ISC Lift Pump Potential Leak BUT It Is Bypassed

[Mile Marker 42]

Hello All, I have recently completed an AirDog only pump installation and I have about 45 highway miles of an uneventful test drive under my belt. Everything seemed perfectly fine and I didn't notice any leaks when I got home. On my installation, I capped off the fuel inlet and outlet of the lift pump with 10 AN caps. I left the return lines as is. I mounted my AirDog pump in the bay where my fuel tank is and I am pushing fuel to my factory 2 filter set up. Pretty straightforward. The AirDog pump was preset at 15 psi and that is what the running pressure actually is as I installed a gauge. Today, I started the engine to reach full air pressure as I wanted to chase an air leak. Once it reached full air, I turned off the engine and got underneath with a bottle of soapy water. While under the engine area with a flashlight, I noticed fuel leaking ever so slightly out of the bottom of the lift pump. What? How could that be when I have it capped off? No banjo lines are leaking on the lift pump and neither is the return line from lift pump to tank. Is it possible the manifold is bad and leaking into the diaphragm area of the lift pump? Or, is it just fuel left over in the lift pump possibly leaking out. I am now trying to understand if there is a way to bypass the complete lift pump manifold with the 2 banjo hoses mounted to it? FYI, I only did this mod as preventive maintenance, not because I had a leak or running issues. Any help would be greatly appreciated. Thanks in advance.

[swampdog2]

does the filter on the engine have a water drain valve ? if so they are really bad about leaking ever so little ,I put a new one on mine and it started leaking also in about a month ,so i put a hose on it and stuck a bolt in the end of it with a hose clamp ! no more leak!

[Mile Marker 42]

Originally Posted by swampdog2

does the filter on the engine have a water drain valve ? if so they are really bad about leaking ever so little ,I put a new one on mine and it started leaking also in about a month ,so i put a hose on it and stuck a bolt in the end of it with a hose clamp ! no more leak!

Filter isn't leaking. Drip is from Lift Pump. Thx.

[RS505]

I installed an Air Dog in almost exactly the same way you did, complete with the 10AN caps. My guess is that it's residual fuel left in the pump leaking out.

I tightened the three bolts on my pump before I capped it off. Haven't seen any leaks after the air dog install, but it was weeping a little beforehand. I carefully cleaned up as much of the leaked fuel and grime as I could. I just checked, no apparent new leakage after a 500 mile trip.

If the pump return manifold is leaking on mine, I'll make a return manifold on my mill to accept the banjo lines, but I really hope it doesn't get to that point.

[Mile Marker 42]

Originally Posted by RS505

I installed an Air Dog in almost exactly the same way you did, complete with the 10AN caps. My guess is that it's residual fuel left in the pump leaking out.

I tightened the three bolts on my pump before I capped it off. Haven't seen any leaks after the air dog install, but it was weeping a little beforehand. I carefully cleaned up as much of the leaked fuel and grime as I could. I just checked, no apparent new leakage after a 500 mile trip.

If the pump return manifold is leaking on mine, I'll make a return manifold on my mill to accept the banjo lines, but I really hope it doesn't get to that point.

I like the mill idea. I'm trying to find a double banjo to 10AN female connector to potentially bypass manifold, but not having any luck. I'll try and tighten the lift pump as you mentioned. Thank you!

[RS505]

I think my return to the tank is size -8. I'm going to replace all of the fuel hoses in the near future. I already bought the hose and it was -10 for the supply and -8 for the return.

By the time I get around to doing that, any leaks should be obvious.

[Mile Marker 42]

Originally Posted by RS505

I think my return to the tank is size -8. I'm going to replace all of the fuel hoses in the near future. I already bought the hose and it was -10 for the supply and -8 for the return.

By the time I get around to doing that, any leaks should be obvious.

So how exactly would you completely bypass the manifold?

[RS505]

I wouldn't bypass it as much as make a new part out of a piece of steel barstock that did the same job as the manifold on top of the stock lift pump, without the pump part, take the factory lift pump off, and add the relay to fool the ECU.

I didn't notice any difference with the Air Dog as far as performance goes, and the Road Relay showed the same HP this trip as the last several. I did notice, and my wife did as well that the smell of diesel in the back of the coach has gone away.

