MFI gremlins... Opinions please!

[docdan200]

I hope our resident MFI gurus can share their wisdom to help pull us out of our present bind...

The car is a 1973 911S. Fuel pump works fine. Ignition is fine. The engine won't fire when cranked though. We disconnected fuel line going into the MFI and fuel flow was fine. We reconnected the line and disconnected the line going to the throttle bodies of cylinders 4, 5 and 6. Fuel did not flow on lines 4 and 6 and barely on 5. We manually primed the stacks by pouring a small amount of fuel, and the engine fired right up...only to die when the fuel runs out. Does this mean the MFI is clogged or defective? Are there other tests we should do before we remove the pump and send it for servicing?

We're all ears...coz we're halfway around the world from where most of you guys are...

[Flieger]

Was the fuel system apart? It takes a whole lot of cranking to prime the system when there is air in it.

[Porboynz]

The engine should fire and start on the cold start fuel delivery. If this is not functioning then it does take a lot of cranking to get fuel flow, especially after a rebuild. I would also check for fuel flow in the return lines indicating good pump pressure. So many things to check really, including making sure you have the fuel lines routed correctly, there are schematics out there that are wrong. My Haynes manual had the fuel lines to the filter housing transposed, which was interesting.



This one is correct

[docdan200]

Thanks for the quick reply guys!

Flieger: The fuel system wasn't taken apart. The car sat for a couple of weeks since it last ran...and at that time, it was running without a hitch.

Porboynz: There was no rebuild. The car is a low-mileage, real cherry "S". Our only shortcoming was not being able to run the car as frequently as it should. Like I mentioned to Flieger, it's been a couple of weeks since it was run...owing to the rainy season that we have here in Southeast Asia.

[47silver]

Sounds like the mfi pump is not turning is the belt intact?

[Flieger]

My car routinely goes for 4 weeks without driving and I've never had a problem. I use 91 octane E10. I don't think the MFI pump could have gotten gummed up inside to the point that the pistons are sticking.

[47silver]

if you have fuel entering the pump and none leaving the pump i would check.
the belt
the shut off solenoid.
unless the pumps is bad it would have to pump some fuel no matter what if these 2 things are working properly.

[docdan200]

That's the unusual thing...this is the first time that an MFI-related issue has cropped up since the car's arrival here. Previous to this, the car ran superbly! It was a joy to drive...as a properly tuned, MFI'd, 2.4S should.

We will go through the fuel system... Tank to MFI to tank.
Porboynz "corrected" diagram will come in handy... Thanks Porboynz!

47silver: We'll check the MFI belt...

Although we too have "ethanol-laced" fuel, this gremlin only surfaced with the 911S.

Flieger: We did notice some corrosion on the bearings of the fuel pump when we dissected it. This was our "first stop" in the process of deduction...when we noticed there was a fuel-related issue.
The fuel pump was newly refurbished unit, done in the West Coast, since new pumps are NLA.
Changed to a new fuel filter too...and voila... No start.
That's why we turned to this forum for help.

That's why this forum is very much appreciated...most especially by us guys on the other side of the globe!

Keep you posted...

[47silver]

so from what I read you changed the mechanical pump and changed the fuel filter and then the car would not start?
did you time the mfi properly?

[docdan200]

We have a new fuel filter...as well as a newly-rebuilt electric fuel pump...since these electric fuel pumps for MFI applications are no longer available NEW nowadays.
The MFI is untouched up to this point.

[DeFreest]

Years ago my 72S MFI did the same thing. It turned out to be the rev limiting rotor in the distrubutor. Check that?

[Porboynz]

I would triple check you connected the correct hoses to the correct outlet/inlets on the new electric pump as you have changed that. I would also drop the fuel filter and check for who knows what mischief as you also changed that item, plus make sure you have good fuel flow from the filter return and the MFI pump return. IMO the motor should at least try to start on the cold start squirters even if the MFI pump was completely mis-timed or broken. They are generally well behaved though so other than a broken belt sudden failure is unlikely. Checking the fuel cutoff solenoid is a good tip. (just pull the wire off so it cannot activate) Good luck, are you in S'pore or KL or similar?

[docdan200]

Porboynz: We'll go through the checklist that you just pointed out.
We'll be pretty busy tomorrow going through the fuel system...while ticking off the inputs from all you generous fellas.

As to our locale, we're in Manila...pretty close to KL / Singapore.

This board absolutely rocks!

[daepp]

Hopefully Grady Clay will chime in. Good luck!

[docdan200]

Paging Grady Clay... MFI problem on aisle no. 1 ...

[356RS]

Have you checked the electric fuel pump quantity delivery from the MFI pump (left side return line) to the fuel filter console? With the ignition on and the fuel line from the MFI pump running into a measuring beaker, you should see 900 to 1000cc of fuel in 30 seconds. If that tests good, I would check to see if you have dirty injector pressure valves (fine rust particles from fuel tank) preventing the injectors form building up the 215 to 255psi pressure needed. It's an easy test to do. Remove the injector lines from the top of the MFI pump so you can see all 6 outlets. Turn on the ignition switch, electric fuel pump running, and see if there is any fuel coming out of the 6 connectors on top of the pump. This might take a 10 seconds or so to see the results. You should not see any fuel coming out of the connectors at all. If you do, that pressure valve is leaking and you will not have enough fuel pressure for that injector to work. You might have 5 or 6 leaking and that will prevent the engine from starting.

[docdan200]

Hi Mark,

Thanks for that tip!
I'll relay this immediately to our wrench...and hopefully (...with fingers crossed...), we'll be able to solve this fueling problem soonest.