Porsche Mechanical Fuel Injection - Bosch MFI

Porsche developed a mechanical fuel injection system, used versions of it on numerous race cars, and also used it on several production models in the early 1970s, most famously the early 911, including the iconic 1973 Carrera RS

The big advantage of the MFI system over the other fuels systems of the time was it's instantaneous throttle response, and it is still favoured today for classic 911 racing.
Being an open-loop system, the quantity of fuel supplied to each injector was regulated simply by engine speed, throttle position, and had some other inputs, such as barometric pressure, and engine operating temperature.

With todays technology, it is possible to use modern Electronic Fuel Injection systems (EFI), but still it is argued that nothing beats the MFI, both for simplicity, and performance.

The drawback with MFI is finding a good mechanic who can work with this system, and get it tweaked/adjusted to its optimum performance.
But all is not lost. At first glance, the system seems complex, and a lot of people simple dump it in favour of either the later CIS injection system, or convert to carburettors, which were also used on the early 911 engines.

In July 2005, after many months/years of searching and researching, I finally took the plunge and bought a 1975 911 Coupe. I was not knowledgable enought at the time to realise what I had just bought! The original 2.7 engine from 1975 would have had the CIS fuel system. However, this car had undergone a detailed restoration around 1995. During that restoration, I was told it had undergone lots of changes.

Upgrade to oil-fed cam chain tensioners (the previous style were prone to failure)
Turbo style lower cam covers
Front mounted 1984 spec copper pipe oil cooler
High Flow oil pump
New Valves
Gearbox oil cooler
Upgraded brakes to later 3.2 carrera spec all round
RS Bilstein suspension
Later 915 gearbox from 3.2 carrera with factory short-shift kit
Stainless brake hoses
Brake servo
Complete Burgundy leather interior from a 930 (911 Turbo)
Stainless Steel Heat exchangers

So, I drove it for a while, figured it was running a bit rich (black smoke, and rough idle) and decided I would read up on the MFI system and find out how to adjust it. Little did I know what lay ahead !

I found lots of useful information about MFI on the Pelican Parts forums, and I needed some special tools to adjust the MFI pump, so I bought those from Pelican Parts in the USA.

I tweaked the idle mixture setting a bit, without much success.

I drove it for another bit, just every now and again, and although it was a fantastic drive, and performing really well, I knew it just wasn't right. It was a bit embarassing sitting in traffic with black smoke bellowing out the exhaust.

I read through the famous Porsche document Check, Measure, Adjust known as the CMA a few times, and decided I'd better get an expert to sort it out for me. Trouble was, I was in the Northwest corner of Ireland, and the closest "god MFI guy" was in the UK. I though about it for a while, and put it on the back burner.

Aside:
In the meantime, I changed my daily driver a few times, ending up with a 1983 MK1 VW Golf GTI 1.8 Campaign. I drove this for a while and found it to be a fantastic little car. Very quick and very good handling. As the suspension was a bit soft, I decided to replace the shock absorbers & springs, but I got carried away, and replaced all the suspension bushes, topmounts, ARB bushes, etc etc. In the process of that, I had bought some more tools.

Since I was in motor-mechanic mode, I decided it was time to sort out the 911.
I again looked around for a professional who could do it for me, and at the same time I read up a lot more of the MFI system, and the CMA procedure, and eventually decided I'd tackle it myself.

I posted a thread on the Pelican parts forums, about what I intended to do, asking for advice, and got some very good feedback, encouraging me that I actually could tackle it myself, together with suggestions on what I should do, and what I shoudl measure, etc, etc.

I started work. I discovered the MFI pump was the correct pump for the 2.7 engine, so that was a good start
I was advised to measure the valve lift, to gauge what camshafts were in my engine. For this I needed a dial gauge which I didn't have, so I ordered one from pelican together with a z-block (P207) which makes it easy to mount the dial gauge in the right place. I also ordered a set of cam cover gaskets which came with new lock nuts and washers. (I would have to remove the cam covers to check valve clearance, and to measure the valve lift)

I was also advised to remove the intake stacks and throttle bodies to measure them also. This would mean new gaskets for refitting, so I ordered them, again from pelican.

