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Fuel Valve / Fuel Cock FAQ

September 27, 2010 | Troubleshooting & Repair

Text and photos by Barry Lee Brisco


The fuel valve or fuel cock (also called a "petcock", shown at right in a 356A) is one of those 356 peculiarities that generates frequent questions from new owners. Not used to having a car that offers control over fuel flow from the driver's seat, they are often either baffled by it, ignore it, or may even decide to remove it! In fact, it is a useful device that serves multiple purposes, and it is worthwhile to take a moment and understand its function. While at first glance it may appear to be dangerously exposed, Bruce Baker notes that after 40 years of seeing a few hundred 356s get mangled in accidents, he cannot remember a single incident of a fuel-related fire or a leaking gas tank (read more about 356 fuel system safety issues).

Q. Which position is "on" and which is "off"?

Q. What do all those funny German names mean?

Q. How do I use the "reserve" setting, and how much fuel is in "reserve"?

Q. Why not just leave the fuel valve in the "on" position all the time?

Q. Should I rebuild my old fuel valve or buy a new one?

Q. How do I remove the fuel valve from the tank, and remove the retaining clip on the front of the fuel valve?

Q. How do I drain the fuel tank?

Q. What color is it supposed to be, or, can I paint it black so it isn't so noticeable?


For 1961 model year cars and earlier (T5 and earlier as shown in the photo at right of a 1958 car), on (open or "Auf") is when the handle points straight down. Off (closed or "Zu") is fully counterclockwise, meaning the handle "points" to the right side (passenger side). Reserve ("Res") is fully clockwise, pointing left towards the driver. This assumes that the petcock was assembled and installed correctly. It is possible to assemble it so that the handle works 180 degrees opposite what is correct. It will still work, but is likely to confuse those who are used to the standard positions.

For 1962 and later model year cars (T6) the on and reserve labels are reversed. "Zu" is shortened to "Z", and "Auf" is shortened to "A". Regardless, off is still with the handle turned fully counterclockwise and reserve is fully clockwise. On is with the handle pointing straight up, as shown below in a page from the 1962 356B Driver's Manual (Sep. 1961 edition).

Below is part of page 33 from the Sep. 1957 edition of the 356A Driver's Manual. Note it says "Fuel cock" instead of "Fuel valve". Why did Porsche decide to call two parts with exactly the same function by different names?

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"Zu" = "Closed" (simplified to "Z" in 1962 model year cars and later). "Auf" = "Open" (yes, really). "Res" = "Reserve" (the easy one to remember).

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If you are driving along and the engine starts to sputter and lose power, and you check your fuel gauge and see that the needle is low on the scale, turn the fuel valve handle fully clockwise to "Res". After a few seconds the engine should run normally if the problem was simply low fuel in the tank.

You now have approximately 1 1/2 gallons of fuel to get you to the nearest gas station, meaning 30 to 45 miles depending on what kind of mileage you get, and assuming that the reserve inlet in the tank is not clogged from years of disuse!

IMPORTANT: after filling up the tank after running on reserve, turn the handle back to "Auf" ("Open"). If you leave it on "Res" you run the risk of driving the car past the reserve amount and, when the engine starts to sputter from no gas, discovering that you now have no reserve at all.

DO NOT rely on the fuel gauge to accurately portray the quantity of fuel in the tank. Some owners find that when the gauge shows a 1/4 full tank (which should be about 3 gallons) they need to switch to reserve because at that point they only have about 1 1/2 gallons left. The 356 fuel gauge is woefully imprecise (and it is normal for the needle to bounce erratically while driving, this is due to fuel sloshing around and moving the float up and down).

Tips on the reserve setting

(From Vic Skirmants) Every now and then, while driving on an expressway, move the lever to "reserve". Let it run a few miles. That way, if there is water in the tank, and it starts to sputter, you have momentum to give you time to switch back to "auf". That way you won't be switching to water if you do need the reserve setting for real someday.

(From Bill Block) Not the most important reason, but perhaps the most useful reason for turning selector to Zu every time the car is parked: if you turn gas off every time car is parked, you will also need to turn the selector to auf -- which means you will never find out that you left the selector on res from the last time you ran the tank down.

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Many do just that, but many always turn it to the "Closed" ("Zu") position each time they shut down the engine. If you have a leaky carb, with the fuel cock on, fuel can leak down into the cylinders, fill them up with gas, and make it impossible to start the engine. This is more likely to occur if the car is parked nose up. Worst case scenario: with a cylinder full of gas, you determinedly attempt to start the car and break something very expensive, resulting in a complete engine rebuild. What to do: remove the spark plugs and turn the engine over by hand several times to get the gas out of the cylinders, then put the plugs back in and start it. The gas in the oil is not a problem.

RECOMMENDATION: why not develop the habit of turning the fuel valve off right after turning off the ignition? As an added bonus, if an ignorant thief hotwires your car and drives off with it, in a few blocks they will run out of gas and abandon the car.

(In the Sep/Oct 97 issue of the Registry magazine, Vic Skirmants wrote) "I keep reminding people to turn off the fuel cock when parked. If the fuel tank is full, it can act as a gravity-fed fuel system, and keep enough nominal pressure on the carburetor needle valves to let the fuel keep dribbling down the intake ports. Also, if you have the front of the car jacked up to do somework on the front end, you really have a gravity-fed fuel system!"

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It is simpler to buy a new one (available from the usual sources, looks exactly like the original, and is not particularly expensive) then to try to rebuild it unless you have a burning desire to exercise your mechanical skills. But it is always admirable to preserve and restore original parts.

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To remove the fuel valve from the car, drain the fuel tank, pull the cotter pin on the petcock shaft and loosen the nut that holds it to the tank. The tank side of the nut is a right hand thread, so counterclockwise to remove. The petcock is a left hand thread so clockwise to remove. Have a rag on the floor under the petcock because you'll still have a few teaspoons of gas to deal with. NOTE: You do not have to remove the gas tank from the car to remove the petcock.

To remove the clip, put the fuel valve in a vise, close the vise a bit to relieve tension on the clip, and take it out. Some use a 14mm socket and soft jaws in the vise to do this. Removing the clip releases the part of the fuel valve that has the hole for the cotter pin in it. Beneath this there is a flat metal plate and a cork gasket that covers the holes through which the fuel passes. This plate will fit in either a "heads" or "tails" orientation. Note which side is up, because if you put the petcock back together with the plate upside down, it will cause the cotter key hole to be in the wrong orientation which will cause the handle to point in the wrong direction, resulting in no end of confusion.

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Look at the gas gauge and estimate how much is left. Get a couple (three if it is full) of five gallon gas cans for the fuel in the tank. Turn off the fuel cock. Disconnect the rubber line at the fuel cock. Get a long enough section of new line to go down into the gas cans sitting on the floor outside the car, and attach the new line to the fuel cock. Drain the tank into the cans. Remember to drain the reserve by turning the fuel cock handle fully clockwise! Be careful of inserting a drop light into the tank because it could be hot enough to ignite the gas. (Contributed by Frank Boyer)

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The fuel valve is bare aluminum and the steel pieces are zinc-plated except for the snap ring (the handle is black semi-gloss), making it fairly visible even under the dash. A few find this objectionable, and paint it black. Most leave it as is and ignore it. A few even think it looks kind of cool: the triumph of function over form.

Even fewer go all the way and remove it. The disadvantage is that now you have no fuel reserve function, no way to conveniently shut off the gas when working on the engine, and draining the tank is a bit more difficult. Experienced 356ers recommend keeping the petcock in place as originally designed by the good doctor.