Car acts like gas tank is empty when plenty of gas is still there

[procan]

So my car will shut down like the gas tank is empty, but there is plenty of gas still in the tank. When I fill up some more gas, she starts right up and runs until it eventually happens again. I know there's plenty of gas still in there because the last time this happened, I pulled the gas tank out and drained over 5 gallons of gas.

Some details about my fuel system:

• The fuel pump has bee whining for some time. It's quite loud.
• Car is a 912 with a 2.0 911E motor. I pulled the fuel pump out, it's a PIERSBURG. Checked online, and it looks like the stock fuel pump for the 912 motor.
• I'm running Zenith 40 TIN carbs
• The fuel pump has been located in the engine bay of the car. It is the only fuel pump.

Questions:

• Maybe the fuel pump is on its last legs, and needs the hydrostatic head of a full gas tank to give it the suction pressure it needs?
• Is the location of the fuel pump ok? Should there be a second one closer to the gas tank?
• Is there a problem with using the 912 fuel pump to feed the 911? All that should matter is that the fuel pressure is right for the carbs, no?

[Targalid]

What is the fuel pressure and flow rate? Is there a clogged fuel filter?

[Christien]

One fuel pump is correct, there shouldn't be a second. Whining is not a good sign at all, but first check your fuel filters. I had a clogged filter on the outlet of the gas tank that would cut fuel flow significantly when the crap on the bottom of the tank got swished around. It was only a problem when I was cornering a lot, like at the track, which is why it baffled me for so long (I thought it was heat related so looked first to ignition stuff). If the filters check out ok, then look at replacing the fuel pump. Always check the cheap and easy stuff first

[procan]

Targalid- I don't know what the fuel pressure and flow rate is since everything is disassembled right now.

Christien- There is a big grey BOSCH canister immediately upstream of the fuel pump in the engine bay, I was assuming that is a fuel filter? There is also another small clear cone shaped fuel filter downstream of the fuel pump that you can see the gasoline flowing through. I haven't popped the the small cylindrical metal mesh fuel filter out of the bottom of the gas tank yet, but I'll do that this week. I'm thinking maybe the fuel pump needs to be rebuilt or replaced.

[sballard]

Did 1969 cars have vented or sealed gas caps? Might try loosening the gas cap to see if it keeps the car from shutting down.

[HarryD]

Originally Posted by procan

Targalid- I don't know what the fuel pressure and flow rate is since everything is disassembled right now.

Christien- There is a big grey BOSCH canister immediately upstream of the fuel pump in the engine bay, I was assuming that is a fuel filter? There is also another small clear cone shaped fuel filter downstream of the fuel pump that you can see the gasoline flowing through. I haven't popped the the small cylindrical metal mesh fuel filter out of the bottom of the gas tank yet, but I'll do that this week. I'm thinking maybe the fuel pump needs to be rebuilt or replaced.

Some pictures would help.

You are describing a fuel starvation issue.

Some thoughts on what you have posted.

I am surprised there would be a fuel filter UPSTREAM of the pump. Generally, that is a big no-no since as the filter plugs, it increases the need for suction pressure until the fuel starts boiling in the line and the pump cavitates (and eventually destroys itself).

Typically, you will have a screen (or sock) upstream of the pump to keep big chucks out of the pump and a proper filter down stream.

Do you have carbs or fuel injection. A clear filter is ok if you have a carb rated fuel pump (3-5 psi). If you have fuel injection, the pressures are much higher and the plastic filters are not rated for this service and can fail catastrophically (think massive fuel leak/fire).

[procan]

HarryD -

I'm running Zenith 40 TIN carbs.

Here is the fuel filter UPSTREAM of the fuel pump:


Here is the fuel filter DOWNSTREAM of the fuel pump:

[mpeastend]

I had this problem with a 70 911T equipped w/zeniths...exactly the same symptoms. Turned out it was gunk in the gas tank. There is a metal mesh filter over the fuel outlet at the bottom back of the tank. It was full of gunk. When you let it sit a while, it'll allow flow, but as the flow increases from driving, anything that settled will again plug up whatever surface area of the mesh that is allowing flow...and starvation.

[procan]

Originally Posted by mpeastend

I had this problem with a 70 911T equipped w/zeniths...exactly the same symptoms. Turned out it was gunk in the gas tank. There is a metal mesh filter over the fuel outlet at the bottom back of the tank. It was full of gunk. When you let it sit a while, it'll allow flow, but as the flow increases from driving, anything that settled will again plug up whatever surface area of the mesh that is allowing flow...and starvation.

Thanks man. I pulled the gas tank already, mainly to address some exterior surface rust around the flanged edge. I still need to pull that metal mesh filter out of the bottom and take a look. I've been doing a little research on how to properly clean out the gas tank as well. Any suggestions?

[HarryD]

Originally Posted by procan

HarryD -

I'm running Zenith 40 TIN carbs.

Here is the fuel filter UPSTREAM of the fuel pump:


Here is the fuel filter DOWNSTREAM of the fuel pump:

Eliminate the Bosch filter. You should only have the screen in the fuel tank and nothing else between there and the pump. This may solve your problem.

When you can, inspect the screen in the tank.

If your fuel system pressure is low (like it needs to be for carbs, the clear filter should be ok.

[bourgeois911]

Originally Posted by sballard

Did 1969 cars have vented or sealed gas caps? Might try loosening the gas cap to see if it keeps the car from shutting down.

Don't rule this one out, I've seen this happen more than once and would explain why it works after a fill.

[procan]

Originally Posted by bourgeois911

Don't rule this one out, I've seen this happen more than once and would explain why it works after a fill.

What am I looking for here?

[Targalid]

When the car stalls, loosen the filler cap and listen for the pfffffff of air rushing into the fuel tank. It could be as simple as a crushed vent line to the fuel tank. Lack of a tank vent leads to a vacuum forming within the tank as the fuel is pumped out. Because of the low pressure of a carb system, it does not take much vacuum to overwhelm the fuel pump. The vent line comes from a charcoal filter box (to trap those nasty gas fumes) which is legendary for becoming plugged after forty years. My old VW actually pumped the tank flat when the vent line became blocked.

[procan]

As far as the fuel line is concerned, my car only has one fuel line that goes from the gas tank to the fuel pump, and then splits off at a "T" to go into each bank of carbs. There is no return line to the tank or anything. Is this correct?

[HarryD]

Originally Posted by procan

As far as the fuel line is concerned, my car only has one fuel line that goes from the gas tank to the fuel pump, and then splits off at a "T" to go into each bank of carbs. There is no return line to the tank or anything. Is this correct?

That is pretty typical for a carb setup as long as you are using a fuel pump designed for carbs and not fuel injection.

Since the pumps do not put out a lot of pressure, the valve on the float valve can cut off the flow when the bowls are full and the pump is deadheaded. when the leveldrips, the valve opens and fuel flow resumes.