73-911E-Targa

Hello all!

On the Pelican site I've created a thread to describe the various components of the MFI system in what I hope is 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. Any help will be greatly appreciated! Here is the link:

MFI simplification thread (1st draft)

Peter Zimmermann

I'm not sure how to approach this subject, but will start by describing the recommended diagnostic sequence. MFI is like your TV, you don't take it apart, after discovering that it won't work, until after you check to be sure that it's plugged in. The 911 is similar in that certain things must be checked prior to performing work on the MFI system. Those checks must be done in the following order:
1. Is the air filter cartridge clean and unobstructed?
2. Does the engine have compression loss? This should be determined by performing a leak down test to establish that a maximum leakage of 10% is not exceeded.
3. Spark plugs are of a satisfactory heat range and gapped correctly, and plug connectors measure within factory resistance requirements.
4. Ignition point dwell angle must be correct.
5. Ignition timing must be correct.
6. Measure fuel pressure and flow.
7. Verify that Injection Timing End of Delivery Stroke is set correctly (this is often done wrong following replacement of the drive belt on the MFI pump).
8. Check spray pattern of the six injector nozzles.

Some of these tests are often assumed to be correct, but I've found that the items listed will fix about 98% of an MFI 911's running problems. Long before messing with the MFI pump, except for cleaning the warm-up thermostat and making sure that the double wall hose for warm air supply to it is in good condition, there are other basic procedures to follow. The first is a test to check the air-correction screws/channels (one screw per cylinder mounted near the bottom of each throttle body and fitted with a spring). The channels that these air-correction screws fit into can become plugged with carbon, the screws should be removed one at a time (don't mix them up!), and Chemtool B-12 should be sprayed, with the help of the thin plastic tube that comes with the can, into the screw hole. If the air channels are clean a noticeable change in idle speed will be noticed before each screw is opened (turned counter-clockwise from its seated position) by no more than eight half turns. This air flow change can be measured using a Synchrometer or a Uni-syn.
Also, before running-related problems are blamed on the MFI pump, throttle valve correlation and push rod adjustment must be checked/set. The basic adjustment of the push rods can be done using a proper vernier caliper, but correlation requires a set of special protractors/indicators.

I also want to make readers of this thread aware that MFI pumps have significant internal differences. Because of that, each pump has an I.D. plate attached to it, and the plate has a Bosch number. Those numbers are as follows:
1969 911 E (early) - 0408 126 002, (late) - 0408 126 006
1969 911 S (early) - 0408 126 001, (late) - 0408 126 005
1970/71 911 E - 0408 126 010
1970/71 911 S - 0408 126 009
1972/73 911 E - 0408 126 014
1972/73 911 S - 0408 126 013 or 021
1972/73 911 T - 0408 126 015

I hope that the above comments help!

73-911E-Targa

Peter,

Thank you very much for your comments. This project is not designed as a repair project, per se. Instead, I'm attempting to identify and describe the major components of the MFI system and their relationship to each other. I've always found that if I understand how something is supposed to work, I'm much better equipped to diagnose problems should the need arise. I'm working off the assumption that there are others like me out there.

I've always found that MFI is a very reliable system once properly set up and, like you, I've found that the MFI pump is rarely the problem and never the place to start a diagnosis. The factory publication MFI Check, Measure, Adjust, followed to the letter, is the best place to start if you've determined that your problem is fuel injection related. The Ultimate MFI resources thread and MFI Message Board Index also contain loads of useful resources.

What I'm requesting is that people with MFI knowledge review the MFI Simplification thread and give it check for correctness and "readability." Once I've got a useful and readable synopsis of the system and it's various components, I intend to include it in the Ultimate MFI resources thread as another resource for folks trying to understand their system. Thanks again in advance for any additional comments or suggestions you have.

David

73-911E-Targa

Now that I've got your attention. Any more corrections, additions or deletions to the MFI Simplification thread?