What would happen if MFI Fuel lines were mixed up? Mechanic screwed up?

[M491Cabriolet]

OK, I need help. I'm nearly positive my mechanic screwed up on me and the result is I'm going nuts. I am afraid he might have hooked up my fuel lines from my gas tank to the MFI electric fuel pump incorrectly. Specifically, I think he might have confused the supply line that's supposed to go to the "S" connection on the electric fuel pump and the return fuel line that's supposed to go to the "R" connection on the electric fuel pump. I think the tank's supply line might be hooked to the R connection and the return line is hooked up to the S connection. By the way, the return line has a T connection in it. The D line is properly connected to the right side of the fuel filter console.

Because the return line is a bit shorter because of the T connection, it isn't that simple to just make the swap and see what happens. I'd be doing it on my driveway. So before I do it, I am wondering if anyone can tell me exactly what the symptoms of mixing the two fuel lines from the gas tank to the electric fuel pump would be. It seems to me that both lines would supply fuel to the pump equally, but I am probably wrong.

In essence, I would like to know if having the return hose to the S hook up and the supply line to the R hookup cause a fuel starvation situation (because that's what I am having)?

Thanks for any advice!!!

[M491Cabriolet]

I reread my post and realized it was really not clear what my question was, so I edited it in the hope someone would be able to help me out. Thanks.

[HarryD]

Check these threads to see if they can help you:

MFI Fuel Starvation...Out of ideas.....help please

MFI Problems

73 ignition + MFI problems HELP PLEASE !!!

There may be more, but this is a good place to start.

[Clay Perrine]

With the supply and return line switched, it probably won't run. There is a check valve in the fuel filter console that would prevent the fuel from flowing backwards.

IF the pump is not working or not generating the correct pressure/volume you will get a car that will run, but stumbles and dies when accellerating.

Oh.. and this actually happened to me. I was able to drive mine until the fuel level got down to 1/2 tank before I started experiencing fuel starvation. A 911 won't do this. The tank is flatter than the 914 tank.

[M491Cabriolet]

Thanks. I am confused by something. My car is a 1972. I've seen diagrams that are different for 1969 to 1971, and for later MFI cars. Is there a different way to hook up This is the way I understand it: In the S (suction) line, the fuel flows from the tank to the pump. In the R (return) line, the fuel flows from the filter console to the pump (R) and to the tank. The O or D line (the center pipe on the electric fuel pump) delivers the fuel through the filter console to the engine (MFI pump). Is this right so far?

OK, if so, this is where I get confused. The O or D line in some diagrams is hooked up to the center connection on the filter console on some diagrams, and to the left connection on others. Then, the line that goes to the R connection on the electric fuel pump is connected to the center or right connection on the filter console, depending on the diagram you look at. The line returning flow from the engine (MFI pump) is consistently ooked up to the left connection on the filter console. The qestion is, which is the correct way to hook up these lines on a 1972 911? Please share the knowledge! Thank you in advance.

[304065]

The diagram as originally published by the factory is WRONG.

Get the corrected diagram, Grady Clay posted it.

And be extremely careful with Gasoline, it can kill you.

[M491Cabriolet]

Thanks for the advice about being careful with gas. It scares me as much as sharks! John, you mentioned Grady's diagram. I have everything hooked up this way now. I have seen both Grady's corrected versionas well as the incorrect version. In my factory manuals, though, there is also a third version. That one has the D line going to the connection on the fuel filter console where the R line goes in Grady's corrected diagram. Is it possible that on 1972 and 1973 models these lines were hooked up differently? Mine is a 1972, BTW.

The car runs OK for about 30 seconds at 4000 rpms, then it stutters and backfires very loudly in the exhaust. If I let it idle for about 10 seconds, the car goes back to running normally. To me, this sounds very much like fuel starvation and like the tank screen being clogged and then when it idles, enough fuel goes into the filter to run OK again. However, I replaced the tank, cleaned the screen and blew the lines out with compressed air. It's possible that it has become dirty again from junk in the return tank and the fuel level sender.

What is the best and safest way to drain the fuel tank on a driveway? This will be the next step if I can't figure this out. Thanks.

[3.2 CAB]

Hi, I wish I could help with your problem, but I know very little about the year models that are like yours.

Just be careful when working on the fuel tank and related parts. It seems that lately there has been several instances of static spark setting fires at fuel tanks, while fueling, or opening up the tanks for service. It may be a good idea to use some type of personal grounding strap, to help prevent a static charge from arcing between you and the fuel tank components. I have one similar to this, that I use when opening a fuel tank for repairs, to prevent a static charge from getting to concentrated fuel vapors like when removing sending units. It also comes in handy for working on sensitive electronic components, computers, PAD's, DME control units, etc.
It is very easy to fry things due to static electricity.

Don't let this happen to you.

I hope you get the info you need to fix it. Good luck!! Tony.

[jcge]

You could verify correct fuel pressure at the inlet to the MFI pump

It should be between 0.8-1.1 bar (or 11.6-16 psi) under all conditions.

Here's a pic of the pressure gauge/double banjo bolt fitting you need to do it.



John