MFI simplification thread (1st draft)
 

The Bosch “timed indirect injection” or mechanical fuel injection (MFI) system used on early 911s up until the factory changed to a mechanical continuous injection system (CIS) half way through the 1973 model year is one of the most complex yet rewarding fuel delivery systems ever used on a production car. Below is an attempt to describe the various components of the MFI system in an easily understood manner. The goal is to make the operation of the system more understandable and thus assist with repair diagnosis. Please view this as an open solicitation to those with knowledge of the MFI system to suggest additions, deletions or modifications to any of these descriptions. I’d also like to add my thanks to all those who contributed to the original post which fleshed out some of these definitions. Here goes ...

MFI Overview
 

MFI Overview: The Bosch Mechanical Fuel Injection system is designed to inject exact quantities of fuel in relation to inducted air to achieve better combustion of the air fuel mixture. The system is comprised of two main parts, the Injection Pump and the Control and Compensating Unit. The main elements of the Injection Pump are the pump camshaft, roller tappets, injection plungers and plunger control rack. All of these elements are lubricated by the engine lubrication system through a connecting line and filter and an oil return line to the crankcase. The Control and Compensating unit’s major elements are the contoured cam and roller sensor, centrifugal governor, shut-off solenoid, enrichment solenoid, thermostat and barometric cell.

Operation: An electric fuel pump designed to deliver fuel at a rate of approximately 29 gallons per hour delivers fuel from the tank to the fuel filter. The fuel filter contains an overflow valve which regulates the fuel flow at approximately 12psi to the injection pump. The fuel pump is specifically designed to provide excess fuel capacity so that the injection pump stays a cool as possible. The excess capacity is routed back to the fuel tank via a return fuel line. The ignition pump is powered by a “spur belt” running off the left engine camshaft. The pump contains six injection plungers which are actuated by the injection pump camshaft. The plungers force fuel through six pressure lines of equal length into the injection nozzles in the cylinder heads at a pressure of approximately 210-265 psi. This system is known as “timed indirect injection” because injection is timed so that fuel is injected into the inlet valves just as they begin to open.

When the car is started and the accelerator pedal pressed, air is drawn into the engine through an air cleaner then down two triple-duct velocity stacks bolted on the throttle bodies. The air/fuel mixture ratio is maintained at 14.8:1 (1 pound of fuel to 14.8 pounds of air) by a complex “compensating unit” featuring a contoured cam mounted on the control unit camshaft and moved axially and laterally by a centrifugal governor and by the accelerator linkage. A roller sensor riding on the cam relays the required fuel volume to the rack via a “control lever.” A barometric cell is included to compensate for outside air pressure, a thermostat reacts to varying engine operating temperatures and an enrichment solenoid (on pre-1971 pumps) or a “thermoswitch” (on post 1970 pumps) provides correct fuel enrichment. Finally, a shut-off solenoid cuts off fuel supply when the car is decelerating in gear. All of these devices work together to move the plunger control rack back and forth to compensate for external variations and assure that appropriate levels of fuel are provided.

Here is a picture of Porsche six cylinder engine with the Bosch MFI system.
http://forums.pelicanparts.com/uploa...1202346303.jpg
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

Barometric Cell
 

BAROMETRIC CELL – compensates for changes in air pressure by varying the amount of fuel injected based on the atmospheric pressure. The barometer used on all timed indirect injection (MFI) Porsches is of the aneroid type and is made up of thin elastic metal disks contained in a chamber which sits on the top of the injection pump. The chamber is partially evacuated of gas and prevented from collapsing by a strong spring. The design of the chamber means that the barometer is not serviceable by a home mechanic. When the elastic metal disks are subjected to small changes in atmospheric pressure they are designed to shrink or expand. This expansion and contraction drives a mechanical lever inside the injection pump which alters the position of the control rack and allows the quantity of injected fuel to be increased when the car is operated at high air pressure (for example, sea-level driving) and decreased when operated at low air pressure (for example, mountain driving). The barometric cell is identified in the diagram of the post-1970 injection pump below.

http://forums.pelicanparts.com/uploa...1201286868.jpg
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

Cold Start Solenoid
 

COLD START SOLENOID – located on the top of the fuel filter, this device is an electromagnetic switch that mechanically opens a circuit allowing fuel to be injected when electric current is run through it. When the starter is engaged, an electric current goes through a thermal sensor located at the top center of the crankcase. This thermal sensor measures engine temperature. If the engine is cold, current is allowed to pass through to the Cold Start Solenoid which activates allowing an injection of raw fuel to be sent to the intake stacks. When power is removed from the starter, the cold start solenoid is shut off to avoid flooding the engine. The purpose of the cold start solenoid is to supply extra fuel only when the weather is cold and the engine is harder to start. The cold start solenoid is identified at #4 in the diagram below.

http://forums.pelicanparts.com/uploa...1235690811.jpg
© Dr. Ing. h.c. F. Porsche A.G., Factory Workshop Manual, Volume III, chapter 2, page 0.1-1/4. Posted here for educational purposes only.

