When the topic of air intrusion comes up, it seems like there are generally two types of responses. The first is people will suggest common sources of air intrusion and parts to replace. It's pretty much a guessing game not really based on evidence. The second is the replacing return lines with clear hose and looking for air bubbles. I don't really like the sounds of this for a couple reasons. You have to disturb a bunch of stuff to install the clear lines, and it seems time consuming. So...can the return lines be pressurized and each connection sprayed with soapy water to pinpoint the source of the problem? You could regulate an air source down to 5ish psi and connect it to schrader valve. Would you have to clamp the return line leading to the tank to hold pressure? Would the lift pump hold pressure on the inlet side (not really sure how they work and if they'd act as a check valve for this). Any risk to damaging the diaphragm in the lift pump? It seems like the regulator, and adapters and stuff to do this could be purchased fairly cheaply. Any thoughts?
Because? Theres plenty of ways to determine where air is coming from, and you can narrow it down by how air intrusion behaves in your particular case. Say you are going down the road and the truck acts like its running out of fuel on every hill and sputters when you try to accelerate. That would be air intrusion before the pump. If your truck runs great once started but its really hard to start after setting, thats air intrusion ahead of the pump thats allowing fuel to drain back to tank.
If the return lines are an issue, they will leak externally. No need to pressurize them to look for leaks. As for pressurizing the feed, you dont want to do it from the filter side because you will rupture the diaphragm in the pump and fill the crankcase with diesel. What are the symptoms you are having with air intrusion?
Each time I had air coming into the fuel system on top of the engine, I had dampness in that area. Some of my o-rings were leaking, I had damp wet fuel around the offending ones. Later on I had the problem crop up again, no dampness around any of the injector o-rings, but by accident I discovered dampness on top of the fuel filter housing. That was the o-ring around the fuel heater leaking and letting air in.
What about pressurizing the system from the filter head inlet? I'm sure there's a reason no one approaches the problem this way, but it seems like it would work. My specific problem has been an on again off again thing for the time I've owned the truck. Symptoms are a very long crank. It usually hits a few times while cranking before finally catching. When it does catch, it runs rough for 30 seconds or so. Not too long ago the issue seemed to be resolved, but then reappeared. I found the inlet to the IP leaking. Olives are pretty fresh, so I tightened the hard line. Issue was improved but not resolved. Now I have no visible fuel leaks but still have hard starts. I've seen multiple posts here saying it's possible for air to be drawn in but no fuel leaks.
Your problem doesn't sound like the classic air intrusion. If the fuel system is totally drained on top of the engine, the hard injector lines going to the injectors still have fuel in them. If you glow the plugs long enough and then crank it over, it should start right up and then it will quit. Then you get into the long cranking before it fires again. You cannot get it to hit at all from the get go. That sounds like another problem. I am having that problem now myself, but I know what it is, the engine is not turning over fast enough. I have had my truck a long time now, and I try to notice differences in it when I go to use it and start it. I had 3 episodes of air intrusion, all 3 times it would fire right off after just a little bit of cranking and then it would die. 1st time was the original o-rings were leaking, 2nd time was about 5 years later, o-rings leaking again(went to the viton units the second time) and the 3rd time it was the o-ring on the filter head heater. Each time the culprit had fuel around it, and engine would fire right off and then quit. Now I have a cranking problem. Never had it before until I messed with the starter. The old starter would whip the engine right over, no problem starting. But it would grind every now and then. I had another starter off another truck laying there, so I swapped it out. The engine cranked noticeably slower, and it would not fire off, even though it was turning over. So I took the gear drive off this starter, put it on the motor part of my old starter, and everything was fine and it cranked over faster and started right up. Now it is turning over slow again. I checked the batteries, they are good. Checked for heat in the cables. can't find any. I think it is something with the starter again. I have had problems with other people's trucks with corrosion between the starter and where it mounts to the bellhousing area. I am going to check that and look the starter over good.
During the long crank, do you have white smoke from the exhaust? If you have the smoke, it could be glow plug issues. Air usually doesn’t fix itself, but electrical can be intermittent.
No smoke during cranking. When it does fire off I sometimes get a large plume and engine speed flares high. Batteries are a year or two old hold voltage well during long periods of sitting (for what little that's worth). Cables are all new heavy gauge a year or so ago. All connections were cleaned and protected with dialectic grease when cables were installed. Crank speed seems good, but I really can't say for sure. I think I have a spare starter but it's of unknown condition. Glow plugs are probably 3ish years old motorcraft parts. All ohm out well. I get a good voltage drop while the plugs are glowing and they seem to glow for the appropriate length of time. I agree with the comment that the hard line leading to the IP and the pump itself must be empty of fuel.
Anything between the pump and injection pump will be under pressure when the engine is running, so no need to pressurize it to check for leaks. If there’s an issue with this portion of the fuel system you will have an external leak when the engine is running. I’d suspect the hardline going to the ip, and the fuel heater.
Agreed, the hard line to the pump seems like the most likely suspect. But like I said, no visible leaks there. I'll be driving the truck a bit this weekend, so I'll wrap the connections in paper towels and check for any leakage after the drive. Like I said too, I've seen several comments that it's possible to leak air in and not leak fuel out. Explanations were that air molecules are smaller than fuel. Also that when warm things can swell and seal, then shrink and leak when cold. Bogus?
You can sort of tell also how bad the leak is. Once you get it started, does it run fine the rest of the day, starting and stopping several times? On my fuel heater leak it was small. I could get it started, and it ran fine the rest of the day. But if I let it sit most of the day and started it later that night, it would start right up, run a little rough and then clear up. If I let it sit over night, it would start right up and then quit, and then with just a little bit of cranking, it would fire off. With the larger leaks I had around the o-rings, after I finally got it started, when using through the day it would run a little rough after each start, and if I let it sit overnight, it would fire and then quit and it was a bear to get going again.
