Over the years, there were some variations in the original vent lines from Cessna, and variations in the location of the exit hole in the wing. The vent line can be bent a little for proper fit. See the Installation Tips for McFarlane Fuel Vent Lines.

We pressure test our repaired valves to a minimum of 5 psi using precision controlled air pressure and a calibrated low pressure indicator. This pressure is above the normal fuel pressure on the valve and below the limits of the springs in the valve. Each valve is submerged in Stoddard solvent while the air pressure is applied to the valve. The valves are then tested in each position to assure that there are no inter-port or external leaks. Leaks are detected by looking for air bubbles escaping from the valve.

Leaking fuel or oil through the fuel pump overboard drain line indicates the rotary seal is running dry or contamination happened in the field. Your fuel pump should be replaced before further flight.

Fluctuating fuel pressure can be the result of the following:

1. A worn fuel pressure gauge
2. A worn or broken O-ring on the inlet side fuel fitting
3. An obstruction in the induction system (air intake)
4. A clogged or dirty fuel strainer
5. Foreign material inside the fuel body (such as fuel cell sealant, thread sealant, or a drill shaving)

The most common reasons for low fuel flow/pressure are the following:

1. A worn or broken O-ring on the inlet side fuel fitting can cause an inlet suction leak. An inlet air leak will result in low Fuel Pump output pressure.
2. A weak operating spring in the Fuel Pump diaphragm assembly
3. A worn fuel pressure gauge
4. A clogged fuel vent or fuel cap
5. A constriction in a fuel line either externally (a kinked hose), or internally (due to a collapsed hose lining)
6. Wrong fuel pump for the installation

Cessna SEB99-18R1 describes periodic checks of the fuel gauging system.

The Viton gasket and screw seals do not require any sealer and must not have any lubricants. Do not over tighten the attachment screws as over tightening can extrude the gasket out from under the transmitter and tear the gasket in the screw holes. This will cause a leak. Tighten the screws evenly until you see the gasket just start to extrude a little. Any lubrication will aggravate the tearing of the gasket.

Diaphragm fuel pumps should be replaced or overhauled:

1. At engine TBO or after ten years in service, whichever occurs first. Replace on condition if leaks develop or performance drops for example.
2. Refer to Lycoming Mandatory Service Bulletin 240W for information regarding engine TBO.

Do you have aftermarket Monarch fuel tanks installed? We have received customer feedback stating that when Monarch or other aftermarket fuel tanks are installed, McFarlane’s fuel vent line does not fit. There is nothing we can do to solve that issue. McFarlane’s fuel vent lines fit well when OEM fuel tanks are installed. Please review the Installation Tips for McFarlane Fuel Vent Lines.

There are several other components in the fuel indicating system which could be the issue or are causing the indication problems:

• Ensure that your aircraft fuel indicating system utilized Stewart Warner style transmitters.
  • Even if your aircraft originally came with that style, there were many Cessna Service Bulletins to replace them with the Rochester style. Our transmitters will not work with those components if they have been changed. This was a very common SB when Cessna switched over to Rochester. 
  • Transmitter Identification shows the difference in the Rochester style and the original Stewert Warner transmitter.
  • McFarlane transmitters and Cessna Stewart Warner transmitters have an electrical resistance range of about 32 ohms when in the full position and 250 ohms in the empty position. This can be measured with an ohm meter connected between the wire terminal and the metal body. Note that the gold dichromate corrosion treatment on the zinc plating is a poor conductor. The metal body must be scratched a little to get a good electrical connection. The Rochester transmitter has a much different resistance pattern than the Stewart Warner or McFarlane transmitter.
• Consult your log books and check the part numbers of the fuel gauges and transmitters. Do not rely on Illustrated Parts Catalog or eligibility.
• With the age of most of the general aviation fleet, the possibility of corrosion in the wires or grounding is very likely. The gauge (indicator) may not be working correctly or properly calibrated. 
  • Since the transmitter gauge system works with very low voltage and very low milliamp electrical flow it is very sensitive to proper grounding and high resistance in electrical connections. Most problems are related to electrical connections.
  • Excess resistance in the transmitter circuit can be detected by measuring the transmitter resistance with the transmitter being installed but without the wire connected at the terminal, and then measuring the resistance at the transmitter wire at the back of the gauge with the transmitter wire connected to the transmitter terminal and the transmitter wire disconnected from the fuel gauge and the other ohm meter connection to a ground at the back of the gauge. The resistance readings should be very close to the first resistance reading. In other words, you are measuring the transmitter resistance first without the airplane circuit and then comparing the reading with the airplane electrical circuit.
  • If the preceding wiring check shows good, the problem is likely in the gauge. The fuel gauge has a brass grounding strap that grounds the internal electrical coils to the gauge case. With years of service this grounding strap can develop a thin layer of corrosion that restricts electrical flow. When this happens the gauge will show more fuel than what is in the tank which is not good! Cleaning this ground strap should fix the problem. Refer to the Cessna service manuals for detailed trouble shooting and maintenance information.

Caution! Never short the battery power to the transmitter wire! It will take only seconds before the stainless steel resistance wire in the Cessna transmitter will glow red hot in the fuel tank. The hot wire could explode the tank! Never have power on when trouble shooting the fuel gauging system.

Electricity in the Fuel Tank. Is it Safe?

Always check the fuel gauge system for proper calibration per the Cessna maintenance instructions.

A common cause for this problem is fuel starvation due to improper installation of the fuel pump. Make sure that the fuel lines are not crossed; that is, the inlet line should be connected to the inlet side of the pump (marked “IN” on the top of the port) and the outlet line should be attached to the fuel pump outlet port. Fuel should be present at the inlet side of the fuel pump.

Improper installation of the fuel pump may result in a misalignment of the operating lever with the operating plunger in the accessory case. If so, there will be no movement of the lever and no pumping motion to provide fuel flow. If misalignment is found upon removal of the pump, the lever is probably damaged or broken. If misalignment of the operating lever has occurred, it will require inspection and repair by a qualified technician.

The most common indication of external fuel valve leaks is the smell of avgas in the cabin. Most valves are located under the cabin floor. Fuel stain on the valve and drain plug or drain valve or on the belly of the aircraft can also indicate external leakage. External leaks are generally around the actuation valve stem. Internal leaks are detected when the fuel is turned off and fuel continues to drip during fuel system maintenance. Inner port leakage will allow fuel from one tank to leak into another fuel tank. This type leak is difficult to detect. Fuel transfer from one tank to another with the fuel valve selected to one tank only will indicate inner-port leakage. An extended period of time is needed to detect fuel transfer from one tank to another. Generally if internal leakage is detected when servicing the fuel system, it is likely there is also inner-port leakage. 

The fuel selector valve works in unscreened fuel. Contaminated fuel can cause selector valve seal damage. When fueling from possibly contaminated fuel sources McFarlane recommends the use of a fuel filter, such as Mr. Funnel or another filtering device, to reduce trash in the fuel system.

Yes, please review the Installation Tips for McFarlane Fuel Vent Lines.

Yes. The fuel screen is removed by removing the standpipe. The rubber tipped plunger must be unseated before the standpipe is loosened. Failure to lift the plunger off of its standpipe seat can damage the plunger. Pull the strainer drain knob as if you were draining the fuel bowl. Block or clamp the knob in this position. Insert a smooth round tool such as a screw driver or punch in the standpipe cross-hole and un-screw the standpipe.

No, the fuel indicating system on a Cessna 152 is unique and McFarlane has not designed a transmitter for them yet.