Never have seen this before...

[bnt83]

http://www.pilotsofamerica.com/forum/showthread.php?t=63184

I got to troubleshoot this today and the results where obvious but I never imagined I would see the old AC type low pressure mechanical fuel pump disable the entire fuel system to a point the engine wont maintain full power in a level attitude, where a high wing should gravity feed, but it did.

#1. No fuel pressure with engine running or off, aux pump on or off. No combination would work. Engine runs about 50% power with no sign of fuel starvation

#2. Remove a fuel line off the engine driven pump plug & cap the lines, to prevent the discharge side of the parallel aux pump reverse flowing thru the engine pump back to the inlet side of the aux, and the fuel pressure quickly runs at normal 5 psi with the aux turned on.

No check valves in the system to prevent backflow thru the engine pump should it fail.... Yikes.

 

[bnt83]

For you blokes that like pictures. This is the type of pump I'm talking about.

 

[Silvaire]

#2. Remove a fuel line off the engine driven pump plug & cap the lines, to prevent the discharge side of the parallel aux pump reverse flowing thru the engine pump back to the inlet side of the aux, and the fuel pressure quickly runs at normal 5 psi with the aux turned on...

Did you try running it again hooked up normal? Because it sounds like a downstream constriction that you may have blown through by doing what you did. I'd take a serious look at the main fuel line to the carb.

 

[bnt83]

Did you try running it again hooked up normal? Because it sounds like a downstream constriction that you may have blown through by doing what you did. I'd take a serious look at the main fuel line to the carb.

No and here's why. The aux pump, when everything was hooked up, sounded like a "free wheeling engine" just hamming away. I bled the system ran the engine with no change. It was pretty obvious it was bypassing through the engine driven pump. When I capped and plugged a line off the engine driven pump, the aux pump sounded normal had the normal pressure rise, and slowed after reaching operating pressure.

I was diligent when installing his new hoses, looking at each hose for any flaps in the liners that may have missed at assembly. Also we flushed all lines noting flows before hooking them up to the carburetor.

These two pumps run in parallel right up to a multi port fitting/block right at the carburetor which the engine pump, aux pump and fuel pressure sense connect to. The system only runs at ~5 PSI FWIW

 

[N801BH]

No and here's why. The aux pump, when everything was hooked up, sounded like a "free wheeling engine" just hamming away. I bled the system ran the engine with no change. It was pretty obvious it was bypassing through the engine driven pump. When I capped and plugged a line off the engine driven pump, the aux pump sounded normal had the normal pressure rise, and slowed after reaching operating pressure.

...

A mechanical fuel pump will bypass if the diaphram inside it is ruptured.. That is the normal failure in a engine driven pump....

 

[bnt83]

A mechanical fuel pump will bypass if the diaphram inside it is ruptured.. That is the normal failure in a engine driven pump....

I would think we would be getting fuel out of the case drain line (drain between the two diaphragms) see the picture above with the little elbow sticking out of it if that was the case.

I think the outlet valve inside this pump is just hardened to the point it won't hold any pressure

Either way the pump is bad.

 

[Silvaire]

I believe the check valves are in the pump. How about opening the old one up and take a look-see. Maybe the answer is there.

 

[Henning]

I believe the check valves are in the pump. How about opening the old one up and take a look-see. Maybe the answer is there.

Correct, diaphragm pumps rely on check valves to operate. If either is stuck, no flow through. I would also recommend taking a sniff of the oil for gas smell.

 

[nrpetersen]

You said the pumps were in parallel. Could one of them (the mechanical one probably) be connected in reverse? We had a homebuilt Volmer Amphib that was incidentally plumbed this way & flew a very very low pattern on its first flight at FCM many years ago.

 

[bnt83]

You said the pumps were in parallel. Could one of them (the mechanical one probably) be connected in reverse? We had a homebuilt Volmer Amphib that was incidentally plumbed this way & flew a very very low pattern on its first flight at FCM many years ago.

Very good point, but its not backwards. I believe the current pump is about 25 years old.

 

[Henning]

Very good point, but its not backwards. I believe the current pump is about 25 years old.

It wouldn't have to be. If the check valve on the intake side of the mechanical pump is failed open, it provides the path of least resistance for the parallel electric pump flow.

 

[Silvaire]

It wouldn't have to be. If the check valve on the intake side of the mechanical pump is failed open, it provides the path of least resistance for the parallel electric pump flow.

It would also prevent the mechanical pump from producing pressure. I think that satisfies all the conditions that were being experienced. Any chance you can open the old pump and see what it looks like?