Control Unit
 

CONTROL UNIT – is located inside the Injection Pump. The control unit consists of a contoured cam mounted on a fixed camshaft together with a centrifugal governor that controls the speed of the engine by regulating the amount of fuel admitted so as to maintain a near constant speed whatever the load or fuel requirements. When the accelerator is pressed the contoured cam is rotated and moved forward, back and axially on the camshaft by the centrifugal governor (a mechanical transfer device) so that fuel delivery can be altered based on engine load or speed. Riding on the contoured cam is a small roller sensor. This sensor maps the information received from the rotary motion of the engine, the accelerator position, the atmospheric pressure (from the barometric cell) and the engine temperature (from the thermostat) and transfers that fuel volume information to the control rack by moving the governor control lever. The purpose of the control unit is to supply the engine with different quantities of fuel under varying engine speeds and loads. The control unit and its various parts are identified in the diagram below.

http://forums.pelicanparts.com/uploa...1201217400.jpg
© Glenn’s Diesel and Gasoline Fuel Injection Manual. Posted here for educational purposes only.

Here is a picture of the three-dimensional space cam which is the heart of the control unit.
http://forums.pelicanparts.com/uploa...1202314230.jpg
© Bruce Anderson's 911 Performance Handbook. Posted here for educational purposes only.



Paul, screen name pjh69911, proposed this clarification:

If this makes more sense to the board I'll be happy to change the definition I suggested or merge the two. I just couldn't make a merger work but I'm open to suggestions!

Enrichment Solenoid
 

ENRICHMENT SOLENOID – is located beside the Thermostat on pre-1971 Injection Pumps. When the ignition is activated, a time-limit relay closes the solenoid circuit for two seconds moving it beyond the full power position and pushing the plunger on the control rack to allow maximum fuel rate delivery for starting. When engine temperature is below 50° Fahrenheit, a thermo-limit switch keeps the circuit closed for an appropriate period of time to allow additional fuel for starting. The purpose of the Enrichment Solenoid is to enrich the combustion mixture in cold or hot starting conditions. The Enrichment Solenoid was removed from the Injection Pump beginning in Model year 1971. On 2.2 and 2.4 liter engines, a thermoswitch in the breather cover operates the cold start valve. The pre-1971 Enrichment Solenoid is pictured below and at # 28 in the diagram of the early Injection Pump.


http://forums.pelicanparts.com/uploa...1201285501.jpg
© Haynes Porsche 911 Automotive Repair manual. Posted here for educational purposes only.

http://forums.pelicanparts.com/uploa...1201285518.jpg
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

Here is a picture of the thermo-time switch which keeps the enrichment solenoid closed longer than the 2 second time limit relay to assist with starting very cold engines. The picture is from this thread: cold start enrich '71E

http://forums.pelicanparts.com/uploa...1202245214.jpg

The function of the thermo-time limit switch is shown in this diagram from the same thread (originally from the CMA manual).
http://forums.pelicanparts.com/uploa...1202245980.jpg
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

Mechanical Fuel Injectors
 

FUEL INJECTORS - The six fuel injectors used in the Bosch MFI system are mechanical in operation. They are threaded into the cylinder heads behind each intake valve. The injectors operate by opening up when fuel is forced through the hard fuel line connected to the injector and through a filtering screen at approximately 210-265 psi. The fuel comes in from the bottom of the injector and is forced upward against a spring, where it forms a cone and is finely atomized and sprayed out of the injector into the intake port (a picture of proper fuel atomization is shown below). The injectors should be tested regularly to assure that they hold pressure and have a perfect spray pattern with a nozzle tester similar to the one shown below.

