MFI Simplification Project - Help wanted!

[David E. Clark]

In my never-ending effort to simplify the mysteries of the Bosch/Porsche MFI system and help us all to be better equipped to diagnose and repair problems with the system, I think it might be helpful to have plain English explanations of how each part of the system works and how it relates to the other parts. The definitions in the Factory Workshop Manual are OK but I think we might all benefit from simpler (or at least better written) explanations. I figure that if we understand exactly how everything is supposed to work we'll be better equipped to diagnose problems when they occur. The various parts I have identified for this project are:

1. Barometric Cell

2. Cold Start Solenoid

3. Control Unit

4. Enrichment Solenoid

5. Fuel Pump

6. Injection Pump
a. Control Rack
b. Camshaft
c. Contoured Cam
d. Roller Tappet
e. Centrifugal Governor

7. Shut-off Solenoid

8. Thermostat

Of course I am open to suggestions - including “no need for this project.” Please consider this an open solicitation to those versed in the MFI system to take a crack at some of these “plain English” definitions. Since it’s my idea I’ll start with the first on the list:

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 servicable 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 below.

[shbop]

bump for more info.

[David E. Clark]

OK guys, I don't want to write all of these myself! Here's what I've come up with for number 2:

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 in the diagram below.



I know that lots of you know more about MFI than I do. Please contribute.

[David E. Clark]

Anyone want to contribute?

[RWebb]

I don't have MFI, but I thin a basic 'theory of operation' paragraph would be nice. Then follow up with the deviations from basic operation (starting; cold start; and changes in pressure, etc.).

This might also be a good place to post the links to the numerous threads on MFI.

[Zeke]

I worked on mine enough to know the basics. Is there something missing? The thing wouldn't return to 900 RPM if the micro switch didn't close on deceleration (along with the fuel shut off). Isn't there something in the system that knows the RPM (other than the flyweights)?

The whole thing is a nightmare, AFAIC, but I did enjoy the song it produced. I'd still take EFI over anything. I know, this doesn't help your thread, but the 6 individual EFI units on carb or MFI bodies fascinates me.

[David E. Clark]

Originally Posted by RWebb

I don't have MFI, but I thin a basic 'theory of operation' paragraph would be nice. Then follow up with the deviations from basic operation (starting; cold start; and changes in pressure, etc.).

This might also be a good place to post the links to the numerous threads on MFI.

I agree that a "theory of operation" introduction will be helpful. My plan for this thread is to get all the definitions, explanations and theories of operation sections written and then I will edit them all into one cohesive document and post that to the Ultimate MFI resources thread.

I've attempted to gather most of the useful MFI threads in the MFI Message Board Index. Any additions or suggestions are always appreciated. I've been rewriting this Index to make it alphabetical by topic but, as you might imagine, gathering all the links and properly posting and cross referencing them takes time.

I should have a definition of "control unit" posted here by tonight. That leaves lots of other definitions ripe for the taking!

[304065]

David, expect some contributions here from me in the near future, I have lots rattling in my head and I know where to find a lot of interesting information. As an evolution of this, we need to assign out test projects e.g. what is the thermostat rod change with temperature to various MFI types to report back the data.

In the open heart surgery thread, there was some info on the travel of the baro cell plunger with pressure, that should be appended to the description.

[David E. Clark]

Here's my try at # 3:

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.

[pjh69911]

David,
Just to clarify a few points. The MFI is not proportional control. It is a very non-linear system. A doubling of speed does not result in a doubling of fuel, because of the non-linear effects of the intake, tuned runners, etc.. The same is true for the throttle position. The governor in your picture is really a mechanical transfer function that converts rotary motion(engine speed)into an axial displacement of the follower on the space cam. The other input is throttle position, which rotates the space cam. These two independent inputs result in a unique mapping on the space cam, then modified by the barometric cell and thermostat to result in a rack displacement=fuel delivered. Paul.

[David E. Clark]

Originally Posted by pjh69911

David,
Just to clarify a few points. The MFI is not proportional control. It is a very non-linear system. A doubling of speed does not result in a doubling of fuel, because of the non-linear effects of the intake, tuned runners, etc.. The same is true for the throttle position. The governor in your picture is really a mechanical transfer function that converts rotary motion(engine speed)into an axial displacement of the follower on the space cam. The other input is throttle position, which rotates the space cam. These two independent inputs result in a unique mapping on the space cam, then modified by the barometric cell and thermostat to result in a rack displacement=fuel delivered. Paul.

