New fuel pump yesterday - can't get below 1/4 tank?

[steve_wilwerding]

Got my passenger side fuel pump replaced under warranty yesterday - it was starting to make a ticking noise.

I was pretty low on gas (~30 miles to empty, according to the dash) when I brought it in, but I figured that might be good for them working on it. In any case, I talked to the service advisor afterwards, and he said that the gas tank has a hump in it, and that the passenger's side, when they went to work on it yesterday, was bone dry (which probably explains why the fuel pump went - previous owner must have driven on E a lot). He said that you should never let the gas get below a quarter of a tank, or it will start burning up the fuel pumps.

I've never heard this before - if it's in the manual, I don't recall reading it. It's not a big deal to start fueling it a little earlier, but that seems like kind of a poor design.

Anyone else heard of this?

[JFP in PA]

The problem is that these pumps are cooled by fuel around them; run the fuel level very low all the time, the pump overheats and its life is shortened. While this is common to a lot of the newer cars, because of the way the pump sits in the Porsche, it is a bit more of an issue. And, no, I have never seen anything from Porsche about this.

[Silver_TT]

JFP -- is what you stated specific to the Cayenne or all Porsche models?

[JFP in PA]

Pretty much all of them, some are worse than others, depending upon how the pump sits, the shape of the tanks, and pump locations, etc. (note the service advisor's comments in Steve's post). Running the tanks low on fuel most of the time is problematic particularly in vehicles with multiple tanks (one will always run low before the other) or strangely shaped tanks (like the four wheel drive 996 cars to get around the driveline), but also in the more normal shaped single tank cars (Boxster or 911 type). Customer's that run the car's low on fuel frequently have more pump problems than those that don't due to the pump getting too hot from lack of cooling and cavitation. And this is not just a Porsche issue, we see it on other makes as well; it is simply an unpleasant fact with "in-the-tank" fuel pumps.

[Silver_TT]

Ok, that's what I thought, thanks. My car is a 4S has the "saddle" gas tank so this info particularly applies -- didn't know that before. Thanks again.

Edited October 11, 2012 by ferrugia

[bigbuzuki]

I researched this a little while back.

The info I received stated that the pump in the tank which had low fuel would be shutoff to prevent such cavitation.

It was unclear if this was a function of the fuel level sensor noting the applicable half of the saddle tank being low or in combination with a sensor monitoring the attitude of the vehicle if it was banked to the left or right whilst offroading over a hill etc.

Edited October 12, 2012 by bigbuzuki

[JFP in PA]

I researched this a little while back.

The info I received stated that the pump in the tank which had low fuel would be shutoff to prevent such cavitation.

It was unclear if this was a function of the fuel level sensor noting the applicable half of the saddle tank being low or in combination with a sensor monitoring the attitude of the vehicle if it was banked to the left or right whilst offroading over a hill etc.

When the car is in motion, and the fuel level state is very low (< 1/4 tank), the fuel can slosh about rather quickly as the car corners and crests hills or simple rises in the road. At these moments, the pump goes into cavitation before the fuel level sensor even moves, and then recovers just as quickly. If it didn't the driver would notice a loss of power due to fuel starvation. But the result is the pump over-runs, gets hot, and wears internally. Eventually this repeated behavior catches up with it and it dies prematurely.

[bigbuzuki]

JFP, would you have any info on the operational logics of the fuel pumps for the Cayenne that you can share, such as which pump runs when etc ?

Thanks in advance.

[JFP in PA]

Not really, I our world fuel pumps are a lot like light bulbs, they either work or they don't. We just watch for common factors on the vehicles we need to change them out on to see if there is any pattern to the whys and whens of how they crap out. And constant low fuel levels are a 'repeat offender".

[RFM]

From the notes i have left: with the activation of drivers outer door handle - pump 2 (left side) running starting procedure: pump 1 (right) and 2 (left) running part load fuel tank < 60 L pump 2 running full load fuel tank < 60 L pump 1 running part load fuel tank > 60 L pump 1 running full load fuel tank > 60 L pump 1 and pump 2 running both together. Hope it helps.

[bigbuzuki]

From the notes i have left: with the activation of drivers outer door handle - pump 2 (left side) running starting procedure: pump 1 (right) and 2 (left) running part load fuel tank < 60 L pump 2 running full load fuel tank < 60 L pump 1 running part load fuel tank > 60 L pump 1 running full load fuel tank > 60 L pump 1 and pump 2 running both together. Hope it helps.

Good info RFM, thanks.

Edited October 13, 2012 by bigbuzuki

[RFM]

A small correction concerns fuel pumps operation: full load fuel tank > 60 L pump 2 running full load fuel tank > 60L pump 1 and 2 running both together if off road mode is engaged. ( Cayenne DFI ) This happened to a more even wear and heat distribution to obtain.

[hahnmgh63]

Many of the newer cars have in-tank fuel pumps and yes they do use the fuel to keep them cool, and yes it will shorten the life of the fuel pump if you always run your tank low all else being equal. I do a lot of work on Audi's & Bimmers and have seen this before.