Below is a picture of a Bosch mechanical fuel injector for the MFI system:
http://forums.pelicanparts.com/uploa...1202166980.jpg

Here is a picture of the appropriate nozzle spray pattern for the Bosch fuel injectors:
http://forums.pelicanparts.com/uploa...1202168217.jpg

Here is the mechanical nozzle tester used to check for leaks and spray pattern.
http://forums.pelicanparts.com/uploa...1202232269.jpg
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

Fuel Pump
 

FUEL PUMP – is an electric roller-cell type with a delivery capacity of up to 33 gallons per hour. The pump is mounted on a bracket under the fuel tank. The fuel is delivered from the tank to the fuel filter. The fuel filter contains an overflow valve which regulates the fuel flow at approximately 12psi to the injection pump. The fuel pump is specifically designed to provide excess fuel capacity so that the excess fuel can be used to cool the injection pump. After flowing through the injection pump, the excess capacity is routed back to the fuel tank via a return fuel line. If fuel pressure increases above 14psi, an independent bypass valve installed in the fuel pump routes excess fuel to the fuel tank.

The Exact Fuel Pump Specifications are:
Regulated pressure = 14.2 PSI, 0.8±0.2 Bar
Flow rate = 125 liters/hour
Porsche P/N = 901.608.105.00
Bosch P/N = 058097 0001
Current draw = 3.5A @ 12V
RPM = 2800

The fuel pump is # 2 in the Fuel Injection System Diagram and its location is shown in the diagram below.

http://forums.pelicanparts.com/uploa...1201391944.jpg
© Chilton's Porsche 2 Repair and Tune-Up Guide (1973). Posted here for educational purposes only.

Injection Pump
 

INJECTION PUMP: The MFI Injection Pump consists of two major parts: (1) the pump assembly which contains the camshaft (#19 below), the six plunger units (#s11-14 below) and the toothed control rack (aligned horizontally behind #27 below); and (2) the control and compensating unit (described above). Each of the six plunger units consists of a plunger and a cylinder. Each plunger has teeth which mesh with the control rack on top and a roller tappet (#20 below) which rides up and down on one of the camshaft lobes on the bottom. As the camshaft rotates, the movement of the lobes forces the tappet into the plunger. At the same time, the various components of the compensating unit (thermostat, barometer, enrichment solenoid, etc) are sending instructions via connecting levers to the toothed control rack causing it to slide back and forth in the teeth of the plunger. This movement of the control rack causes the plunger to rotate exposing a slanted "metering land" in each plunger. This metering land is designed to close or open the inlet port to the fuel injector (see diagram below). The amount that this inlet port is open determines the quantity of fuel to be delivered with each stroke. As a result of these complex movements, the fuel contained in the plunger chamber is forced out in appropriate quantities through the hard fuel pressure lines then through the injectors and into the intake port.


pre-1971 MFI Injection Pump
http://forums.pelicanparts.com/uploa...1201285518.jpg
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.


Pump Plunger and Cylinder showing fuel delivery.
http://forums.pelicanparts.com/uploa...1202165988.jpg
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

Thermostat
 

THERMOSTAT - The thermostat is located on the front of the injection pump and is one of the compensation units on the pump along with the barometric cell and the enrichment solenoid. Its purpose is to compensate for various engine operating temperatures. The unit consists of several pairs of thermal expansion discs which shrink or expand based on engine operating temperature. The thermostat receives its heat signals from a double walled hose connected to the driver's side heat exchanger. When the engine is cold, the control rack is set to fully enrich the fuel mixture for easier starting. As the engine warms up, hot air is feed through the hose to the thermostat. This causes the discs to expand moving a lever in the injection pump which alters the control rack to lean out the mixture. Thermostat has no effect on the control rack after a temperature of approximately 113°. The thermostat is number 1 in the diagram of the Ignition pump above. Here is a detailed schematic of the thermostat and the expansion disks:

Schematic drawing of MFI Thermostat.
http://forums.pelicanparts.com/uploa...1201367276.jpg
Copied from the MFI thermostat spacers thread. Posted here for educational purposes only.

Shut-Off Solenoid, Micro-Switch and RPM Transducer
 

SHUT-OFF SOLENOID, MICRO-SWITCH AND RPM TRANSDUCER CIRCUIT: The shut-off solenoid is part of the control and compensation unit (on the lower right in the diagram below). Its function is to move the control rack to the fuel off position when the vehicle is decelerating in gear. The shut-off solenoid is controlled by a micro-switch in the accelerator linkage and an RPM Transducer on the relay panel which receives its RPM signal from the distributor. The RPM transducer senses engine speed. It is supposed to activate the shut-off solenoid cutting off the fuel supply when the throttle is closed (for example when the car is decelerating) and engine speed is above 1,500 RPM. If everything is working correctly, the solenoid is supposed to release when the RPM transducer senses engine speed dropping below 1,300 RPM regardless of throttle position. This allows the fuel supply to resume and the car to idle correctly. If engine speed is again increased, the solenoid circuit is interrupted by the micro-switch allowing the RPM transducer to again become activated at 1,500 RPM.

http://forums.pelicanparts.com/uploa...1201217400.jpg
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

The RPM Transducer is #5 in the diagram below and wired through #30b on the terminal.
http://forums.pelicanparts.com/uploa...1201317163.jpg
© Dr. Ing. h.c. F. Porsche A.G. Posted here for educational purposes only.