 

[Henning]

It would also prevent the mechanical pump from producing pressure. I think that satisfies all the conditions that were being experienced. Any chance you can open the old pump and see what it looks like?

Yep, they come right apart, and I would definitely inspect to see what was keeping the check valve open, it may have a chunk of deteriorating fuel line in it.

 

[N801BH]

It would also prevent the mechanical pump from producing pressure. I think that satisfies all the conditions that were being experienced. Any chance you can open the old pump and see what it looks like?

Hmmm.. maybe I read it wrong but the OP said the aux pump would not pass pressure unless the mech pump was bypassed..


I got to troubleshoot this today and the results where obvious but I never imagined I would see the old AC type low pressure mechanical fuel pump disable the entire fuel system to a point the engine wont maintain full power in a level attitude, where a high wing should gravity feed, but it did.

#1. No fuel pressure with engine running or off, aux pump on or off. No combination would work. Engine runs about 50% power with no sign of fuel starvation

#2. Remove a fuel line off the engine driven pump plug & cap the lines, to prevent the discharge side of the parallel aux pump reverse flowing thru the engine pump back to the inlet side of the aux, and the fuel pressure quickly runs at normal 5 psi with the aux turned on.

 

[Henning]

Hmmm.. maybe I read it wrong but the OP said the aux pump would not pass pressure unless the mech pump was bypassed..


I got to troubleshoot this today and the results where obvious but I never imagined I would see the old AC type low pressure mechanical fuel pump disable the entire fuel system to a point the engine wont maintain full power in a level attitude, where a high wing should gravity feed, but it did.

#1. No fuel pressure with engine running or off, aux pump on or off. No combination would work. Engine runs about 50% power with no sign of fuel starvation

#2. Remove a fuel line off the engine driven pump plug & cap the lines, to prevent the discharge side of the parallel aux pump reverse flowing thru the engine pump back to the inlet side of the aux, and the fuel pressure quickly runs at normal 5 psi with the aux turned on.

Right, the fuel just pumped straight back through the mechanical pump into the feed, no pressure because no restriction.

 

[bnt83]

I've experienced wondering and low pressure from a very old pump before. The valves internal to the fuel pump are just round flaps of rubber, as they age and get hard they don't seal quickly as the pump pulses up and down. The new pump in my 177 holds very steady pressure about 6 PSI, the old pump I had was all over the place but it never stayed at 0 for much more than a second or two on takeoff. The bad pump on this 177B won't even do that. The most I've seen was 2 PSI and it seems the gauge kinda sticks on the low side >2 and doesn't zero out well every time.

 

[bnt83]

http://www.vansairforce.com/community/showthread.php?t=73721&highlight=lycoming+fuel+pump

 

[Silvaire]

Hmmm.. maybe I read it wrong but the OP said the aux pump would not pass pressure unless the mech pump was bypassed..

No, he capped the line to completely remove the mechanical pump from the system. The two pumps are in parallel so the aux is always bypassing the mechanical pump. If the inlet check valve of the mechanical pump were failed open the outlet pressure of the aux pump would just go backwards through the mechanical pump and back to the inlet side of itself again. But to think of it seems like both check valves would have to fail for that to happen.

It would be nice to know what those check valves look like.

 

[N801BH]

No, he capped the line to completely remove the mechanical pump from the system. The two pumps are in parallel so the aux is always bypassing the mechanical pump. If the inlet check valve of the mechanical pump were failed open the outlet pressure of the aux pump would just go backwards through the mechanical pump and back to the inlet side of itself again. But to think of it seems like both check valves would have to fail for that to happen.

It would be nice to know what those check valves look like.

I must be getting confused..... Most fuel system arcitectures I have seen have the aux electric fuel pump upstream of the mech pump.... In theory, if the check valves fail in the mech pump, the aux pump will still provide adaquate fuel pressure to keep the motor running because it is boosting the pressure before the mech pump..

 

[Henning]

I must be getting confused..... Most fuel system arcitectures I have seen have the aux electric fuel pump upstream of the mech pump.... In theory, if the check valves fail in the mech pump, the aux pump will still provide adaquate fuel pressure to keep the motor running because it is boosting the pressure before the mech pump..

That would be plumbed in series and the way it normally works.

 

[bnt83]

That would be plumbed in series and the way it normally works.

But Cessna 177 & 177B (maybe RG's IDK) for whatever reason are plumbed in parallel. There is a steel line that comes off the strainer and connects to a T screwed into the aux pump. The other fitting on the T is the engine driven suction-side hose. Then two hoses, one from the outlet of each pump, connect to a three port block that is screwed into the carburetor. The third fitting on that block at the carburetor is the fuel pressure sense line that feeds the gauge in the cockpit.


