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Fuel drip after shutdown


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Wondering if anyone has any ideas here..

Shut down after the second flight of the day.  About to push back into the hangar, and noticed fuel dripping out of one of the breather tubes coming out of the left cowl flap.  The fuel dripped at about a 4 sec/ drip rate.  Some steam (I think steam...probably not smoke) was coming out too.

This is the first time I’ve seen something like this.  It was pretty hot today...low 90’s, and unsure if that had anything to do with it.

I switched the fuel selector to off, and the drip stopped about 10 mins later.   Any ideas??

 

 

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Are you sure it’s not coming out of the the fuel sump drain. The O ring goes bad in that once in a while. Sometimes just pulling the ring on the floor board up and down a few times will reseat it. Maybe not the problem at all but thought I’d mention it.

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Re a several vent hoses in the left cowl, the crankcase breather about 3/4” diameter aluminum tube. A 1/4” rubber vent hose from the engine fuel pump fuel leaking from here is bad and means fuel pump failure.  A 1/4” rubber vent hose from the bottom of the engine induction system, it drain raw fuel from the induction system, some draining from here would be OK as long as it stops fairly quickly.

Clarence

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It’s definitely not the crankcase breather.  It’s one of the ¼ inch rubber hoses.  Gonna pull the cowling tomorrow.  I hope the hose is connected to the sniffle valve, in which case, all should be ok.  Will post the results.

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If after landing you did not turn the electric fuel pump off excess fuel will be drained.

Fuel may be coming from the engine fuel pump breather line. To check for pump leak do the following:

1. Open throttle and mixture (full forward)

2. Turn on the electric pump and watch fuel pressure.

3. With the pump running close mixture and throttle.

4. Turn pump off.

5. Check fuel pressure is holding for at least a minute.

If fuel pressure drops in less than a minute the engine fuel pump may have a cracked diaphragm. Check fuel pump breather line for fuel.

Fuel pressure drop could also be caused by a leak on the lines or the fuel servo.

José 

 

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My plane did the exact same thing and those few drops slowley increased to more and more drips. It was exactly as piloto explained. Mechanical fuel pump diaphragm had cracked and the pump needed to be replaced. Verified by not being able to hold fuel pressure. 

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Each drain should be pretty easy to identify as a source...

Both fuel pumps, each have a drain... fuel comes out in drips when something has gone wrong inside...

The sniffle valve is a popular place to have some fuel exit... after shutdown...

Fuel selector drain, if it leaks fuel out, it is also leaking air in while flying...  air bubbles interrupt the accuracy of the FF data... a hint of what may be happening...

PP thoughts. Not a mechanic...

Best regards,

-a-

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To provide an update.... The original drip was in fact, coming from the sniffle valve.   I did a runup today, and it did not drip.  Although, I wouldn't expect it to drip, as the engine probably didn't get hot enough by just running on the ground.

I did the test as Jose suggested.  Fuel PSI got to just over 14 psi, and then did drop after I turned off the pump and closed the throttle/mixture.  I'd say it dropped about 2 psi in 20-30 seconds.  

I then started the engine and did a mag check.  The left mag was normal, about 60rpm drop.  However, the right mag, which also usually drops 60 rpm, now drops 120 rpm.  I tried running up and leaning to burn off any lead, but it did not resolve the issue.  I have Tempest fine wires all around.  

I doubt the issues are related, and obviously will not fly it until I can talk to my A&P.  Definitely have some gremlins to work out.

 

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New and OH’d mags have a break-in period(?) where the timing has a tendency to drift...

Star washers have a tendency to dissapoint when re-used... a loose mag might change things a bit...

Stuff I have only read about. Not first hand experience...

Best regards,

-a-

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8 hours ago, jasona900 said:

To provide an update.... The original drip was in fact, coming from the sniffle valve.   I did a runup today, and it did not drip.  Although, I wouldn't expect it to drip, as the engine probably didn't get hot enough by just running on the ground.

I did the test as Jose suggested.  Fuel PSI got to just over 14 psi, and then did drop after I turned off the pump and closed the throttle/mixture.  I'd say it dropped about 2 psi in 20-30 seconds.  

I then started the engine and did a mag check.  The left mag was normal, about 60rpm drop.  However, the right mag, which also usually drops 60 rpm, now drops 120 rpm.  I tried running up and leaning to burn off any lead, but it did not resolve the issue.  I have Tempest fine wires all around.  

I doubt the issues are related, and obviously will not fly it until I can talk to my A&P.  Definitely have some gremlins to work out.

 

With the engine off, full throttle and mixture you should see fuel dripping down from the sniffle valve hose when the electric pump is running. 

Fuel pressure drop is an indication of a minor crack on the engine pump diaphragm. This crack will grow.