Here's a schematic of the RPM transducer thanks to Warren Hall - "Early S. Man" (may he rest in peace) and Mike Gillies of Brisbane, Australia.
http://forums.pelicanparts.com/uploa...1201317196.jpg

Finally, below is a simplified wiring diagram and description for the micro-switch and RPM transducer (#5 in the schematic). * Provided by Pelican member Jim727.

Pin 1: Output to shutoff solenoid;
Pin 2: Power from engine fusebox;
Pin 3: Ground and pin 31 of time limit switch;
Pin 4: CD Ignition and distributor.

The RPM Transducer (also known as the "Speed Switch") receives a signal proportional to the engine RPM from the distributor (contact 1 on component 1 in the diagram); this signal is also used to trigger the CD Ignition pulses (contact "C" on component 3 in the diagram). Circuitry in the RPM Transducer bridges contact 1 to contact 2 (on component 5 in the diagram) when engine RPM is above 1500 and opens the connection when engine RPM is at or below 1300 and falling. The final link in the chain is the microswitch which allows the output of the RPM Transducer to energize the shutoff solenoid when the throttle is closed and the above conditions are met.

http://forums.pelicanparts.com/uploa...1201379656.jpg
© Intereurope Workshop Manual, Porsche 912, 911T, 911E, 911S, page 62 (October 1972). Posted here for educational purposes only.

Very nice. Thanks for all the work to put this info together.:)

David -

It still says "...RPM Transducer on the relay panel which receives its RPM signal from the ignition coil". Your diagram indicates othewise.

Jim

In the "Fuel System" figure, the fuel pump in the diagram is #2, not #1

Jim,

Can you please explain what you are talking about in more detail? As I understand the system, the RPM Transducer gets it's RPM signal from the ground connection on the coil. I agree that it was confusing to say that they are "connected" but unless I've been misinformed (or read my reference materials wrong) the RPM signal is from the coil. This is what I'm trying to say anyway! Can you think of a better (or less confusing) way to say this?

I found this description of the RPM Transducer on the Question and Answer page but it doesn't help with this issue:

Thanks Steve. Even I should be able to tell the difference between the fuel pump and the fuel tank! ;)

Pin 1: Output to shutoff solenoid;
Pin 2: Power from engine fusebox;
Pin 3: Ground and pin 31 of time limit switch;
Pin 4: CD Ignition and distributor.

The RPM Transducer (also known as the "Speed Switch" receives a signal proportional to the engine RPM from the distributor (contact 1 on component 1 in the diagram); this signal is also used to trigger the CD Ignition pulses (contact "C" on component 3 in the diagram). Circuitry in the RPM Transducer bridges contact 1 to contact 2 (on component 5 in the diagram) when engine RPM is above 1500 and opens the connection when engine RPM is at or below 1300 and falling. The final link in the chain is the microswitch which allows the output of the RPM Transducer to energize the shutoff solenoid when the throttle is closed and the above conditions are met.

Great explanation! Thanks. I made the changes. Is it clearer now?

Works for me. Good stuff.

Jim

http://forums.pelicanparts.com/uploa...1202556859.gif

Now that I've got your attention. :D Any more corrections, additions or deletions?

What is the function of the injectors which go into the intake stacks?

From another thread, I believe that they are responsible for enrichment for cold start, etc.

My stacks are plastic. The injector is on the inside on each runner. Each is connected to a braided fabric line which eventually connect together and then lead to the top of the fuel filter. Is it clear what I mean?:confused:

I want to know how these work and whether mine need to be modified, as I have no heater, therefore thermostat/cold enrichment circuit. I have a screw type adjustment. Also, I have no plenum on the motor presently.

