Main.TrailIndexPage | Fuel System | Fuel Injection

Fuel Injection

This component is part of the Fuel System. For a comprehensive look at how this all works together, see Engine Starting Aid Tour.


Bosch six piston injection pump, with their port-type injection.

Pump Types

Typical Pump ID Plate

Bosch TypeM B Part #Comments
PES 6 KL 70 B 120 R11127 070 07016 Element Injection Pump with 3 dimensional Cam
PES 6 KL 70 B 120 R18129 070 0301Up to Eng 10/20 000196 and 12/22 000136
PES 6 KL 70 B 120 R18z129 070 0601From Eng 10/20 000197 and 12/22 000137
PES 6 KL 70 B 120 R18y129 070 0801USA Spec with Additional Fuel Stop Solenoid
280SL (non-USA cars)  
PES 6 KL 70 B 120 R21130 070 02 01 
PES 6 KL 70 B 120 R23130 070 14 01 
PES 6 KL 70 B 120 R25130 070 14 01 


Download the parts list for PES6KLB120 injection pumps:

See also:

For German speakers here is a nice description with cut-open views:

Injection Pump Function


Mercedes-Benz began using fuel injection in their passenger production cars in 1955. The 300-SL gullwing used a direct injection, like diesel engines, it squirted fuel directly into the cylinders. It used a six piston Bosch injection pump. With this type of injection, timing was extremely critical. A degree or two deviation would make a noticeable difference and could even cause engine damage from improper combustion with unburned raw fuel ending up in the motor oil. Timing was carried out using a complex but extremely accurate process with a drip tube and fine pump adjustment to with-in one degree. The result was amazing performance, the trade-off was a very sophisticated, expensive, and sensitive injection system which required precise timing and careful upkeep.

Around 1960 Mercedes-Benz began using an indirect or port type injection. The fuel, instead of being squirted directly into the cylinders, was squirted into the intake above the intake valves. The fuel injected 220-SE and 300-SE engines of the era used a Bosch two piston injection pump with port type injection. The pumps actually squirted fuel for three engine cylinders at one time. The fuel simply waited around in the intake ports until an intake valve would open and then was drawn in for combustion. The system was amazingly smooth and efficient. Although less precise, it was much simpler and safer. Timing could be carried out without the sophisticated drip tube process. Some variation in pump timing would not effect performance noticeably. Naturally horsepower output was sacrificed for the simpler and more reliable system.

Around 1963 Mercedes-Benz began using a Bosch six piston injection pump, with their port-type injection. These 230-SL and 300-SE injected engines, squirted one cylinder at a time above their respective cylinders. The injectors were moved from the intake manifold and were relocated in the cylinder head right above the intake valves. The result was a horsepower increase over the two piston injection pump systems without the complexities and hazards of the early direct injection systems used on the 300-SL gullwings and roadsters. Timing again became a little more critical. The 113 cars with their Bosch six piston, port-type injection require a course (visual) setting of the pump during installation. This six piston injection pump squirts fuel to the intake of each cylinder. This coarse or visual method of setting the pump is probably accurate to within ten or twelve degrees.

These injection pumps will normally stay within adjustment specs, unless the timing is disturbed by mechanical failure or improper adjustment. The timing process does not need to be as exact as direct injection, however idle and power output, begin to degrade as the injection timing deviates more and more from the factory specs. These engines can run at 180 Deg off specs, but run very poorly. A healthy well tuned engine will thrive on a precise and well metered fuel supply. Be certain to maintain or re-set pump timing to specs during re-assembly.

It is not too surprising that the injection system works even if its timing is off a bit. After all, as RPM's increase, there is no way the injector spray could be perfectly timed at every RPM. As is the case in valve timing (perhaps with the exception of the new variable versions), a compromise is acceptable. However, precise injector timing should be considered a performance variable. The linkages must be correct, so you can begin your adjustments from a correct starting point.


The engine revolution does drive the pump, and as the engine rpms increase, the pump deliveries of fuel to the injectors increase respectively. The centrifical flyweights in the pump will increase the "volume" of fuel to the injectors as the rpms increase. So engine rpms increase frequency of fuel injection as well as volume of fuel injected via the centrifical flyweights.

The "rack" inside the injection pump is the member which rotates the helicentrical cut plungers in the injection pump. These devices change the "volume" of the fuel injected. Most adjustments, and external inputs to the injection pump, move the "rack" changing fuel volume injected.

The barometric compensator moves the rack, the "WRD" warm running device moves the rack, the centrifugal flyweights move the rack, and depressing the accellerator pedal moves the rack via the engine linkages. Remember, the rack changes "volume" of fuel injected. The frequency of injection is constantly varying with engine rpms. All these devices are doing their thing at the same time!

  • The small slotted screw with 10mm lock nut on the intake venturi is not an adjustment, but a factory stop for the venturi valve. It is frequently mis-adjusted. The large slotted air screw on the intake manifold is for idle air adjustment.
  • The idle fuel mixture adjustment knob on the back of the injection pump changes fuel mixture at low rpms only. (Turning clockwise richens fuel mixture, as mentioned, engine must be off, pay attention to the "clicks")
  • A rack adjustment screw can be accessed by removing a cap screw on the back of the pump (turning it clockwise leans the mixture at all rpm ranges).
  • Two other pairs of screws on the centrifical governor will change fuel mixture at different speed ranges (mid range and high rpm range). The attached photograph shows a view of the rear on the fip with it's cover removed. I've identified the main rack adjustment rod, the throttle rod, space cam, stylus and fly weights. The operation is very, very complex so I'm going to just discuss what happenes when the throttle linkage is moved on the outside of the pump. Imagine if you can that the engine is idling in neutral. The flyweights are in and restricted by there springs (more about that later). As you step on the gas pedal the throttle linkage inside the pump rotates the space cam up to approx 90 degrees clockwise. 90 degrees clockwise would have the pump at full throttle. As the space cam rotates because of throttle changes the stylus reads unjulations in the surface of the space cam and directly adjiust the rack and fuel volume to the engine.