I removed the throttle bodies, and found they were very dirty. The butterflies and the inner bores were covered/caked with carbon deposits.

After a lot of scrubbing, and a few cans of brake cleaner, I got them cleaned up, and measured as the correct throttle bodies for an S engine, and went through an advised adjustment procedure, using a vacuum cleaner and a synchrometer, to get equal aiflow through all the butterflies, setting the stop positions as required.

I had bought a leakdown tester, so I measured the cylinder leakage across all 6 cylinders. All but one were very good. Jury still out on the reason for that one. Could be carbon build-up preventing the exhaust valve from seating properly.

I got delivery of the dial gauge, so I remove the inlet (top) cam cover, adjusted the valve clearance, and set about measuring the valve lift.
I had a figure of 5.4mm in my head, so I assumed my 10mm dial gauge was more than adequate. I was wrong. I started to measure the inlet valve lift, and got 10.7mm. This confused me, and I wondered about the relationship between 10.7mm and 5.4mm.

Then I realised my gauge only read to 10.7mm maximum, so it was possible the valve lift was more than that. I was annoyed that pelican would sell a dial gauge which was too small for the task.

After a bit more reading up, I found that I should have been measuring the valve lift at TDC overlap - This is 1 turn after TDC compression, where the exhaust valve is closing, and the inlet valve is opening in preparation for the intake stroke. Specified at 5.0mm to 5.4mm

Silly me.

I found the specs for valve lift for an S cam, and decided I may as well measure total valve lift, as well as the TDC overlap measurement, so I searched and found a Draper 25mm dial gauge on ebay.

While waiting for this gauge to be delivered, I went ahead and refitted the throttle bodies, and stacks.

While under the car to adjust valve clearances & measure the exhaust valve lift, I noticed I had SSI's and I had a 7R case.

Measured valve lift at TDC overlap - Perfect for an early 911 S cam. Measured total valve lift - close to early 911S cam - still trying to verify this. So far so good.
Bought some new oil, Silkolene Pro S, 10w50 and put 10L of that in to start with.
Bought a compression gauge and did compression tests. This matched the results of the leakage test, showing #5 as being 18% down on the highest compression figure of the other cylinders.

Fitted new rev-limiting rotor arm, distributor cap, and new magnecor kv-85 plug leads.

Long time since an update here, so this is from memory.....

May 2008: I finished the wiring to points, coil & cd box, fitted the hot air supply to the mfi stat, did a quick double check I hadn't forgotten anything and turned the key.
It cranked for about 4 or 5 seconds, and sprang to life :)
(This is without the cold start solenoid connected.)
Then I remembered I hadn't opened the air bypass screws at all ! The Engine started very easily.

Next, I adjusted the timing to TDC @ 900rpm.

I couldnt get a smooth idle so, knowing it was very rich, (and I didn't have much time) I leaned the part-load by about 2 full turns, and got a lovely smooth idle so I could adjust the timing. (that did make it harder to start with no cold-start connected)

Timing was way off, about 11 degrees advanced.
Didn't get the 6k advance measured yet though.
Went for a quick spin a mile or 2 up and empty back road and its going better than ever !

By the time I got back, it had started lean popping, so I guess that means the stat is working
I had a couple of oil drips from the lower cam covers, and one upper cover so need to try tightening those & see if it will cure it.

I decided to have my injectors tested, but that proved more difficult than I imagined. They have a very high opening pressure, which apparently gives better fuel atomisation, and a better burn, but the pressure is much higher than normal modern injectors, so nobody was able to test them for me.

I contacted a company called Koller & Schwemmer in Germany, who tested them & found they were all opening at too low a pressure
I took the plunge and bought 6 new Bosch injectors.

I spent half an hour adjusting the dwell, Something I had never done before on any car !
( Used to just set the points gap & that was it.)
I manged to get it from 26 deg to 39.5 deg, but guess I'm just too ham-fisted to get any closer.
There must be a knack to getting minute adjustments
(My gauge reads in %, & 66.6% translates to around 39.5 degrees as far as I can tell.)