Thanks for any information!:)http://forums.pelicanparts.com/uploa...1202688616.jpg

Yup, it's a supplementary cold start nozzle fitted to the lower portion of the intake stacks. If the enrichment solenoid is operating correctly, this supplementary starting device is activated below 14°. Here is a picture:

http://forums.pelicanparts.com/uploa...1202690708.jpg

Thanks for the reply.

Since I do not have the heat exchanger hose connection to the enrichment circuit, do I need to block these supplementary injectors off?

The cold start enrichtment nozzles in your stacks are actuated by a solenoid on top of your fuel filter. This in turn is controlled by a thermo-time switch that sits in the breather cover on your engine case. It has no relation to the (missing) heat exchanger hose. It is just to help the engine start, after that, during the warm up phase the hot air from the heat exchanger comes into play. This controls the mixture during warm up.

So to answer your question: there is no need to block them

Peter's explanation is correct as is his conclusion that you do not need to block the nozzles. The cold start nozzle just gives an extra squirt of fuel to help start a very cold engine (under 14°). Even though the control rack is set to full enrichment when the engine is cold, the extra bit of fuel from these nozzles helps start a very cold engine. After you've started, the thermostat regulates the mixture up to about 113°. Since you live in Southern California I don't imagine these nozzles really ever come into play for you anyway!

Now if you remove them you might save ½ an ounce each. Think of the performance improvement. ;)

Thanks David and Peter for your technical explanations. I like learning more about the workings of mechanical fuel injection.

This is an excellent MFI thread, is there another one for CIS?

David, great posting, thanks for the leadership here..
Small correction in your second post, third paragraph, you state: The air/fuel mixture ratio is maintained at 14.8 psi
Of course you meant to say the ratio is 14.8:1, and the psi slipped in there someway...:)

A couple of questions on a fantastic thread...

- I assume the temps are in degrees farenheit, so the cold start temp of 14° is way below freezing, not the balmy temperature of our English summers?

- for a '73 MFI setup, as far as I can see there's nothing specific that happens when starting a warm engine, no "warm start" mechanism, no enrichment. Is that correct?

Thanks
Graham

Correct. For you Brits that's about -10°.

Correct again. On a properly running MFI system you don't need to enrich the mixture on a engine that is already warm (above approximately 113° Fahrenheit, 45° Celsius).

That'll be the problem then "properly running"! My current list of things to fix:

- injection pump (completely out of calibration) - now calibrated
- injectors (one completely duff and 5 v poor spray pattern) - new ones to come
- throttle bodies (shockingly poor) - to be replaced or refurbed
- fuel pressure only a few PSI - to be fixed

Hopefully getting that lot sorted will get me properly running and able to start a warm engine without pouring gas down the trumpets!

Graham

Good job. In addition to the MFI resources thread by DEC, this is as good as it gets. I recommended it for the Wiki.

We now ordain you Dr. Injection.

An awful lot of copyrighted information posted here. I hope this doesn't ruffle up any Pelican feathers.

It is absolutely not my intent to violate anyone's copyright. In fact, by noting the copyright and the source after each posted picture it is my hope that readers will seek additional information and go to the copyright source for additional reading. The purpose of this and my other MFI threads is solely to help educate the Porsche community about a very complicated system. By writing this stuff I learned quite a lot about MFI - not only from the sources cited but also from members of this board who have forgotten more MFI information than I'll ever know!

I do not claim to be an expert but I believe that limited use of copyrighted material - like in this and my other posts - is authorized for educational purposes:

The Copyright Act of 1976 specifically states:

picky, picky.

+1 on the PelicanWiki Great thread!!!!

Not at all! I'm not complaining, just pointing out the fact that there is copyright material presented. Wayne has flogged others for doing this. I wouldn't care if the entire book were posted here.

Thanks, really outstanding MFI documentation!

All MFI owners have at least the tools to become true MFI experts (and we need to...)

Regards from Prague.

Ed

I think I will raise this from the dead. I've looked at allot of information, but I don't see anything about adjusting the governor? The mixture screw sits on a round plate with four other screws; two silver and two black. Anybody know anything about adjusting these, or baseline settings?

BTW: PacificFI was the last ones into this pump and it looks like they assembled it incorrectly.

Try this thread for the screws: http://forums.pelicanparts.com/porsche-911-technical-forum/386414-mfi-pump-open-heart-surgery.html

What makes you think Pacific put it back together incorrectly? No offense intended, but I trust Gus more than you. SmileWavy

Yes, Gus does have a good reputation, but everybody makes mistakes. THe control lever from the barometric cell does not go into the lever. Its just sitting on top controlling nothing.