Bleeding a MFI system?

Gearbox · 04-12-2009, 02:45 PM

The sun is shining and the warm weather is just right around the corner. So the cover came off of the 71 911S and everything checked and I tried turning it over. Normally it would start right up after a few cranks. But not this time. After many more cranks, not a thing. Spark looks good, and the fuel pump is humming away, but no joy. A quick shot of starter fluid and the engine revs up and then dies. No fuel smell anywhere. Somehow I don't think the fuel is getting to the MFI Pump and subsequently the cylinders. Checked the MFI belt and it's there. Does these MFI's require some kind of bleeding once in a while? Any other ideas? Thanks Allan

FenderBender · FenderBender's Garage

Check for fuel, clean or replace the fuel filter that is in the tank. I say check for fuel because the sending unit can stick and give a false reading. Good Luck, Keep us updated.

Eagledriver · 04-12-2009, 09:12 PM

You can disconnect the return fuel line from the MFI pump and see if you get fuel flow out of the line when you turn on the fuel pump. If you get fuel flow then the problem is with your MFI pump.

-Andy

Flieger · Flieger's Garage

Your electric fuel pump that supplies fuel to the MFI pump may be worn out after all these years.

Rusty 356 · 04-13-2009, 06:01 AM

Fuel supply is the first place to look. The roller type fuel pump can be disassembled and cleaned. If you have been getting fuel and you have the cold start in the air cleaner i would disconnect the cold start solenoid and use the starting fluid to minimize a nice warm fire in the air box. If you have the capability check fuel pressure around 14psi and flow at about 2 quarts per minute max. It isn't necessary to bleed the pump.
Check fuel shut off solenoid as well. Good luck and don't miss many good days to drive that sweet machine.

Gearbox · 04-13-2009, 01:22 PM

Rusty LOL, The fire had already reared it's ugly head, outside of a toasted air filter box, no other damage. Plus it gave me an excuse to polish up the intake trumpets. That's why I didn't think it was a fuel pump problem, the cold start loaded up my air cleaner baskets up real well. I'll disconnect the fuel line and see if is pumping out, sound like the problem (I hope) as a MFI unit would really be painful. My car is a 71, was there a fuel shut off valve somewhere? Thanks Allan

Rusty 356 · 04-14-2009, 05:42 AM

Alan, fuel shut off is on the mechanical fp. Solenoid that moves the rack. I'm pretty sure it is an energize to run solenoid. Check it for power. I believe it is the only electrical piece on the MFI pump. Good luck.

304065 · 04-14-2009, 06:06 AM

Quote:

Originally Posted by Rusty 356

Alan, fuel shut off is on the mechanical fp. Solenoid that moves the rack. I'm pretty sure it is an energize to run solenoid. Check it for power. I believe it is the only electrical piece on the MFI pump. Good luck.

No, when the RPM is between 1500 and 1300 AND the throttle is closed, current flows to the solenoid which pushes the rack and shuts the fuel OFF. So it's an "energize to shut off" solenoid.

Alan, you need to verify that you have fuel to the pump. Be extremely careful working with gasoline, it can kill you. Verify that you have 1.0 bar fuel pressure in the loop between the pump and the hose that comes out of the BOTTOM of the fuel filter.

The system does not need bleeding, even if it drained back, it would pressurize with a few cranks. Unless, of course, there's no fuel filling the gallery in the pump.

Zeke · 04-14-2009, 07:39 AM

Remove the center fuse on the rear relay panel to deactivate the cold start valve. Something seems strange. You are not getting fuel or enough fuel to the cylinders because the car runs for a second on cold start. Or, you've simply flooded it. We do know fuel is reaching the fuel canister because of the cold start gas.

Now, how about the t-stat? Is that stuck on lean, the out position? Have you done the hair dryer test and clean up?

Gearbox · 04-14-2009, 08:50 AM

Hey Zeke nice hearing from you again. And again you got my drift. If I got fuel to the cold start, I assume the pump is pumping. Actually, the previous owner had installed a manual by pass momentary button to activate the cold start. Normally, a push or two would crank it over nicely, even after winter storage. But it seemed that the only thing giving gas to the motor was the cold start. Everytime I pushed it, it would try to start and then die. Unfortunately, after 10 times, and due to a mis-aligned cold start tube in the air filter, it loaded enough fuel in the housing base to catch fire. Now with the melted housing off the car, the only thing that will fire it up is a squirt of starting fluid. I can't smell any fuel in the stacks even with the throttle wide open. I checked the plugs and they are dry, so flooding isn't the problem. Now how do I check the T-stat? All the linkages are working and connected.

John, Rusty, Yes I know the soleniod. Had problems with backfiring last year and traced it to the relay and got a rebuilt one from Otto. As John states, it only works when deaccelerating via micro switch on the throttle so it shouldn't effect start up, but I will see if it is mechnaically jambed in the on position which may explain the symptom.

The good news is that it gave me the excuse to polish up the intake trumpets. I thought these things were stainless steel, but they attract a magnet, so I guess they are chromed mild steel. I was surprised to see them polish up so nicely with no rusting, even after almost 4 decades and the fire. Gotta look on the bright side