So, dwell & timing set.

I got the LM-1 hooked up to make sure it was working & got and AFR around 12.8 at idle.

Must say it was much harder to start now without the cold-start hooked up !

I do have two oil leaks from the lower cam covers, dripping down onto the ssi's so I guess I'd better cure those before I smoke myself out of the garage !
I have an oil leak at the upper cam cover beside #1, and I can't tell if it's from the banjo fitting or from that corner of the cam cover.
I'm thinking banjo, mabye it should have had new crush washers ? I simply refitted the banjo with the old washers - what looked like plain flat aluminium washers. Although, thinking about that, the banjo at #4 doesn't leak, and it was fitted the same way.

I had an aborted attempt today at adjusting part-load mixture.

I had the car idling nice & smoothly, a bit rich on startup until the stat got some heat, then settled into a nice smooth idle around 900/950 rpm. I don't think the car has ever idled for so long before !
I still had a few oil drips from the lower cam covers, but nothing too bad.

So, I hooked up the LM1 with the exhaust clamp & went off for a drive.
Bit of smoke from oil being burned off the heat exchangers - OK, expected that.
Kept an eye on the afr, seemed a bit lean & was getting lean popping.
Stopped at the side of the road & richened the part load 2 clicks.

Thought there seemed to be a lot of smoke in the engine bay, but OK, probably coming off the heat exchangers. Just let it burn off.
Back in the car, turned around & headed for home. Loads of smoke coming from the air vents. I mean loads. I had to stop twice to let the smoke clear before I got home - only about 2 or 3 miles !
So, where is all the smoke is coming from.
Before I started on this CMA route, I wanted to check if hot air was getting to the stat, so took off the air filter housing and went for a drive to warm it up.
Unfortunately, I didn't plug the two pipes that I removed from the rear & underside of the housing. One large pipe from the oil tank, one small breather of some sort. During that drive, quite a bit of oil was spat out onto the top of the engine block. (so much so that when I returned & parked in the garage, I nearly had a heart attack to see so much oil dripping onto the floor, until I realised my stipidity)
I simpy mopped up the excess oil but I never removed the tinwork.
I thought mabye there was still a lot of oil sitting on top of the block, which was smoking when it got heated, and that is the same air which is getting blown through the heat exchangers, into the cabin.

Well, I had a good poke around this evening in search of my oil leak(s).
As far as I can tell, & see through the fan, there is no pool of oil on the top of the engine block. It all seems quite clean in there, so I'm going to hold off on removing the mfi pump and throttle bodies just yet.
On the 4-5-6 side I have a substantial drip from lower cam cover. There is a big drip about every second or so onto the ssi. The smoke from this is passing up into the engine bay past the rubber seal (beside the upper cam cover).
I did tighten all the nuts on that cover, a few times, now they are as tight as I dare, but still seem to have the leak from the top edge of the cover, which then finds its way down along the cover surface & eventually drips off the ends of the lower studs/nuts. Looks like the studs themselves are leaking, but its coming from higher up.
Don't really know what to do about it. Should I have used some form of sealant when replacing the cover ? It was new gaskets, and the mating surfaces were cleaned. I keep thinking of the rubbery sealant stuff I removed from inside the cover - as if a bead of sealant had been applied the last time the gaskets were done. I know the car had a professional valve adjust shortly before I bought it. I also have wetness from one of the nuts on the upper cam cover, but it seems very slight. more an oil dampness than a proper leak.
On the 1-2-3 side, I have a similar leak from the lower cover, again dripping down onto the ssi. What I don't understand, though, is why I am seeing so much smoke coming into the cabin from this side. ( and none from the 4-5-6 side). The ssi's look in perfect condition, but I guess its possible there is a hole in the top that I cannot see, which is sucking in the oil smoke. The amount of smoke is simply amazing. It just wafts up out of the windscreen vent. You would think there was a fire in the dash somewhere. (I checked, there's not !)
I also have a small leak on the upper side - It's either from the bottom edge of the upper cam cover, or its still from that banjo where I had the trouble with the washers. It's not a lot, but enough to wet the length of the sheet metal that seals between the upper cam cover and the bodywork. It could still be the banjo, although I checked for leaks there with some tissue paper under the banjo, but only at idle. I think the leak only shows up at higher revs.

