MFI Problems

[Grady Clay]

Here is a diagram of the fuel filter console.
Fitting #9 is the “overflow valve” that is a restriction in the return.
The one in the pipe/hose assembly seems to be in addition.
"
"
(C) 1972 Dr. Ing. h.c. F. Porsche A.G.


Best,
Grady

[Grady Clay]

Terry,

To really properly inspect the screen in the tank fitting, you should remove it from the tank. Looking at it through the fuel level sending opening can fool you. The weave of the screen can clearly show even when the screen is almost completely restricted. The screen fitting is easiest to loosen when the tank is bolted in place.

I’m surprised that the pipes were that restricted. Good that you found that. I have always worried about that but never found one so restricted. I think a rifle bore brush for a .22 caliber or close can fit the pipes. That might be a good cleaning tool. Are your pipes in the tunnel factory or other? Plastic or metal? If you feel they need replacing, I would recommend stainless steel with a swage like the factory fittings.

When you disassemble the fuel filter console, investigate all the fittings and passages.


Your question about the electrical connection to the pump is a good one. With the engine running, you should have very close to battery voltage (~14.2V) at the pump. If there is more than a few tenths of a volt difference, I would inspect and clean all the connections. An easy measurement is to put one lead of your voltmeter at the battery + and the other at the electric fuel pump +, you are measuring the drop in the positive wiring. You should also confirm there is very little voltage between the negative pump terminal and the chassis ground.

The MFI electric Bosch fuel pump (Porsche 901.608.105.00, Bosch 058097 0001) is rated at 3.5A @ 12V. It turns approximately 2800 RPM and delivers (free flow) 125 liters per hour. The safety pressure relief valve in the pump opens at 28 PSI but the system pressure is regulated at 14.2 PSI by the restrictions in the return line.

Best,
Grady

[thastings]

Grady,
I had the screen out of the tank and in a parts cleaner and could not see thru it. The mesh was almost impossible to see. As a matter of fact, at first i thought it was a piece of brass tubing. Not having seen a new one, I was not certain as to what was "clean". I wanted to replace it, but I could not find a listing for a replacement anywhere, so I was left with giving it a good scrubbing in the cleaner. I had to re-install it since I did not want to tie up a lift while searching for a new one. I could not find it in the factory parts manual. I need to know thia parts name/part number or something just to order one (I just got a part number of 901-201-023-02 form Doug on another thread). I was hoping our host had it listed, but it appears that I will have to give them a call. With a lift, it's not a hard job pulling it out, after you drain the gas. And now that I cleaned out the sending unit, my gas gauge works, I won't have to drain 8 gallons next time.

I was surprised to find crap in the factory tubes. Just a couple of months ago I cleaned them by pulling a wire attached to a small nylon parts brush (the 20+ .22 cal brushes I own were all at the house, not the garage). Maybe the fresh gas, and all the cleaners that I've put into the gas finally started cleaning the walls.

What really surprised me was the amount of sediment that was in the tank. I have had only 50 gal of gas since I had the tank steamed cleaned. I wonder if it came out of the sending unit for the tank, which I just cleaned and it was "full" of crap.

Terry

[Grady Clay]

Terry,

I agree it is a very fine screen. If you have it out again, try some soaking in carb cleaner and a fine wire or tooth brush. Lacquer thinner works well for a final clean.

I looked on PET6 (’73 MFI) and there are two screen fittings listed.
With a 6 mm outlet is 901.201.023.01.
With an 8 mm outlet is 901.201.023.02.
Yes, they are called “gas caps.” The translation hasn’t gotten any better in 40 years or they outsourced to India.
These are the same part numbers listed in my original paper ’72-’73 parts manual only they are called “Tankverschraubung – screwing.”

I have a bunch of rifle cleaning brushes that have a ring on each end so you can pass them back and forth in the bore with a string. Some date to WWII and earlier with my dad. I think these are currently available and are great for all sorts of cleaning applications.

Looking at this prompts me to ask; what is the return fitting in your tank? Have you had that out for cleaning also? I think Porsche occasionally just used a normal supply screen fitting on the return side. That could be restricted also, just from the inside of the screen. Or, is your return to the bottom drain fitting with a banjo? That keeps the debris stirred up while the engine is running.

You might consider a new screen fitting for the fuel pump supply and use the old one (with screen removed) in the other port with just a plug in the drain position. This will give any debris a place to settle without being stirred up by the return flow.

There is a 914 screen fitting that is plastic and I wouldn’t recommend using that part.

The junk in your fuel tank can only come from two places: In with the fuel or from corrosion of the tank. I would recommend you get a funnel with a screen filter in it for fueling the 911. These are readily available at the roundy-round parts suppliers. That will eliminate a potential source. You might consider periodically draining your fuel tank through a coffee filter and see what you find. You may need to seal or replace your tank.

I will be really interested as to what you find in the fuel filter console.


Best,
Grady

[thastings]

Grady,
Thanks for the part numbers. I was wondering what a "screwing" was in the parts book.
It appears our host no longer carrys these parts in stock. They are special order at $40 each???????????? Are they available anywhere else?