Paul,

Thanks for the reply and the clarification of the relationship between engine speed and throttle position. My goal is to simplify these definitions. The Control Unit is a complex system and I'm sure I may have simplified too much! That said, I have always understood "proportional control" to mean that the system has the ability to expand or decrease inputs in proportion to its needs. Said in the simplest way I know, the fuel delivered is in proportion to the engine's needs. Thus, when the car is accelerating a proportional control fuel system has the ability to deliver appropriate amounts of fuel to allow the driver to reach the desired speed notwithstanding whether the car is going uphill or downhill or is fully loaded or not. I agree that this is absolutely a non-linear system. This is what I meant in the above definition. If this is incorrect, please help clarify. In addition, I would be happy if you would cut and paste your clarifications into my definition above so that it is more complete (keeping in mind the goal of simple, easy to understand, explanations). As I said earlier, there are many people on this board that know a lot more about MFI than I do!

[pjh69911]

David,
Any control system relies on feedback. In MFI there is no feedback. It is strictly open loop. A feedback parameter would be exhaust oxygen content via a sensor. If the measured parameter gets too high, a proportional control system then makes a proportional adjustment to an input parameter, which then changes the fuel output, for example. The MFI system doesn't know what is the appropriate amount of fuel to give, it just responds to the inputs(speed, throttle, temp and baro) and returns a fuel quantity. Paul.

[David E. Clark]

# 4:

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.

[David E. Clark]

Originally Posted by RWebb

I don't have MFI, but I thin a basic 'theory of operation' paragraph would be nice. Then follow up with the deviations from basic operation (starting; cold start; and changes in pressure, etc.).

This might also be a good place to post the links to the numerous threads on MFI.

Please take a look at this overview summary and let me know if it makes sense (especially in conjunction with the individual element summaries). It's surprisingly difficult to simplify a system as complex as MFI into a few paragraphs! Any suggestions or corrections are appreciated.

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 injection 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 psi 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.

[David E. Clark]

Originally Posted by milt

... The thing wouldn't return to 900 RPM if the micro switch didn't close on deceleration (along with the fuel shut off). Isn't there something in the system that knows the RPM (other than the flyweights)? ...

Yes, connected to the ignition coil (on the relay panel) is an "RPM transducer" which "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. Hope this helps and thanks for any help you can provide for this project.

['71 911E Coupe]

Originally Posted by David E. Clark

Yes, connected to the ignition coil (on the relay panel) is an "RPM transducer" which "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. Hope this helps and thanks for any help you can provide for this project.

Does anybody have a picture of this? Does anyone still sell this part?

I am having a problem with my MFI, where just out of the blue it stalls randomly and then you can start it right back up and its fine, or sometimes when you run the engine, the RPM doesn't go quite down to 950 and then stalls. Figured this could be a suspect of the problem, since it has to do with the RPM sometimes.

[David E. Clark]

Here you go, it's number 3 and/or 5 in this diagram.



Here's a schematic thanks to Warren Hall - "Early S. Man" (may he rest in peace).



From your brief description, this doesn't sound like your problem. Usually if the RPM transducer is faulty you will get lots of backfire. Since you didn't mention this, I'm assuming that you are not getting backfire. I'd start by checking the more common culprits for stalling like a faulty coil, points/plugs fouled or not properly gapped or pitted, fuel supply interruption (clogged filter or screen, etc) or intermittent vacuum leaks. Hope this helps and thanks for any help you can provide for this project.

['71 911E Coupe]

Thanks David! I'm gonna look into the problem tomorrow and check things like you said, fuel filter, inginition timing, injection timing, etc. Correct, I do not have any backfiring at this time, thankfully. So I believe what you said is true, based on all that I have read. Maybe this can help some others out too, I hope.

By the way, this thread is a good idea guys, and I hope to contribute as much as I can.

['71 911E Coupe]

A little bit our of order but here is my attempt at # 8.

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. The thermostat is feed by a hose from the driver's side heat exchanger with hot air. While hot air is being feed to the thermostat, the metal discs spread apart from each other and a rod that the discs ride on, is pushed into the injection pump, leaning out the mixture. This is because you do not need as much fuel after the engine warms up. These discs should be cleaned up every now and then since they can get all cruded up. DO NOT MIX THE DISCS UP THOUGH. Put them back on, in the order they came off. Make sure you are getting hot air to the thermostat! Some racing applications replace this with a screw so that the mixture can be changed much faster.

[David E. Clark]

Thanks! The thermostat is number 1 in the diagram of the Ignition pump above. Here is a detailed schematic of the thermostat:



Anthony,

I agree that all the experts say don't mix up the thermal disks but do you (or anyone else) know why? They seem identical to me. I realize that the spacers are different (one thicker than the other) and that the shim must not be left out but the disks look identical to me. If they are put back correctly paired against each other I don't understand why they need to go back in the exact order they came out?