The eye opener is, there is no check valve external to the engine driven pump to prevent failure of that pump from messing the hole system up.

This is the 150hp C177 not the 180hp 177B we are having problems with, but they are both set up the same:

 

[bnt83]

No, he capped the line to completely remove the mechanical pump from the system.


It would be nice to know what those check valves look like.

Correct. To elimate the flow path, all I did was remove one end of hose plug & cap the fittings in the engine driven pump circuit.

Dissassembly of this type of pump is here : http://www.vansairforce.com/community/showthread.php?t=73721&highlight=lycoming+fuel+pump

The bad pump is still installed till we get a replacement.

 

[N801BH]

I understand the issue now.... I always thought the aux feeds the mech pump....

I am guessing if someone wanted to install a fuel flow gauge, that the flowscan transducer would have to mount right where that 3 way adaptor screws into the carb.

 

[bnt83]

I understand the issue now.... I always thought the aux feeds the mech pump....

I am guessing if someone wanted to install a fuel flow gauge, that the flowscan transducer would have to mount right where that 3 way adaptor screws into the carb.

Or, shorten the hoses and put a cross in near the firewall that connects the fuel pressure transducer, two fuel hoses from the pumps, and a single hose that comes off the cross to the flow transducer near the carb, and a short hose from the transducer to the carb fitting.

A mess....

 

[Silvaire]

I understand the issue now.... I always thought the aux feeds the mech pump....

I think in line is usually referred to as a boost pump but most of the Cessnas and Pipers have parallel aux pumps. As for the lack of an external check valve, I'm seeing two check valves in the mechanical pump and both would have to fail open for this to happen so definitely do open that thing up when you get a new one and let us know what you find. I don't think this is a normal failure mode.

 

[bnt83]

I think in line is usually referred to as a boost pump but most of the Cessnas and Pipers have parallel aux pumps. As for the lack of an external check valve, I'm seeing two check valves in the mechanical pump and both would have to fail open for this to happen so definitely do open that thing up when you get a new one and let us know what you find. I don't think this is a normal failure mode.

Maybe the diaphram is ruptured as previously suggested.

 

[Silvaire]

Maybe the diaphram is ruptured as previously suggested.

Isn't the diaphragm on the back side of the check valves? I would think a ruptured diaphragm would be a normal and expected failure mode and shouldn't allow this back flow to happen.

 

[bnt83]

I capped the hose on the suction side of the engine driven pump and with aux on get 6 PSI. When I get the confirmed 25 year old pump off, I'll take it apart.

 

[bnt83]

Isn't the diaphragm on the back side of the check valves? I would think a ruptured diaphragm would be a normal and expected failure mode and shouldn't allow this back flow to happen.

That picture isn't the same. http://www.vansairforce.com/community/showthread.php?t=73721&highlight=lycoming+fuel+pump

 

[Silvaire]

That picture isn't the same. http://www.vansairforce.com/community/showthread.php?t=73721&highlight=lycoming+fuel+pump

Same concept, a ruptured diaphragm would result in fuel entering the back side of the diaphragm and, as Henning pointed out if the seal were also compromised it could get into the oil sump. But if that were the case you'd have had a visual indication of fuel leaking out the vent line above the diaphragm. At any rate, it wouldn't result in free back flow like you were experiencing. Fuel pump diaphragms rupture all the time, I would think that's the most common failure mode.

 

[Henning]

Isn't the diaphragm on the back side of the check valves? I would think a ruptured diaphragm would be a normal and expected failure mode and shouldn't allow this back flow to happen.

The inlet valve is nothing but a check valve and is likely the one that went bad.

 

[Silvaire]

The inlet valve is nothing but a check valve and is likely the one that went bad.

The outlet is also a check valve, they are both pointing the same direction. I'd say both would have had to fail for this to happen. Perhaps this is just one seriously worn out pump.

 

[bnt83]

The outlet is also a check valve, they are both pointing the same direction. I'd say both would have had to fail for this to happen. Perhaps this is just one seriously worn out pump.

Judging from how hard the old fuel lines were, I'm guessing the check valves are hard as a rock too and possibly shrunk.

 

[Henning]

The outlet is also a check valve, they are both pointing the same direction. I'd say both would have had to fail for this to happen. Perhaps this is just one seriously worn out pump.

Or there is a deteriorated fuel line upstream that has fouled both, but yes, this is a 25 year old pump, that's old...