Weird fuel delivery problem?

BBShriver · 06-14-2011, 08:56 AM

Last week I decided to drive my 1973 911T (CIS 2.4) to work. Did not quite make it out of my development before it stalled, and would not restart. It would fire, sputter and die. Did that several times. Though my fuel gauge was just under 1/2 tank, my first thought was that the gauges must be off, and I ran it out of fuel, so I walked back to my house, got my lawn mower gas can, came back and put whatever was in the can into the Porsche (it's a 2 gallon can, and I expect was less than half full). Took a little cranking, but it fired up with no sputter/die ritual and I drove it back and put it in the garage and drove the truck to work. Came home and took the Porsche to a gas station to fill up. But total (including the fill from the lawn mower supply) only took ~10 gallons.

Drove it to work the next day with no problems, running great. So it still seems like low fuel is what the symptoms indicate, but it seems like it should have larger than a 10 gal tank (can't find capacity online).

Can anyone give suggestions on what might cause this? When it was not starting I could still hear the fuel pump, it was cranking fine.. any ideas?

psalt · 06-14-2011, 09:33 AM

Brandon,

The 911 fuel gauge is notoriously inaccurate, especially when it gets to the bottom. Your tank is probably 62 liters/ 16.4 gallons, so you should have had fuel. If the car is new to you, it is always a good idea to drain the tank and clean out the pickup screen and replace the fuel filter. Another possibilities is that it stalled for a different reason and would not restart. If you are new to CIS, there is a hot restart issue, the system needs residual pressure to fire. It is also a good idea to check the system, control and residual pressure if you have never done it. Next time it happens, try lifting the air flow sensor for a second with your hand while the key is ON. You should hear and feel the fuel purge the lines.

Paul

BBShriver · 06-14-2011, 10:09 AM

Had the car about 3 years and never had this type of problem. How do I get to the pickup screen and fuel filter?

where is the airflow sensor?

T77911S · 06-14-2011, 10:36 AM

Quote:

Originally Posted by psalt

Brandon,

The 911 fuel gauge is notoriously inaccurate, especially when it gets to the bottom. Your tank is probably 62 liters/ 16.4 gallons, so you should have had fuel. If the car is new to you, it is always a good idea to drain the tank and clean out the pickup screen and replace the fuel filter. Another possibilities is that it stalled for a different reason and would not restart. If you are new to CIS, there is a hot restart issue, the system needs residual pressure to fire. It is also a good idea to check the system, control and residual pressure if you have never done it. Next time it happens, try lifting the air flow sensor for a second with your hand while the key is ON. You should hear and feel the fuel purge the lines.

Paul

+1
i dont know if it would have cooled down enough to make a difference while you went to get gas. things like this can also be a failing coil or CD unit or maybe even FP.
did the 73 have a FP relay?

drain the tank, sucks now that you just filled it. you can get to it from the bottom of the tank. also, pull the fuel sender and shine a light in there to inpsect the tank. i also used a small mirror and light to inspect from the bottom.

ossiblue · 04:53 PM

Quote:

Originally Posted by BBShriver

Had the car about 3 years and never had this type of problem. How do I get to the pickup screen and fuel filter?

where is the airflow sensor?

What Paul meant by "air flow sensor" is actually the plunger inside the intake--lifting it will cause the injectors to shoot fuel and purge the lines of any air. Your 73 also does not have a fuel pump relay.

The pick up screen is in the bottom of the tank, behind the hex bolt. The fuel filter is inside the engine compartment near the CDI unit. Your problem may not have been low fuel as others suspected so clean and replace the screen and filter, test your system pressures and flow volumes, and if they all check out and the problem returns, look to the ignition.

BBShriver · 06-15-2011, 05:48 AM

Can the fuel filter be replaced without draining the tank? What is the best method to test fuel flow and pressure?

I drove it to work today and it ran fine.

Anyone have a picture of the plunger in the intake? One of these days I need to find a good service manual, but the factory ones are $$$ and Bentley's doesn't have one before 78 I think..

ossiblue · 06-15-2011, 07:17 AM

Quote:

Originally Posted by BBShriver

Can the fuel filter be replaced without draining the tank? What is the best method to test fuel flow and pressure?

I drove it to work today and it ran fine.

Anyone have a picture of the plunger in the intake? One of these days I need to find a good service manual, but the factory ones are $$$ and Bentley's doesn't have one before 78 I think..

The fuel filter is in the engine compartment by the CDI and can be easily changed without draining the tank. The fuel screen, however, is inside the tank and the tank must be drained to remove/replace it.

The plunger can easily be seen. Remove the air cleaner from the air box and look inside the opening. You will see a metal disk/plate in the upper portion of the opening. Gently push up on it to activate the injectors.

BBShriver · 06-15-2011, 07:23 AM

Thanks I will give both a look. Not sure what to do with the screen since I don't have any way to store/dispose of ~16 gal of fuel!

T77911S · 06-15-2011, 08:20 AM

either run a line to another car and run the FP to pump it out, or drive it, and hope it does not leave you stranded

Flat Six · Flat Six's Garage

Quote:

Originally Posted by BBShriver

Anyone have a picture of the plunger in the intake? One of these days I need to find a good service manual, but the factory ones are $$$ and Bentley's doesn't have one before 78 I think..

I'm a really big fan of:

Bosch Fuel Injection & Engine Management by Charles O. Probst. Available new & used (PP offers it here: Pelican Parts.com - Bosch Fuel Injection & Engine Management).

While not brand/model specific, it's been an indispensible source of information to help me understand the various CIS/K-Jet, DME/L-Jet systems on the German cars I've owned over the years.

A must for any technical reference library, IMHO.

Dale