Right, I got under this evening and had a good look at the oil return tubes, and everything else under there. Both return tubes, on both sides had a good coating of fresh oil on them. The #2 and #5 cylinders seem to have a lot of burned oil on the fins - quite black & sticky. I cleaned them a bit, and cleaned the oil return tubes, and also cleaned as much of the heat exchangers as I could get to. I did notice the access holes (for the studs) through the exchangers were very oily, and the bottom surface of the exchangers had a fresh oil coating spreading outwards from these holes. Guess I need new oil return tubes. It does have the collapsable tubes, so I thought mabye just replace the seals, but I don't think I'd be able to get a circlip pliers onto the clips to remove the tubes intact, or is that possible ? I can see the clips & reach them with my fingers. The nuts on the exhaust studs looked quite rusty, so I hope I can install new return tubes without having to remove them. I also have a drip from a couple of the lower cam cover nuts. It seems to be leaking out the stud, but I can't be sure. The studs did have some sort of sealant on them which I noticed when removing the covers.

got some great advice about my oil leak possibilities from the Pelican Parts forum: You can easily fix everything that might be leaking oil onto the heat exchangers with the motor in the car. The first thing to do is to get those heat exchangers out of your way. Drop the whole exhaust system; it will make some of the oil leak fixes easier. Hit all of the nuts with PB Blaster or something for a few days before you try to loosen them.

You should be able to spin the return tubes so that you gain access to the circlips. Use a well padded pair of channel locks or vise grips if you have to. Just put new o-rings in the ones you have and reuse them; they should remain sealed for a good long time. Use some high temp silicone grease on all of the o-rings.

While the return tubes are out, go ahead and reseal the crossover oil return line where it goes into the forward left case. This thing is darn near impossible to work on with the return tube in the way, so take advantage of the fact it isn't. Take the compression fitting apart, then take the fitting out of the case and put some plumber's teflon tape and RTV sealer on the threads and screw it back into the case. The compression fitting doesn't use any sealer, but rather some anti-seize so you can get it apart later if you ever have to.

On the right side, remove the long piece of tin, the forward piece of tin (next to the bell housing) and the shroud screws that go into the oil cooler. Take the "J" pipe off that goes to the tank, and remove the oil cooler. Replace the seals inside of there and remount it. The seals like to fall off when they are horizontal in the car, so "glue" them in with some of that sticky silicone grease you used on the return tube o-rings.

If you have not replaced the thermostat o-ring, do that too. And the breather cover gasket right next to it. Take the oil temp and pressure senders out and put some plumber's tape on their threads, being careful not to cover the openings in the ends. The pressure line to the MFI pump under the temperature sender likes to leak, too, so put a couple of new compression washers on it.

When your valve covers are off, you have an opportunity to seal up the rocker shafts. If they don't already have them, putting in some RSR seals would be a good idea. With any given cylinder at TDC as if you are setting its valves, you can loosen the rocker shafts on it and slide them out into the "dry" areas of the cam towers and put those seals into the grooves. Remember to turn the nut only, not the screw, when loosening or tightening those rocker shafts.

When putting the valve covers back on, don't over tighten the nuts. They have a very low torque value (I think something like 8 ft lbs?). That's why they have the nylock nuts - so they don't rattle loose from that low value.

Make sure the MFI pump is not leaking as well. The baseplates sometimes leak. The oil supply and return lines sometimes leak. If the baseplate is leaking, you have to remove the pump to fix it. I'm not sure that base gaskets are even available; maybe Pacific Fuel Injection or Henry Schmidt has them. I just reuse mine with a healthy coating of silicone RTV.

