)

Diagnosing/Repairing the Pump

• The lower half of the pump is lubricated by engine oil.

• Filters out contaminants in the oil.

• Oil finds its way back to the motor by an overflow fitting.

Pump Timing/Metering

• Pump is mechanically timed.

• Each injector will squirt fuel once every two revolutions or at the beginning of each cylinder's intake event.

• Delivery is dependent on two algorithms, engine speed, and throttle opening.

• Metering

• Is related to both engine speed (linear) and throttle opening (est. of needed fuel per degree of throttle opening).

• Space cam wasn't an accurate model of metering at part-throttle conditions.

• Car would develop a lean surge at light or part throttle.

Parts of the System

History of Mechanical Fuel Injection

• Invented by Jonas Hesselman in 1925

• Built the world's first direct-injected engine

• Primarily used in Aircraft. (Around WW2)

• Effectively prevented stalling of aircraft during high speed maneuvers.

• Bosch, during the 1950's, developed a MFI system for gas-powered automobiles.

• First used on the 1955 Mercedes-Benz 300 SLR.

• Later used on MB road cars and various models of the 1968-1973 Porsche 911.

Bosch D-Jetronic Mechanical Fuel Injection

Advantages over Carbs

• Had a closed, non-vented, fuel supply.

By Sawyer DeHart

• Carbs used open float-bowl vents

• Adjustments can be easily made.

• Carbs would require removing the whole upper portion to adjust float level.

• Has automatic cold-start enrichment.

• Carbs would have to use a choke for cold-start.

• Also made cars emissions legal in the U.S. (and made more power too)