Who knew that my remarkably reliable 1st gen Cummins would develop a chronic fuel starve issue over the past 3 years. During this time I've tried many fixes, seen at least three mechanics and spent a lot of cash to try and figure out what the root cause is. I've down to three variables, the final one that I'm most confused by.

The truck fails when:

1) Water in the fuel system which is easy enough to fix with new filters and bleeding the system etc.

2) A faulty fuel solenoid valve which I finally deactivated and replaced with a manual shutoff control.

3) Excessive back pressure in the fuel system caused initially by a non-vented fuel cap (which I replaced with a vented version) and now something where when I opened my secondary fuel filter system (under the truck between the tank and engine) a "whoosh" of air comes out.To get the truck running again, I manually bled the file system via the fuel filter on the engine and then again at the secondary fuel filter until all I saw was clean diesel. After that it started right right up and I drove it home.

What causes so much back-pressure to make the truck fuel starve? Diagnosing air in the fuel system seems to be a several step process (one mechanic said they tested it but clearly didn't do a good enough job) and I only have so many tools to do a comprehensive job.

So, does anyone know a sure fire way to diagnose how that much air can get into a fuel system and/or does anyone know of a really good, old-school diesel mechanic in the Boston, MA metro area?

This issue is 3+ years old now. I've done and spent so much money on this that while all the work has made several small improvements to the truck, it remains unreliable given I never know when it will stall and what is the definitive reason for it doing so.

Any and all help appreciated.

 

I have a similar issue. When my truck gets down below 3/4 full it doesn't want to start. I have been told it could be a leak in the fuel line causing it to suck air or else a hole in the pickup tube inside the tank. I plan to cut a hole in the bed this evening or tomorrow and look things over. I'll post back what I find.

Edwin

 

Is there a secondary vent tube, other than the vented cap?...If not can you add one?...Somewhere near the top of the filler hose would seem to be a good spot...Just thinking out loud......Ben

 

I've heard of your problem and a lot of the times they say that air is coming in from the top of the tank where your lift pump inlet is. I was going to pull my tank today but though the better of it since it's fairly complicated and my tools are limited. Good luck with your diagnostics.

 

There is a secondary tube and when I look at it I'm tempted, as mentioned in the other post, to pull the take, inspect and potentially re-seal the lift pump inlet and potentially even replace all of the fuel lines and other tubing. Without knowing what the exact problem is, I could do all that and still experience the same issue.

 

So am I understanding this right? It's not air leaking INTO the system - it's air NOT getting into the tank to replace fuel thus causing a vacuum as the pump trys to pull fuel to the engine?

 

I guess I'm not really sure. For a while I thought it was a non-vented fuel cap. One day during a hot spell I removed the cap to hear a whoosh of air escaping the tank. That seemed to point towards air pressure in the tank building up to the point that my OEM (3-5psi) fuel pump just not being strong enough to over come that. One YouTube video I saw says to put a comparable electronic fuel pump between your tank and the secondary fuel filter to compensate for that. It may work but it certainly doesn't show where air pressure is coming into the system.

This is all new to me in that I never had any air/vacuum pressure issues when I lived on the west coast. On the east coast, hat and humidity seem to playing a factor here.

 

Well, I just got the hole cut in the bed. Now I'm resting before I go try to find out why there seems to be a leak. LAst time I pressurized the tank and it seemed like there was air coming out somewhere in the vicinity. Need to figure out which line is the fuel line and which is the return line.

Stay tuned.

Edwin

 

I took the pickup unit out of the top of the tank. No obvious issues other than the fuel screen at the end of the pickup tube was covered in gook. I cleaned it off with wd40 and brake cleaner. I examined the rubber line going from the supply spigot down to the pickup screen and it was still flexible after all these years. It still had the original crimp bands on it so I think it is original. All in all the unit looked in good shape. Mine is the 89 version with the spigots that stick straight up and then there are push on fittings that have the barbs for rubber lines to clamp on. I haven't seen any o-rings that could be replaced. Just the nylon clips that hold the angled fittings on. I'll put it all together and run it for a while to see if cleaning the gook off cured the problem.

Edwin

 

67IHC: Did you find anything on yours? IN looking at mine there is a vent in the pickup assembly that had a biot of dirt in it. I cleaned it out. It looks like a small 1/8" hose barb with nothing hooked to it. When I pressurized my tank there was leakage out of the vent and leakage out of the clamp where the assembly is clamped to the top of the tank. If you take out the whole assembly to check your fuel strainer be sure to clock the arrow on the assembly with the mark on top of the tank. IDK what would happen if it's not aligned properly.

I put it all back together and went for cheese burgers. Had no problems but the tank is nearly full. I'll have to drive it a while to get the level down to where it starts acting up before my "repairs".

Edwin

 

Edwin,
The outlet and return lines to the tank are labeled with an arrow on the top of the sending unit. The return line is also a smaller diameter.

The clip on fuel lines that clip onto the top of the sending unit have O-rings on / in them.

As far as not lining the sender up properly in the tank will result in the fuel float to hitting the sides of the tank and not reading correctly.

 

Thanks Oliver, very helpful info.

 

Oliver:

Once I cleaned all the crud off the top of the unit the plastic label as to which line is which is plainly visible. I'll have to look for the o-rings in the angle adapters assuming the problem isn't fixed by cleaning the crud off the strainer. I haven't put the section of the bed I cut out back in yet so I just need to drive out some more fuel to see what happens.

BTW, is the purpose of the big coil of tubing on the return line to warm the fuel in the tank?

Edwin

 

67IHC, I assume this is the same Cornbinder you were having fuel issues with back last summer? But the "fix" didn't last or didn't really fix it?

 

Yes, same truck. But I think I've been running ionto two issues related to the fuel system: a faulty fuel solenoid shutoff valve (fixed!) and back pressure in rhe fuel system which negates what my OEM fuel punmp can handle.