The pump drive on the forward end of the left cam can leak as well. Yes, it can be replaced with the motor in the car; I've done mine and several buddies'. Take the left rear wheel off. Remove the drive pulley and belt, marking the pulley's orientation (after lining up the "FE" mark on the crank pulley). Remove the forward piece of tin (you have to take the throttle rod off first; it passes through a slot in the tin). From there it is a "simple matter" of prying out the old seal and pressing the new one in. I use a bunch of hooked dental picks to pry the old one out. It doesn't have to be pretty... The new one just presses in with finger pressure. Just make sure it is straight and flush all around. Make sure you line the pulley up the same way it came off; it only uses two of the four holes, and they are not 90 degrees to one another. They are clocked half a tooth off, to allow for that fine of an alignment with the timing marks.

Anyway, that should take care of most, if not all, of your leaks. At least the ones dripping on the heat exchangers. Once you are done with all of that, your tuning efforts will be much more pleasant. So, I ordered a load of O-rings & gaskets.

Sep 2008
woohoo.

Today was a good day.

I've been dreading taking off the SSI's, after reading about the possible/probable problems with the studs. I have no heat source, so though I'd stick with the spray-spray-spray method & see.

I've been spraying the nuts with Plusgas for the past week or so. I ran out of plusgas last night so used WD40 instead.

Today I went and sprayed them all again, and thought, I'd try one nut to see if there was any chance the PO had used anti-seize when fitting them.

Sure enough, absolutley no problem. One barrell nut was a bit stiff, but forward & back about 10 times freed it up & off it came.

I stood the ssi up on its end, and about a quarter cup of oil came out

I got brave and decided I may as well take both off. The 4-5-6 side was a bit more awkward. I managed to bend a ring spanner using my bfh, to get at the nuts on #4 and #5. Had awful trouble with teh #6 barrell nut. Not that it was seized, just could not get the hex tool seated properly no matter what way I moved the ssi itself (all other nuts removed at this stage).

I ended up using a different hex bit, with a 1/4 drive small socket that fitted up the access hole in the ssi, a long extension and a ratchet.

From looking up at the engine, without the SSI's in the way, I'm pretty sure the oil must have come from teh time I drove it without plugging the breather pipes. There was a lot fo oil on top of the engine at that time, and a lot dripped onto the floor, so I'm sure it made a mess.

I now have all the various o-rings & gaskets, so I may as well replace them while I have access.

I'm delighted those ssi's came off so easily. I was very apprehensive about getting them off.

Today I got the collapsable oil return tubes out, and got the tinware removed to get at the oil cooler to do those seals.
The more I see the more I am convinced I don't really have an oil leak, apart from the cam covers. It does really look like the oil that got dumped on top of the engine has found its way down by various routes.
I intend to replace the seals & gaskets anyway, for peace of mind.
When I dropped the oil, I was amazed at the strong petrol smell. I've hardly done 10 miles in it, but a lot of idling, while setting dwell , timing, etc.

She lives
I put on the exhaust/silencer & fired up last night
I thought I might have to bleed the fuel lines to the injectors, but it sorted itself out after a bit of cranking.
I have a small oil leak at the breather cover. I did renew the gasket but maybe I didnt clean the mating surfaces properly before refitting
No sign of any leaks from the return tubes or cam covers

December 2008 ok, finally all the oil leaks are sorted.
Now, I'm intent on getting the afr sorted at part load. I'm determined to get there before 12 months have passed !!

May 2009
After a LONG delay, I finally got back to this.
I measured a fuel flow rate from the filter to the MFI pump at 1150ml/30 seconds. Thats fine. Fuel pressure, measured at return from MFI pump to fuel filter housing, measured 0.8bar. Spec is 11.8psi +/- 3 psi
Converting 1 bar = 14.5 psi, thats perfect.
Checked it again a day later, as the gauge was still plumbed in place and am getting closer to 0.65bar.
Still in spec, as 0.65 * 14.5 = 9.43 psi. Spec is from 8.8psi to 14.8psi.
Wonder why there is a difference ?

Next step is to start the engine & see what the pressure is like. Maybe there is a problem there. Hope not.

To Be Continued.....