A 14psi electric pump fuel pressure is an indication of pump wear. When the pressure is just below 14psi you may have a hard time starting the engine in cold weather. If the engine pump fails to deliver enough pressure a weak electric pump may be unable to keep the engine running at high power during takeoff.

José

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10 hours ago, Piloto said:

With the engine off, full throttle and mixture you should see fuel dripping down from the sniffle valve hose when the electric pump is running. 

Fuel pressure drop is an indication of a minor crack on the engine pump diaphragm. This crack will grow.

A 14psi electric pump fuel pressure is an indication of pump wear. When the pressure is just below 14psi you may have a hard time starting the engine in cold weather. If the engine pump fails to deliver enough pressure a weak electric pump may be unable to keep the engine running at high power during takeoff.

José

I have not had any issues with cold starts... however, it has become more difficult to hot start.  Again, I think the hot weather has something to do with it, but is likely exacerbating another issue.

What sort of fuel PSI should one expect from the electric fuel pump alone?  I have never thought to check..  

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15 minutes ago, jasona900 said:

I have not had any issues with cold starts... however, it has become more difficult to hot start.  Again, I think the hot weather has something to do with it, but is likely exacerbating another issue.

What sort of fuel PSI should one expect from the electric fuel pump alone?  I have never thought to check..  

The pressure should be in the middle of the green arc or about 20psi. As the electric pump ages the pressure will drop. During run up always check fuel pressure. A low fuel pressure can cause engine stall during takeoff. This is why the electric pump must be on during takeoff, specially at high altitude. On the turbo M20M the fuel pressure goes down at altitude and the electric pump must be turn on to keep the pressure high.

José

Edited by Piloto
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40 minutes ago, Piloto said:

The pressure should be in the middle of the green arc or about 20psi. As the electric pump ages the pressure will drop. During run up always check fuel pressure. A low fuel pressure can cause engine stall during takeoff. This is why the electric pump must be on during takeoff, specially at high altitude. On the turbo M20M the fuel pressure goes down at altitude and the electric pump must be turn on to keep the pressure high.

José

Good info, thank you.  I do monitor it during flight, and it is around 25psi. 

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I have not had any issues with cold starts... however, it has become more difficult to hot start.  Again, I think the hot weather has something to do with it, but is likely exacerbating another issue.
What sort of fuel PSI should one expect from the electric fuel pump alone?  I have never thought to check..  


My Weldon will typically produce 26 to 27 PSI during a start. If I am running, the engine driven fuel pump will show 24 or 25 PSI. If I turn on the boost pump while flying, I will see the 26 or 27 PSI.


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I just checked it again, and again PSI peaked at about 14.2 psi, and dropped as soon as I closed the throttle and mixture.  I spoke with Eric at Oasis, and he said it is possible the mixture isn't closing all the way, which can also cause the PSI to drop.  Obviously I am hoping that is the case.  He is suppose to be in Nebraska later this week, and may stop by for a look.   Will continue to post any updates for those interested.

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I just checked it again, and again PSI peaked at about 14.2 psi, and dropped as soon as I closed the throttle and mixture.  I spoke with Eric at Oasis, and he said it is possible the mixture isn't closing all the way, which can also cause the PSI to drop.  Obviously I am hoping that is the case.  Either way, it sounds like my He is suppose to be in Nebraska later this week, and may stop by for a look.   Will continue to post any updates for those interested.


I’ll run Jose’s test this evening to verify one holds pressure. When I do a cold start, I have the mixture rich, throttle quarter open (roughly 1,000 RPMs). With this configuration, I see th 26 to 27 psi.


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First post here, regarding fuel pressure I’ve noticed during the initial climb after shutting off the electric pump the pressure will read in the bottom quarter of the green.  The electric pump brings it up to the middle and during cruise it normally sits in the middle.  Is this typical?  Another anomaly I noticed once was fuel pressure was quite slow to get to the green after starting.  The electric pump brought it up but went back down after turning off.  After a couple min it was in the middle of the green.

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Step one should be to verify the fuel pressure with a known test gauge to determine if you have a pump issue or a gauge issue.  Old gauges and pressure transducers with old wiring aren’t always accurate.

Clarence

 

Edited by M20Doc
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On ‎6‎/‎2‎/‎2018 at 1:59 PM, marcusku said:

First post here, regarding fuel pressure I’ve noticed during the initial climb after shutting off the electric pump the pressure will read in the bottom quarter of the green.  The electric pump brings it up to the middle and during cruise it normally sits in the middle.  Is this typical?  Another anomaly I noticed once was fuel pressure was quite slow to get to the green after starting.  The electric pump brought it up but went back down after turning off.  After a couple min it was in the middle of the green.

Do the fuel pump pressure test described above. You may have a cracked pump diaphragm.

José 

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