To add another layer of complexity to the operation of the stylus and space cam as the rpms increase the space cam is allowed to move towards the rear of the pump and the stylus reads another track so to speak on the space cam. Working together the space cam, stylus, rack and flyweights are in effect similar to a modern engine management system. The fly weights help determine engine load by constantly being updated with rpm inputs. At various load factors the fly weights which are rpm sensative adjust the space cam so that it moves forward or backwards. The throttle rotates the space cam up to 90 degrees cw so the stylus will read a different section of the space cam and adjust the air fuel ratio accordingly.

I came to the realization concerning the roles of the three separate screws: they are anything but independent of one another. In other words, it is altogether too easy to over-adjust one set of screws, and overwhelm the adjustment on the next set in the progression. In general the white screws adjust high rpm partial load fuel delivery. Black screws adjust low to mid range partial load fuel delivery. The main rack adjusts fuel volume at all rpm's. The white, black and idle screws set the preload on the governor flyweights by adjusting three springs of different weights and lengths. The longest and lightest spring being the idle, and the shortest and heaviest the high speed. Adjusting the fuel injection pump is a balancing act with a constant need to compromise.

From their CW stop the white and black screw can turn nine CCW turns before the screw heads are flush with the aft edges of the flat springs that provide the "detent" action. Therefore the white and black screws have an effective range of 54 clicks.

The further the flyweights are allowed to open, the leaner it will run. "Left is lean" on that externally adjustable center idle screw. Left backs it out, and allows the flyweights to open further and pull the space cam further back. It would stand to reason that holds true for the mid and high speed screws as well. Space "cam" is actually a bit of a misnomer; it does not spin as one would expect a cam to spin. It looks nothing like a cam, either. More like a small metal hockey puck. It rotates about its axis for about 90 degrees or so when pulled up and down by an arm connected to the throttle lever. It moves fore and aft on its axis in response to the opening of the flyweights which is influenced by the three spring settings.

  • Shims under the barometric compensator and the WRD change the position of the rack or fuel mixture at all speed ranges.

All these inputs devices are acting on the "rack" at the same time. The back end of the injection pump is full of springs, levers, linkages, compound cams, and all sorts of intricacy that ties everything together so the "rack" can be moved to deliver precisely the correct amount of fuel.

Normal tuning may involve adjustment of the intake idle air screw and possibly an adjustment of the injection pump idle mixture knob. The small intake venturi slotted screw with 10mm lock nut is, unfortunately, often used as an adle adjustment by the uniformed. This is where years improper adjustment begins.

The actual "engine load" varies independently of rpms. In other words, road conditions, (hills, down hills), wind resistance, climate, altitude, temperature are just some of the external factors which alter the "load" or fuel requirement on the engine. With the very early mechanical injection systems, engine vacuum was used to determine engine load. A leather diaphram in the injection pump changed the position of the rack.

For those who can read German here is a nice description of an injection pump and its functional elements but even if one cannot understand the words there is still a lot of information in the detailed pictures:


Pin length in the WRD, is set to make sure that the extra cold idle air is closed off at the correct temperature. This is "ground zero" for the WRD adjustment. The oval shims adjust mixture during warm-up. The barometric compensator has round shims which can adjust the fuel injection at all speed ranges and temperatures. The more difficult "rack adjustment" also adjusts the injection at all speed ranges and temperatures. The large slotted screw on the intake, adjust idle air (all temperatures) but mostly used for warm engine idle. The thumb screw on the back of the injection pump adjust fuel mixture up to about 1,500 rpms (all temps), two dark color screws inside the injection pump (rear governor springs) adjust mid-range (1500-3500 rpms) all temps, and two light colored screws inside the injection pump (rear governor springs) adjust high range(above 3500 rpms) all temps

Before making injection pump settings check make sure your engine is in good tune. Make sure your engine linkages are correct, your ignition timing is correct, your distributor is working properly (vacuum retard, advance etc), and the fuel pressure and volume is correct. The barometric compensator on the IP, and the "heat feeler" in the WRD can also be defective. When the baro compensator goes bad, the injection usually goes lean. The WRD "heat feeler" can be measured or tested to make sure the cold engine idle air operates and closes off when it should.

Start by checking all tuning items and make sure the cold engine idle air closes off when it should. Next determine and adjust the fuel mixture on a warmed up engine all speed ranges. Normally a rack adjustment or baro compensator shim adjustment, idle mixture thumb screw adjustment or intake air screw will be sufficient. Internal adjustment of the pairs of governor screws is a fine tweek, but not normally needed. Lastly adjust the WRD for cold running. Add or remove oval shims to adjust fuel mixture during warm up period. Changing pin length will also alter the fuel mixture during warm-up, however you may wrongly change the shut-off temperature of the cold engine idle air.

There is a compound shaped cam inside the injection pump governor unit which develops wear grooves over time. This cam moves the rack depending on engine rpm. If the cam is badly worn, all these injection pump adjustments can compensate to an extent, but may not be able to solve every issue perfectly.

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