I have the banjo fitting on the return line.

I "think" I know where the sediment in the tank came from. The tank, after the original steam cleaning, was filled with new gas from old plastic gas cans that we used in our race car days. When I went to drain the tank this time I noticed that there was a lot of junk in the bottom of these same cans. There is something to be said about doing it yourself.

Terry

[72doug2,2S]

Strange that, I just ordered 2 of these in July 2004 for $9.50 each, from our host via phone. Perhaps I got the last two stocked?

[thastings]

They say that they have not had them for a long time. Of course the sales rep was reading from the online catalog which says this.
I will try them again.
Terry

[thastings]

Doug,
I just called again and this time the price was $9.????????????
Terry

[Grady Clay]

Terry,

What is the progress?

What have you found with the fuel filter console?

Have you found a new brass screen fitting? I’ll bet a $9 screen fitting is the 914 plastic version. Stay away from that one to supply screened fuel to the electric pump. It might work as a diffuser in the return circuit.

The more I think about this I would use a complete screen fitting in the return line to the tank. The fuel is filtered by the big MFI filter and the screen diffuses the return flow for less disturbance of debris sediment. You can install a drain petcock in the bottom port since you seem to be draining the tank regularly.

Only the Porsche translators would offer a 901.201.023.01 “Screwing” in their parts system. I thought that was only available from another department.

Best,
Grady

[ed martin]

Here we go again. I've been sort of buying and selling these cars and this is problem is very typical. For some reason the old Porsches seem to be prone to rust in the tanks and the lines more than any other car. I think they are more prone to rust in general. I've been through the process of trying to isolate the problem, only to find it will eventually reoccur at some other time and place. I've cleaned the lines with solvent, blown compressed air through them and even would push some stiff wire through. For some mysterious reason these measures were not always enough. I have concluded that the only sure and final way to deal with this, and I know it sounds extreme, but just change out all the lines and the tank. It is the only sure way of not being haunted by this problem and wanting to drive your car over a cliff. This includes all the metal lines particularly the ones through the tunnel, that is the suction and return. If you repair or replace sections it will solve the problem temporarily only to have it return at some future date. The alternative in my opinion would be to just sell the car -that's what the previous owner did.

[thastings]

Thats why they put guard rails at all cliffs.
Ed, it runs soooo...good when its right, and since I've had this car for 20 yrs, theres only one way we are going to part company.
So, now it is clean up to the gas filter unit, so I take that apart and clean it out next. I also found that one hose was not replaced, the one from the filter to the MFI. That will come off too.
If this does not clear up the problem, once again it is time to go back to looking at the replacement MFI unit I got from UncleZak as being the cause. One thing is for sure, over the last year, I've learned a lot more about the MFI system then I ever wanted to know.
Terry

[pjv911]

Terry any new updates ? Did you fix it ?

Kurt Williams

[thastings]

Kurt,
I no longer have the "every 3 miles no fuel pressure for 10 seconds" problem. After cleaning the fuses and blocks, and replacing the fuel pump fuse with a "new and improved" one, it improved immediately. This increased the flow out of the inline filter from under 900 ml to 1200 ml.

There is still the occasional burp, that that I have a hard time feeling. If I did not have a pressure gauge in the car, I would not even feel or know about most of them. No more stopping along the road shoulder waiting for the pump to fill the MFI back up.

Of course, I did that AFTER cleaning the tank again, replacing the tank filter and replacing that one hose.

The more I drive the better it seem to get.

It is running richer since the fuse change and I have not had a chance to adjust it.

I'm still going to look for a good rebuilt, fresh, MFI pump or get one of those EMI setups for the regulator end of the pump. I think there may be a problem in the regulator end of the pump. If you hear of one let me know.

Terry Hastings

[304065]

Hey, great news!

You could still have some junk in there, you might splice in a cheapo plastic filter and let the fuel circulate for a few hours (you can do that with the engine off, just have the key on so the fuel circulates in the loop between tank and pump) although you might want to have a battery charger hooked up to keep the batteries strong.

I would very much doubt that your MFI pump is in need of a rebuild in the absence of some major symptoms-- give 'er the old Check, Measure Adjust and see if that richness can be corrected. See my thread in the archives on installing an LM-1 to monitor mixture. . .

Great news.

[thastings]

John,
I have already installed, run and removed a second little inline filter. It was clean after a month.
I had been adjusting the mixture trying to solve my problem, but now that I improved the flow after installing a NEW pump (your old one is still sitting on top of the 'frig as my wife just reminded me) it's time to un-adjust back to where it was.
I have a CO2 gauge installed in the car now. Its in the red only on startup and NOW when I let it idle for several minutes. That should change with the adjustment.
terry Hastings

[pjv911]

Sounds great . I currently have a problem with my mfi setup . I found this thread in a search and read the whole thing . I am going to start a new post which will be specific to my problem . If you have any idea`s feel free to add any input .

Kurt Williams