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Mechanical Fuel Injection Troubleshooting and Adjustment
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Pelican Technical Article:

Mechanical Fuel Injection Troubleshooting and Adjustment




MFI tune up tools

Applicable Models:

Porsche 911 (1969-73)

Hot Tip:

Make sure that all your systems are properly checked.

Performance Gain:

Lots of extra horsepower from a well tuned MFI system

Complementary Modification:

101 Projects for Your Porsche 911

This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.

One of the most complicated fuel delivery systems ever created was the Bosch mechanical fuel injection system, or MFI. The system was a precursor to the more modern electronic controlled fuel injection systems, and is actually a marvel of mechanical engineering. The system functions in a similar manner to the electronic systems that followed it, yet all the controls were almost completely mechanical. As a result, there is lots of tuning and adjustment required for the complicated system.

Unfortunately, there isn't a whole lot of information available on the subject. This project will give an overview of the MFI system, and also some pointers on tuning and adjusting it. For more information, it is recommended that you consult the factory workshop manuals, or if you can find it, the Porsche technical bulletin "4532.20 Check, Measure, Adjust" for the mechanical fuel injection system.

The MFI system consists of two main parts, the injection/distribution pump, and the throttle bodies. The electric fuel pump located in the front of the car supplies pressurized fuel to the injection/distribution pump. The injection pump is the heart of the system, and is responsible for metering and providing the fuel to each cylinder. The pump itself consists of two main parts, the fuel pump assembly, and the compensating unit. The fuel pump assembly is responsible for the actual delivery of fuel to each of the cylinders, while the compensating unit is responsible for adjusting the fuel delivery according to the levels set by both the accelerator and the speed of the engine. Precise coordination of all the elements of the engine are required for the system to function properly.

The MFI system was mostly used on Diesel cars where a timed injection system is absolutely necessary (diesel cars don't have ignition systems). In 1966 Porsche first used an MFI system on the 225 Horsepower Carrera 906 with a 2.0L engine. This engine was later installed in the venerable 911R. The advantage of an MFI system is that the fuel delivery is metered to the RPM of the engine. In general, the system supplies fuel to the cylinders in a more precise and aggressive manner than carburetors can deliver. Because the system was designed with a closed, non-vented fuel supply, the emissions were also much better than carburetors. Even compared to the electronic fuel injection systems of today, the MFI system has a very high injection pressure. Fuel is squirted out of the injectors at a remarkably high 220-250 PSI. This high pressure aids in the atomization of the fuel, which in turn increases the surface area of the mixture. This results in a more efficient and complete burn of the fuel.

The entire goal of any fuel injection system is to provide the proper mixture of air and fuel to the engine. The MFI system uses the injection pump to coordinate the amount and delivery of the fuel. The velocity stacks and throttle bodies mounted to the top of the heads regulate the airflow into the cylinders. In order to achieve the ideal air/fuel ratio, these two systems must be carefully coordinated. There is a lot that can go wrong with the MFI system, but when it is working it performs very well.

The fuel pump half of the injection pump works similar to the valve train in the engine. A small camshaft inside the pump rotates around, and pushes on small cylinders. These cylinders act as plungers that push fuel through the lines to each cylinder. As the engine rotates, the camshaft pushes the small fuel pistons up and down as each cylinder is fired. Close coordination with the timing of the engine is required to get the process just right. In a similar manner to the method in which the ignition system fires sparks, the fuel pump side of the injection pump pushes fuel through the lines to each cylinder in a timed and controlled manner. When the fuel reaches the cylinder, it exits through an injector embedded inside the cylinder heads. This side of the fuel pump is lubricated with oil fed from the engine. Two oil lines that connect to the top of the engine provide an oil supply and return for the internal mechanicals of the pump.

