In figure 1, the small tube is a “weep” or tattle tale drain fitting for the engine driven mechanical fuel pump. Figure 2a is an AC fuel pump common to Lycoming powered Mooney’s. This pump is very similar in appearance to pumps used on Chevrolet engines. One difference is the center section. This section separates the fuel side of the pump from the engine so that in case a diaphragm is perforated, the leaking fuel will be pumped overboard instead of into the engine. A fitting and hose connect the fuel pump to the tube located in the lower cowl. Without this protection, fuel could enter the crankcase and dilute the engine oil resulting in engine damage or failure.
The larger clear hose protruding thru the cowl bottom is a fuel drain for the intake system. Excess fuel from over priming, flooding and normal shut downs, drain from this tube.
In Fig. 2, we find a larger tube located in the cowl flap area. This tube is for crankcase breathing or venting, and is connected to the top portion of the accessory housing.
Protruding from the panel just aft of the cowl flap is a threaded fitting. This fitting is installed into the electric fuel pump. (Fig.3) The purpose of this fitting is also as a “weep” or tattle tale for the seals in the fuel pump. As in the mechanical pump, the electric pump has a center section that separates the fuel pump side from the electric motor side. Any leakage from the pump will be pumped overboard thru this fitting and not into the electric motor.
E, F, & J Models
On “E” & “F” models with original cowls, the lower cowl, (fig.4) will have one tube protruding. Some models have two separate tubes protruding. Inside the cowl, this line becomes a “T”. Attached to this “T” is a “weep” line from the fuel pump as in the earlier models and a fuel sump drain line for the fuel injection system. Since the fuel injection servo is mounted on the front of the engine sump, an induction drain is mounted at the lowest point in the induction system. Figure 5 shows the location of the induction drain valve. This valve is manufactured by Mooney, p/n 610122-501. Fig 6, is a disassembled view of this part. It consists of a standard AN fitting that has been modified to accept a nylon ball and retainer pin. Current cost of this valve from Mooney is approximately $88.00.
Since this valve appears to be a standard AN fitting, it is often overlooked when an engine is exchanged. The purpose of this valve is to close and seal the induction system when the engine is running, and to open and allow excess fuel to drain when the engine is not running. On “E”, “F” & “J” models, it is common for this line to “weep” a small amount of fuel after each engine shutdown.
Lycoming powered, fuel injected Mooney’s, share a distinctive, “loping” sound at low power settings during taxi and idle operation. The loping sound is caused by system leakage at the induction drain valve. A small amount of “lope” is normal. Excessive “lope” can be cured by removing the drain valve hose at the drain fitting and spraying the inside of the fitting with Tri-flow, corrosion X or solvent. Fuel dye and oil cause the ball to stick and not seal properly causing rough low rpm engine operation.
These models also have the breather/vent hose in the cowl flap area and the electric fuel pump “weep” fitting in the aft panel. “J” models have the breather/vent, mechanical pump “weep” drain and induction drain all located in the left cowl flap area.
Located in the right cowl flap area on the Bravo, (fig.7) we find three tubes. The larger tube is the breather/ vent line that is connected to the air/oil separator and then to the accessory housing as in the other engines. The two smaller lines are connected to the exhaust side of the primary and standby vacuum pumps.Just ahead of the nose gear in the lower cowl are two smaller diameter hoses. One hose is connected by a Tee to the lower fuel sump drain as the other Lycoming engines and to a fuel drain in the lower part of the inter cooler. The other hose is connected to a spacer between the engine driven vane pump and the engine. Again, this prevents a leaking fuel pump seal from diluting engine oil. Located in the aft panel, as the other models, the electric fuel pump “weep’ fitting protrudes thru the skin.
On all models, leakage from any of the “Weep” fittings is cause for replacement of the electric or mechanical fuel pumps. An addition to your preflight check list would be to check each of these protuberances for obstruction. In our area, these are favorites for dirt daubers.
MAPA Log, Volume 25, Number 4 (April 2002)