Fuel drip from common drain after engine shut down

[Immelman]

Aircraft is a 1966 E model, engine lyc IO-360-A1A.

Yesterday I went flying and after shut down noticed a fuel drip (one drop every 5 sec or so) coming from the common drain in the left-rear of the engine compartment. I don't think I've seen this before so it caught my attention... I'll be going to my mechanic about this shortly but wanted to see what mooneyspace had to say.

To be clear: Inside the cowling, that drain goes to a tee and one half goes to a drain on the engine oil sump (I believe this serves as an induction system drain/overflow from the fuel servo, i.e. in a flooded/over primed scenario). The other side goes up to an overboard vent for the engine driven fuel pump.

The other interesting thing is that on this particular engine shut down the prop stopped not in its usual spot. Take that for what little (if anything) its worth. I de-cowled the airplane and looked for anything obviously wrong and didn't see anything. At this point the drip stopped, and I cowled it, did a normal hot-start, and ran the engine in my parking spot and checked fuel pressure with the engine driven and boost pumps. I ran the mixture through its range of motion and noted normal fuel servo operation with about a 75-100rpm rise at idle before the engine started to quit, as the mixture was leaned. I then shut the engine down from ~1300rpm using the mixture and it quit normally, no sputtering. This time the prop stopped in its usual spot, and no more drip.

Ideas?

Edited February 28, 2016 by Immelman

[Yetti]

Sometimes the drains are tide together some times they are separate.

What model plane?

http://www.donmaxwell.com/publications/MAPA_TEXT/External_Hoses/External_Hoses_Tubes.htm

 

To test if it is the mech pump.

Boost pump on,

Open mixture full

Count to six

Close mixture

boost pump off

see if pressure holds on FP guage

If not, replace mechanical pump

 

[Immelman]

Thanks Yeti.... sorry I forgot the most basic info! 1966E. Lycoming IO-360-A1A.

Thanks for that procedure... thinking about that: That's more or less my normal pre-start routine for a cold engine. How long should the fuel pressure hold before you see any sort of appreciable drop?

Edited February 28, 2016 by Immelman

[Yetti]

Since that is the normal start procedure, I don't know how long it should hold. cause I am usually hitting the starter.  Say at least 30 seconds to a minute.  Mine was falling off immediately after I turned off the boost pump.

Depending on how you shut down the engine, according to Maxwell article some fuel can come out of the induction area

 

[Yetti]

Oh and the pump became suspect when I took it up to 5000 feet after annual and I saw the FP stumble a bit on the guage

[DrBill]

make sure mixture is at cutoff when parked... Mine did that Tues and Mech noticed it. 

came from manifold... Cutoff mixture and it stopped.

Bill

 

[Immelman]

Just an update on this issue... I spoke with my mechanic briefly and tried a couple more things. Airplane is grounded pending figuring this out of course.. but here is some more data:

1. The fuel pressure does NOT hold as it normally did afer Yeti's suggestion

2. It does NOT appear to be the engine driven fuel pump leaking. I isolated the fuel to be coming from the drain coming out of the oil sump (induction system side of the "Tee").
Tried several bouts of running the electric fuel pump, and it would build pressure, and then fuel would leak out from the sump after the fuel pump was turned back off and the pressure dropped.

3. After that experiment was done with everything buttoned up, I started the engine normally for a brief ground run, shut down normally, FUEL PRESSURE HELD, and no leak.

The drip is, I think, consistent with a change in behavior where the system is releasing pressure more rapidly than normal. For whatever reason, that did not occur after my brief ground run... So I am thinking, something in the servo? Fuel flow divider? I need to get back with my mechanic on this and/or get it into the shop. My system knowledge of the servo and fuel flow divider, and things such as any check valves, seals, and associated failure modes is not very strong.

Edited March 8, 2016 by Immelman

[Yetti]

From another thread. Servo Test.

 

 

6.  The fuel servo is not related to this issue but i asked about his thoughts since it was 1985 since being replaced.  The way to test it is to run the engine up and get it warm, do mag checks, then to idle down to 900 or so RPM, turn on the boost pump.  If the pressure from the boost pump pressure changes the mixture then the servo needs to be looked at.