In addition to providing spurts of fuel that pulse with the speed of the engine, the injection pump also has to regulate the amount of fuel pushed during each spurt. In order to accomplish this, the plungers themselves are designed with a corkscrew-like groove cut into their side. The plungers are allowed to rotate in their bores, amounting to only half of a turn. As the plunger turns, the corkscrew effect basically allows more fuel to enter each plunger. The rotation of these plungers is tied into the throttle position. The greater the throttle position is, the more the pistons will turn, and more fuel will be delivered.

To recap, the up and down motion of the fuel pistons occurs in time with the RPM of the engine. The rotation of the fuel pistons is connected to the position of the throttle or accelerator. While this relationship is fine for many operating conditions, to obtain the required air/fuel mixture ratio, the engine must be supplied with different quantities of fuel under varying engine speeds and loads. Internal to the injection pump, a centrifugal governor works in conjunction with the throttle position lever to meter the fuel fed into the system. In addition, a barometric compensator is used to adjust to changes in altitude that might affect the air/fuel ratio. A warm up thermostat linked to the heat exchangers senses when the car needs a richer mixture upon cold startup. Finally, a shut-off solenoid reduces fuel delivery to the injection system when the throttle is closed, and the engine is coasting in gear.

Because the internal mechanisms of the MFI pumps are coordinated precisely with the timing of the engine, each pump is specifically designed to work with a specific engine. Swapping pumps from engine to engine is not a wise idea, and the car will probably never run correctly. The pump, distributor, and camshafts of the engines must be matched together for the MFI system to operate properly.

In many ways, the MFI system combines elements of the modern fuel injection systems, and the older style carbureted systems. Like carburetors, the MFI system uses a set of throttle bodies to meter and control the air flow intake into the system. Since the entire goal of the fuel injection system is to maintain the proper air/fuel mixture, the throttle bodies must be properly synchronized with the fuel injection pump. All of the linkage rods in the entire system must be properly aligned and synchronized. If one or more are off, then the car will not run properly. Of paramount importance is the length of the connecting rod that runs between the pump regulator lever and the cross shaft. The length of this rod must be set at exactly 114(0.2mm from ball center to ball center, in order for the fuel injection system to function properly.

There are a whole set of Porsche protractor tools that are used to adjust these rods. The tools are attached to the rods on the throttle bodies, and also on the injector pump. Changes in the angles of the throttle body butterfly valves have to correspond with angular changes on the pump lever. It's very difficult to accurately check and measure these settings without these tools.

Also very important is the adjustment of the air bypass screws. These are used to balance the amount of air entering each cylinder when the car is idling. To check these values, use a synchrometer similar to the one used to adjust carburetors (Pelican Technical Article: "Carburetor Adjustment, Balance, and Tuning"). Close the air correction valves, and then open them up 5 half-turns for 2.0 and 2.2 engines, and 3 half-turns for 2.4 engines. Start the car and let it warm up. Using the hand throttle, set the car to idle at 3000 RPM. Measure the air flow over each of the velocity stacks, and then write down the average of all the measurements. Now, using the air correction screws on the throttle bodies, adjust each cylinder until the average number is reached. If the screws open up more than 4 full turns, then there is probably carbon buildup on the inside of the passageways, and the throttle bodies need to be cleaned.

The adjustment of the idling speed is not performed at the injection pump, but rather with the air correction screws. To change the idle speed, evenly turn each of the six air-correction screws. Be careful to turn each one the same amount. Adjust the screws until the idle reaches 900 ( 50 RPM.

There are two different mixture adjustments on the MFI system. One adjusts for idle and low rpm, and the other for higher rpm when the engine is under load. You should not adjust these unless you have a CO meter handy and hooked up to the car. Make sure that the car is warm when performing the adjustments, and also make sure that the engine is off when you turn any of the adjustment screws, or you can damage the pump.

