Published in the September 2018 issue of “Die Porsche Kassette”
Ⓒ2018 Technolab / PedrosGarage.com
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On a previous Tech Article we discussed the fuel injection system, now we delve deeper into it and discuss the fuel pump in particular.
The fuel pump, as it name indicates, pumps fuel from the tank to the engine. It uses pressurized fuel lines which end at the fuel rail* where the injectors vaporize the liquid gasoline so it mixes with the incoming air from the intake plenums. The mix then goes through the intake valves and into the cylinders where the spark plugs make it all ignite.
Most of the older (fuel injected) Porsches (up to the 993) had an external fuel pump, meaning that it was located between the tank (up front) and the engine (at the rear). In some cases like on the 924 and 928 there were two pumps. A pre-pump and an external pump.
But starting with the 1997 water-cooled Boxster 986 and then with the 1999 Carrera 996, Porsche installed a single submerged pump. Yes, it is inside the gas tank!
Although it may seem counter-intuitive, submerging the electric fuel pump in the gasoline tank puts the component least likely to handle fuel vapors well farthest from the engine and submersed in a cool liquid which is less likely to start a fire. Liquid gasoline does not ignite, only its vapors in the presence of oxygen.
(*) In direct fuel injection (DFI) cars the injectors are located right on the cylinder walls and vaporize fuel straight into the combustion chambers.
Fuel pump immersed in gasoline
If the fuel pump stops working your Porsche is not moving. When a fuel pump starts to go, the engine may still get enough gas to keep it going, but a failing pump will not heal itself and will eventually stop operating altogether. If you are alert, you may be able to catch a failing pump before it goes out completely.
So what are the signs?
The most common one is when you’re on the road at higher speeds (60+ MPH), the car starts jerking but then resumes normal driving. Check your gauges because this could mean that you are running out of gas literally, or it could signal a failing fuel pump.
A failing pump can also result in lower MPGs, so be alert if all of the sudden your car becomes a gas hog.
Porsche fuel pumps from 1997 through 2008 generally have a humming sound that can be heard by the driver if your hearing is better than mine. Many times though, a pump that’s starting to fail will develop a VERY loud, high-pitched whirling noise which even I can hear!
If you suspect that your fuel pump is on its last legs a simple pressure test can be performed to determine if the correct fuel pressure exists at the rails. But even if there is less than optimal pressure it may not be just the pump. A bad fuel pressure regulator and or bad fuel lines can also affect a fuel pressure reading.
Most of the newer Porsche fuel pumps are integrated into the fuel sending unit which consists of the electric fuel pump, the filter, the strainer and the electronic sensor used to determine the amount of fuel in the tank. In the newer Porsche sports cars it is located under the car’s battery and accessed via a panel on top of the fuel tank.
Fuel sender unit with pump (in aluminum)
If you decide to tackle it yourself, let me remind you that you will be dealing with fuel and fuel vapors. You should be in a very well ventilated area and be extremely cautious of sparks that could be generated from the use of power tools.
It may be more advisable to let your trusted Porsche techs tackle the job as they will run diagnostic tests to make sure that the issue has been precisely identified and they also have specialized tools for each particular job. Replacing one of the least expensive will cost several hundred dollars, newer ones are way more than that.
As you can see above, it is not fun work replacing the fuel pump, especially at 3:30 AM in one of the pits at Sebring during a DE weekend.
A bad fuel pump can certainly ruin your day!