Hey guys, I need help.
While I had my truck in the garage, I noticed a small fuel leak between the tank and the frame, so I repaired it and of course it wouldn't start and for the life of me I could not get the air bled out of the lines no matter how hard I tried. I finally spliced in an old electric pump at the tank temporarily just to get fuel up to the filter. That worked. So I took the elec. pump out and hooked up the fuel lines to the tank normally(quickly of course to keep as much air out as possible). Now the truck starts, and idles fine. It'll even go about 50 ft on the rd, then starts missing like crazy and dies unless you hold the pedal to the floor and even then it runs on 2 or 3 cylinders and sounds like an old sick john deer tractor. eventually while I was trying to get it back in the driveway, it came to life again and ran fine as I pulled in. Just to be sure, I loosened one injector line at a time and tried to bleed any air out. After that, another test drive same thing. I've done this over and over, and no change. It's gotten a tiny bit better, but I'm afraid to keep doing it for fear I might damage something letting it run on a few cylinders.
I even pulled up to a tree and let it spin the tires at about 25mph on wet grass hoping a load on the engine would get the same results as driving on the street without having to fight traffic in a truck that's barely running. It ran fine. But then again took it out, sputters and dies. I don't know what I'm doing wrong, I've checked for more leaks, made sure that theres enough fuel coming from the lift pump. Is there a specific way to bleed these fuel lines? Or could it be something else?

Just to cover all the bases, the truck is a 1990, 94 motor, 96 Man. P7100 pump, 375 injectors, there's no computers on it anymore, it's all manually controlled including the fuel shutoff (which I checked, it's working fine)

I'm stumped. And my patience has worn out so it's been sitting for a week and I haven't touched it.

 

Sounds like it's still sucking air somewhere. The way I hunt down the hard ones is to pressurize the tank to no more than 10 psi with an air blow gun with a rag wrapped around it. Friend has a fuel tank cap with a tire valve tapped into it just for this purpose.
Besides finding leaks this is a great way to fill and bleed air out of the filter. Heck with the thumb pump.

 

The 94 pump has a poorly designed overflow valve (the little guy in the valley between the pump and engine, to which the return line connects. Check to see if yours still has the one piece valve, if so be thankful you got this far and replace it with the current two piece valve.

Also, if any of your fuel inlet and return hoses are over two years old, replace them all - from the tank to the frame, and from the frame to the engine. They may look good, but they will develop microscopic holes and suck air under any load at all.

Check your fuel heater electrical connection and the screen and gaskets in the fuel heater. The electrical connection will eventually leak air, and using dielectric grease on it generally stops the air leaks. Most fuel heater screens never get cleaned or replaced, and develop corrosion and quit flowing fuel. The gaskets at those unions eventually will develop air leaks and allow the system to pull air and air lock the injection pump.

Since you have the later engine, pick up a copy of Haynes Manual #30041, it is about $15, and it will explain all the bleeding procedures in detail.

However, the best way to bleed these is to loosen the small plug in the center of the fitting on the filter housing, on the line to the injection pump, and just keep pushing the primer until the bubbles quit. Then tighten and start the truck. Sometimes it is necessary to also crack a line to an injector barely loose at the injector end and have someone crank the engine until that line starts blowing fuel. Just be real careful as this has so much pressure it can blow right through your skin and kill you, and never do it to a hot engine or you will end up a crispy critter.

 

I've tried the pressure test, while I had the temporary electrical pump hooked up I looked for leaks, couldn't find any.
The lines are within a year old, I replaced them when I rebuilt the motor about a year ago the original leak came from a holddown that came loose and let the line vibrate against the tank crossmember, which I rerouted it this time and added another hold down to prevent it from reoccuring. The overflow valve I believe is the one piece, it has the '96inj pump on it but I believe the fuel system was all from the '94. what years did the two piece come on? I'll look into the heater too, I never thought of that to be honest.
Well funny you should mention replacing lines. What I would absolutley love to do is redo the entire fuel system, pre filter. Rather than use the mechanical lift pump would a high output inline pump work? I do have the haynes manual for the 94 up trucks, and I read there that minimum line pressure is 25psi. Holley # 12920 flows 43gph at 45psi. I have no idea what the gph minimum is? anyone have an idea? Is there a blockoff plate for the mechanical pump? Should I even take it off? Another thing, since the pump makes 45psi would I need a regulator or would it benefit from the extra fuel pressure?
Last question, tanks. I still have the plastic frame side tank, but I'd like to convert to a rear fuel cell. Will any aftermarket fuel cell work? I wouldn't see why it would differ, but thought I'd make sure. Thanks for the input guys, greatly appriciated.

 

The mechanical pump is pretty darn reliable I'd stick with it.
If you go to electric remove the mech pump and put a plate over the hole. If the your electric pump is before the fuel heater it will have to be removed, it's designed for suction, not pressure and will leak.
20 gph minimum. Pressure above the factory recommended 35 psi max has more potential to cause leaks and fuel filter splitting. The overflow valve will dump excess pressure but not sure at what volume and if the return line can handle it.
Seems like a lot of work without much gain.

 

The Carter 4600, pump is a 7 to 15psi pump and I have one on my 95. Has worked for two years with no problem. What I need to do though is install a ball/check valve in a runaround line to bypass the pump in case it fails. I still have the mechanical on the engine and no problems pushing fuel through it but the truck will not run well if the carter is turned off. It will do a fast idle but starts to starve past that. Carter makes a check valve just for this #169-1002.

 

Hey did you know that a cummins won't run without the pickup in the fuel tank below the fuel level?
I feel stupid. Apparently when I dropped the tank the pickup got screwed up in the tank and wasn't where it should be. It's running fine now. Thanks for the suggestions though guys.

 

Most won;t admit to mistakes but I know all have made some. Myself included!! Glad it was something simple.