To adjust the idle mixture of the MFI system, you will need to turn the small adjustment screw that is located on the back of the pump. You must use a special long tool to reach through the blades of the cooling fan, in order to turn the adjustment screw. The adjustment screw is spring-loaded and must be pushed in towards the pump in order to engage the mechanism that controls the mixture. The adjustment screw is indexed, and you should feel it click as you turn it. To richen up the mixture, turn it clockwise. To lean it out, turn it counterclockwise. Turn it only about one or two clicks at a time, and only when the engine is off. Make sure that you monitor your readings with a CO meter after you change the mixture, and only take measurements when the engine is warm. On the 2.0L engines, you may have to use a flexible shaft to reach inside and access the mixture adjustment screw.

To adjust the higher rpm, or part-load mixture, you need to remove a small hex socket-head bolt from the rear of the pump. Inside the pump, there will be another socket head receptacle that you can turn to adjust the mixture. Turn the screw clockwise to lean out the mixture, and counterclockwise to richen it up. Note that this is in the opposite direction from the other mixture adjustment. For the 2.0L engines, you may have to loosen up the starter solenoid lever on the pump in order to gain access to the screw.

In general, when you are adjusting the mixture, make sure you take detailed notes of where you started, so that you can restore the car back to its original condition. Failure to do this may seriously disturb the balance of the MFI system, and make your car run poorly.

When installing the pump back onto the engine, there are a few things to be concerned about. Firstly the pump should always be carried by the bottom. The gold tin can that looks like a good handle on the top of the pump is actually the barometric compensator, and can be seriously damaged if you pick the pump up by it.

Once the pump is mounted on the engine, the toothed drive belt must be properly set in order to synchronize the pump with the main crankshaft. There is a mark located on the pulley hub of the MFI pump. Make sure that this mark is aligned with the small notch on the case of the pump. Then place the engine at TDC for cylinder number one. Refer to Project 18 for more details on this procedure. Then rotate the engine 360 degrees. The engine should now be at TDC for cylinder number four. Then rotate the engine until the FE mark on the crankshaft pulley aligns with the mark at the bottom of the fan housing. At this point, install the toothed belt. The pump should now be synchronized with the engine.

So what can go wrong with the MFI system? Apparently a few potential problems have been isolated over the past 30 years. The MFI systems are often known for running too rich. One of the causes of this problem is that the heater hose that connects to the thermostat on the back of the pump is not properly installed. This hose is connected to the heat exchangers, and heats the element inside the pump to lean out the mixture. When the car is cold, the mixture is rich to aid in starting and warm up. If the hose is disconnected or damaged, then the element does not get heated, and the pump is set to run rich all the time. Make sure that the hose on your car is firmly connected, and installed properly. There is also another hose that is connected to the heat exchangers that is attached to the air cleaner on 1972-73 911s. This hose is part of the emissions control, and doesn't affect the performance of the engine.

The shut-off solenoid is used to stop fuel delivery when the engine's RPM is high, yet the accelerator pedal is not pressed. This occurs when coasting along in gear, most commonly down a hill or along the highway. The shut-off solenoid significantly reduces the delivery of fuel to the engine during this moment. Without the shut-off solenoid in operation, raw fuel would be dumped down into the cylinders with the throttle bodies closed. In this condition, stepping on the accelerator can generate a huge backfire in all cylinders. The shut-off solenoid is controlled by a small microswitch linked to the main throttle rod. Make sure that this switch is properly adjusted to be electrically closed when the throttle is completely closed (foot off of gas pedal).

Another source of problems with the MFI systems is the throttle bodies wearing out, similar to carburetors. The bearings on the butterfly valves in the throttle bodies can become worn out after about 100,000 miles. Sometimes the valves are so loose that they can be heard flapping when the engine is running. If the bearings on the butterfly valves in the throttle bodies become too loose they can cut a groove in the sides of the chambers. If a groove is scored, then the throttle bodies need to be scrapped. It's a wise idea to get the throttle bodies rebuilt and new bearings installed before significant damage occurs.

