So i replace my old style 3/8 fass system with a new airdog II-4g model due to no lift pump pressure and couldn't get system to prime and get rid of the air. Now even with the new system in, I go a few blocks down the road and my low fuel pressure light turns on and starts stumbling when going to a stop and sounds awful. What the ;lkjasdf;ljasd;lfkja is going on here? Im getting fed up...ive been so frustrated trying to figure this out the past couple months. I love cummins but this one is beating me up. And don't want to risk trying to drive it to a cummins shop. This is the greatest site ever btw.

Two things could be happening...... One is the tank vent can get clogged and a vacuum is building inside the tank. Ashop nearby says they have seen several fuel tanks almost completely collapsed.

Second, is the problem could be in the tank with debri build up on the module screens or module fuel filter. Fuel filter inside the module, at least on my 02 was a small white plastic cylinder shaped container with screens inside. Or the pick up tube is sucking debri. One or the other.

 

I cut out the screens, removed the module filter and installed a larger accessable filter before the lift pump. I can change it out anytime I need. Sometimes you get bad fuel and need access to it for a ready change out.

 

Get a vented fuel cap that will fit on the filler neck.

 

This site should be selling those out of the store.

Edited June 19, 20169 yr by JAG1

The kit did come with a mod for the in tank fuel module. And it did have me put holes in the side of the module. I didn't take out the screen but the straw should be able to suck up what ever fuel flows into the bowl from the sides i would think. I did take it down the road literally today with the cap off (funny you mentioned the cap) and no change.

Okay..... what about the small plastic cylinder shape that on or inline with the pick up tube? Did you remove that and go straight tubing for a new drawstraw?

 

Is there a screen inside the new fuel pump inlet? I removed that too and let my big prefilter now do the work. Some of the old ones had screens. Don't know about the new ones.

Edited June 19, 20169 yr by JAG1

for the suction line it is straight draw straw. the return is hooked up to original oe suction port and i drilled a 1/4 hole in that tube for return. So...the module has new suction port drilled with bulkhead and old suction port has airdog pump return, then the return line from tee near engine is at the same original return port on module (not messed with).

Edited June 19, 20169 yr by Cumminz

Not understanding everything described there but at any rate you are sure both return and supply is clear and free flowing?

 

What about the factory fuel filter.... could you have disturbed some debris that is now overwhelmed the filter? After I moded my fuel system I found that I had to tape small tubing to a shop vac and suck out a bunch of stuff out of the filter bowl and provide a new filter.

As far as the first question, no Im not 100% sure. I WILL check that. The original filter is taken out. the airdog system replaced that. goes from tank, to airdog system, to injection pump. This truck does start and it does fire right up every time. I might buy that pre-modded fuel module from vulcan so I know i won't have issues with that stupid thing haha

Edited June 19, 20169 yr by Cumminz

Easy way to check for vacuum issues to leave the fuel cap loose.

 

If you had an in tank pump it is the same fuel basket that Vulcan has. I used an ADII with the kit to remove the in tank pump, sounds just like what you describe came with yours. Mine has worked flawlessly for 6 years. Is this the same problem you had with the Fass system?

Yeah. Now im thinking that the screen on the fuel module might be plugged up (like JAG1 was saying). I modded the fuel module and added the new airdog system recently. The old fass system had the 3/8 lines and were dry rotting anyway. The OEM fuel canister is removed completely. The injection pump, connecting tubes, and injectors have also been upgraded recently as well. I only have a "low pressure" light at the moment. But when in park it will stumble a little but light will not be lit. Go down the road a few blocks then the light will go on and will stumble worse the more its driven like it is starving for fuel.

Edited June 23, 20168 yr by Cumminz

Is your fuel tank full?

 

When you modded your fuel module did the kit replace the in tank lift pump or did you just add a drawstraw?

 

The screens at the very bottom of the fuel module stay in, but did you remove them and clean them before reinstall?

 

Was the drawstraw installed to low or not have an angle or something to keep it from being suctioned to the bottom?

