Fuel starvation

[BK911]

1972 911E W/MFI

My engine starts sputtering whenever I have less than ¼ tank of gas and make a hard turn. It has even left me on the side of the road a few times. I just have to crank it for a few seconds then everything is fine. But I’ve never had this problem with my other 911’s.

I was just curious if anybody else had this problem.

Thanks!
BK

[ken_xman]

your gauge is off by 1/4

[ttweed]

Fuel slosh in the tank can cause your pump to draw air in hard cornering. Don't let it run lower than 1/4 full or install a surge tank to insure an uninterrupted supply of fuel to the pump.

TT

[BK911]

Never heard of a surge tank. Does that replace the existing tank? Or is it an add on?

I was mainly curious to see if anybody else had this problem. If so, then I will live with it. If not, then I will start troubleshooting.

Thanks again!
BK

[Grady Clay]

Check for rust in the tank plugging the filter screen in the bottom of the tank.

Best,
Grady

[ttweed]

Originally posted by BK911
Never heard of a surge tank. Does that replace the existing tank? Or is it an add on?

I was mainly curious to see if anybody else had this problem. If so, then I will live with it.

Yes, I experienced this with a CIS 3.0 engine running hard on the track w/ 1/4 tank or less. My solution was to just treat a 1/3-full tank as empty and fill it more often.

You should check that your whole fuel supply system is up to spec, though, as Grady suggests. Restrictions in fuel delivery could make the problem more severe than it should be.

Surge tanks can be internal or external. Most fuel cells incorporate at least a "surge well" or cup around the pickup (as well as using the foam to baffle slosh), to retain a pool of fuel for the pickup point in hard cornering. Routing the return line from the engine into this cup will help prevent it emptying in long sweepers. A stock tank can be modified in this way as well.

EFI systems can be much more sensitive to fuel pressure fluctuations than MFI, as the MFI pump itself makes the final step up in pressure to the injectors, but it still needs a good uninterrupted supply of fuel from the electric pump. With EFI, the amount of fuel delivered to each cylinder is a function of the pressure in the fuel rail times the injector pulse width, so it is very dependent on the high pressure fuel pump. On my LL7 EFI car, the tank is just an open aluminum box, and I would get enough sloshing in the tank at 1/2-full to cause problems. I fabricated an external 1.5 Qt. surge tank to deal with this according to this schematic. This requires an additional low-pressure, high-volume electric fuel pump in the system. There is a picture of my installation on this page at my website if you scroll down to the update from 02/13/04.

HTH,
TT