Sometimes the oil seals in the pump fail, causing gasoline to leak out of the pump and into the crankcase of the engine. If you are checking the oil on an MFI car, and the oil level appears to be rising, it is most certain that your pump is leaking gas into the oil. The oil will also smell a bit like gasoline. Empty and change the oil at once: as the gasoline will just eat up the bearings inside of the engine and force you to perform a rebuild.

The timing of the engine is very critical for the proper operation of the MFI systems. Since the fuel is delivered at a very high pressure within a specific moment of time, the ignition spark must be correctly coordinated with the injection system, otherwise the car will perform poorly. Double and triple check the timing on MFI cars to make sure that it is set correctly.

The factory technical publication has a pretty good checklist to follow when you are looking to find problems with your MFI system:

Check the air cleaner for blockages and clogs

Check the engine to make sure that you have appropriate and even compression in all the cylinders

Examine the spark plugs for deposits and also inspect the ignition wires (See Pelican Technical Article: Ignition Tune-Up)

Check the dwell angle of the points (See Pelican Technical Article: "Setting the Timing, Dwell, and Idle Speed")

Check and set the ignition timing (See Pelican Technical Article: "Setting the Timing, Dwell, and Idle Speed")

Check the fuel pressure and flow to the injectors

Inspect the injectors for dirt and buildup

Check the injection timing and synchronization with the engine crankshaft

Measure and adjust the linkage arms of the MFI system for their proper correlation to the MFI pump

Perform an emissions test to check CO levels at idle and also under load

With a little bit of know-how and the right tools, the MFI system can perform quite well. The 911S in 1969 was the one of the first production Porsches to use the MFI system and was rated at 170 Horsepower: quite a feat for a small 2.0L motor. If you are planning on performing your own tune up and maintenance work on your MFI system, it is suggested that you obtain a set of the measurement tools, a set of factory manuals, and also the MFI technical bulletin (4532.20).

The three primary components of the Mechanical Fuel Injection system are the injection/distribution pump (upper left), the velocity stacks (upper right), and the throttle bodies (lower right).
Figure 1

The three primary components of the Mechanical Fuel Injection system are the injection/distribution pump (upper left), the velocity stacks (upper right), and the throttle bodies (lower right). This is a recently rebuilt pump for a 2.4S motor. The large gold can on the top is the barometric compensator, which should never be used to pick up the pump. The large silver cylinder hanging off of the rear of the pump is the shut-off solenoid. The large black cylinder on the back of the pump is the thermostat that is connected to the hose that provides hot air from the heat exchangers. The velocity stacks are made of plastic and have bearings pressed into them for the throttle body linkage. The throttle body shown here has new butterfly valves, and newly adjusted linkage arms.

Shown installed on the 911, the throttle bodies and velocity stacks are placed directly on top of the cylinder heads.
Figure 2

Shown installed on the 911, the throttle bodies and velocity stacks are placed directly on top of the cylinder heads. The complexity of the throttle linkage is shown here. Each of these rods must be carefully measured and adjusted or the mixture for one or more of the cylinders will be off. Also shown here is the microswitch that indicates the throttle is completely released (shown by arrow). This switch activates the shut-off solenoid which prevents the pump from dumping raw fuel into the cylinders when there is no air to mix it with. The injectors for the MFI system are screwed directly into the cylinder heads, making these heads somewhat unique.

Shown here are the locations of the two mixture adjustment screws.
Figure 3

Shown here are the locations of the two mixture adjustment screws. The one on the left is the part-load mixture adjustment, or higher-rpm adjustment. The hex socket-head screw shown here needs to be removed before the adjustment socket inside can be accessed. The photo on the right shows the low-rpm or idle mixture adjustment screw. This screw needs to be pushed in and turned in order to change the mixture. You should feel distinct clicks when you turn the screw in either direction.

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Comments and Suggestions:
gigclick Comments: Thank you for this detailed article, it was very helpful.
November 23, 2013
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts

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