[153stars]

Rubber gasket that shrinks after change to ULSD fuel has probably really shrunk now that it's drying out.
I have even pulled some new old stock paper gaskets off the shelf in plant that shrunk with age and became brittle.

[Mile Marker 42]

Originally Posted by 153stars

Rubber gasket that shrinks after change to ULSD fuel has probably really shrunk now that it's drying out.
I have even pulled some new old stock paper gaskets off the shelf in plant that shrunk with age and became brittle.

Fuel inlet and outlet are capped. Why would gasket matter at that point?

[Ljwt330]

Originally Posted by Mile Marker 42

Hello All, I have recently completed an AirDog only pump installation and I have about 45 highway miles of an uneventful test drive under my belt. Everything seemed perfectly fine and I didn't notice any leaks when I got home. On my installation, I capped off the fuel inlet and outlet of the lift pump with 10 AN caps. I left the return lines as is. I mounted my AirDog pump in the bay where my fuel tank is and I am pushing fuel to my factory 2 filter set up. Pretty straightforward. The AirDog pump was preset at 15 psi and that is what the running pressure actually is as I installed a gauge. Today, I started the engine to reach full air pressure as I wanted to chase an air leak. Once it reached full air, I turned off the engine and got underneath with a bottle of soapy water. While under the engine area with a flashlight, I noticed fuel leaking ever so slightly out of the bottom of the lift pump. What? How could that be when I have it capped off? No banjo lines are leaking on the lift pump and neither is the return line from lift pump to tank. Is it possible the manifold is bad and leaking into the diaphragm area of the lift pump? Or, is it just fuel left over in the lift pump possibly leaking out. I am now trying to understand if there is a way to bypass the complete lift pump manifold with the 2 banjo hoses mounted to it? FYI, I only did this mod as preventive maintenance, not because I had a leak or running issues. Any help would be greatly appreciated. Thanks in advance.

I don’t think there is any way for the return passage in the fuel manifold to leak into the passage exposed to the lift pump. I tried to find some photo or cutaway view but could not. My understanding is the manifold is a solid piece with separate passages cast/drilled into it. There is no seam or gasket between the passage supplied by the banjo fittings and the passage supplied by the fuel inlet/outlet lines.

My guess would be residual fuel, possibly seeping through the lift pump motor seal itself. It’s happened before on an unmodified lift pump.

[Mile Marker 42]

Originally Posted by Ljwt330

I don’t think there is any way for the return passage in the fuel manifold to leak into the passage exposed to the lift pump. I tried to find some photo or cutaway view but could not. My understanding is the manifold is a solid piece with separate passages cast/drilled into it. There is no seam or gasket between the passage supplied by the banjo fittings and the passage supplied by the fuel inlet/outlet lines.

My guess would be residual fuel, possibly seeping through the lift pump motor seal itself. It’s happened before on an unmodified lift pump.

My guess as well. Thank you.

[TR4]

Originally Posted by Ljwt330

I don’t think there is any way for the return passage in the fuel manifold to leak into the passage exposed to the lift pump. I tried to find some photo or cutaway view but could not. My understanding is the manifold is a solid piece with separate passages cast/drilled into it. There is no seam or gasket between the passage supplied by the banjo fittings and the passage supplied by the fuel inlet/outlet lines.

My guess would be residual fuel, possibly seeping through the lift pump motor seal itself. It’s happened before on an unmodified lift pump.

I agree with Ljwt330. Could it be motor oil leaking from somewhere? When my valve cover and engine brake spacer were leaking, oil would run down near the lift pump.

Bill

[Mile Marker 42]

Originally Posted by TR4

I agree with Ljwt330. Could it be motor oil leaking from somewhere? When my valve cover and engine brake spacer were leaking, oil would run down near the lift pump.

Bill

No, luckily I don't have any oil leaking. It was diesel fuel. I am hoping someone has a exploded view of a lift pump to confirm the return side doesn't intersect the fuel side. You think I should remove the 3 bolts that hold the pump part of the lift pump to the body and remove the pump and fire up my engine to see if any fuel leaks out? That would solve my mystery.