You might want to get a fuel pressure gauge. The problem may be more than just low fuel pressure. Just to be sure anyway. Kind of strange that it is happening with both the old pump and the new. Maybe the VP44 return is worn out? Not holding enough pressure maybe?

I do think ILOF is on to something if the complete change of the fuel system is netting the same results.

So i ordered the airdog pump to have a low fuel pressure light and a mod for the in tank fuel module. My fuel module does not have the pump in the module and the draw straw does have the angle on it. The sensor for the low fuel pressure is mounted on the airdog unit. So it is either the filters or the fuel module itself somehow. But if you think about it, the lift pump has new filters so that only leaves the module left to cause this since it has to be something between the lift pump and the tank. So....I bought a new fuel module (that is already modified) and see if that works. That way I wont have the pre-screen on the bottom and i can also fix my fuel level sensor ( it is empty at half way mark on gauge in dash). So win-win. If this fixes it then it need to be brought to attention the fuel module can be a source of low fuel pressure if it is an old unit. What do you think guys? I will post back with results as "solved" if my idea is right. Thanks to JAG1 for the fuel module idea.

You R welcome.... keeping my fingers crossed now that you spent some money.

So here is my update. The blue hose is the 3/8 pump return to tank, the clear hose is attached to the stock 5/16 engine fuel return to tank. The 1/2 inch suction line from tank to pump, and 1/2 inch pump to vp44 are hooked up. If I am correct, the blue 3/8 inch hose should be pouring out fuel that is excess from the airdog, and the clear hose should be excess from the injection pump right? Why would it only be trickling out of both instead of pouring out. I unhooked the line from the injection pump at one point and it poured out into the bucket like a fire hose so supply pressure and volume is good. But how come there is no excess fuel coming out of these hoses? Is there a return restriction or did i even do this test right? Something fishy is going on here...

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Edited July 9, 20168 yr by Cumminz

If the clear hose is connected to the return from the engine, I am not sure you would see any fuel there without the engine running. This line returns fuel from the VP and the injectors. Since nothing is going to the injectors there would be no return fuel. I also dont know how much fuel returns from the VP when the engine is not running. Crank the engine and see what difference it makes. 

 

I would think there would be more fuel returning from the pump than what you see but that is an assumption my part since I have never checked mine to see what it does. Never had any reason to. 

 

Have you tried starting it?

I think for just bumping the starter that looks about right.

 

Without the motor running I wouldn't imagine much (if any) fuel coming out of the return line (clear). The blue looks like a decent amount again for just bumping it and we're talking return fuel FROM the Airdog right? So it should be a ton of fuel, at least not as much as it would be when un-hooking the supply line at the VP.

 

Try starting it for a sec and see how it looks then.

The blue hose is the return from airdog. but if the pump is pushing 165 gph through the supply to the injection pump, wouldn't that fuel need to go through the return so it doesn't keep building up pressure? See when i run the pump for 25 seconds and the supply is primed, the pump no longer sucks fuel from the suction line from the tank to the pump. I thought it still suck fuel and the circulate to the return if it is not needed for the supply. Shouldn't the fuel just circulate through the supply and return if the engine isn't running? The overflow valve on the injection pump should open up to let the excess fuel pass through to the return of the engine still though right? 

Edited July 10, 20168 yr by Cumminz

Ok. First let me show you a picture.

 

VP44 return is on the left, then the VP44 supply is on the right. So as you can see the return side is restricted fairly well. So even if the fuel pressure coming out of the VP44 is above 14 PSI its only flow the amount of the port will allow. Since AirDog 165 has a reduced return flow compared the old school AirDog 100/150's it tends to recirculate within the AirDog itself. I don't like this method much because it can have issues with cavitation just like the old OEM Carter and Holley pumps have to this day. If you dead head a pump it will create its own cavitation issues. So with AirDog 100/150's they are flowing majority of the fuel back on the return hose.

If the vp44 is restricted not only by that small orifice as well as the overflow valve, then no wonder these things have issues with heat. So even with the engine running the injection pump is only trickling out small amount of hot fuel. Terrible design to me. Might as well go with the p-pump then.