Air in fuel, vacuum side is NOT leaking. Advice?

[captain mike]

Here goes...
2016 lml duramax, silverado 2500hd automatic 4x4 ltz. 240k miles
My truck had zero fuel issues. After replacing my head gaskets, I can't keep it primed. Will lose prime in minutes. Truck runs fine after started.
I started diagnosing the typical places. Replaced fuel filter, and seals in the housing/primer. No change. I then disconnected the fuel lines at the tank, the drivers side quick-connect, the filter inlet, and outlet. Sealed one end Applied 10 inhg vacuum to each section individually, and had no leaks after an hour at each section. That should rule out air intrusion at any of these locations. I then reconnected everything, replacing all the clamps with new. Blocked the end of the rubber outlet hose on the filter assembly. Hand primed, and it held prime and the plunger was firm for one hour. This should indicate that the vacuum side is sealed and functioning properly. I hooked the fuel filter outlet back to the engine pump inlet fitting and immediately cannot keep it primed. Plunger loses firmness after seconds, and i can bleed air out of the test port above the cp4.
I then blocked the injector return line fittings and checked the return flow from each injector after a 15 second crank. The #7 injector returned 10 ml of fuel, which is higher than spec, but not bad enough in my opinion to cause a no start and loss of prime.The other injectors all returned minimal amounts of fuel.
I removed the rubber hose at the pressure relief on the drivers side rail. No fuel coming from the fitting after 30 seconds cranking. This should indicate that the pressure relief is functioning properly.
Truck runs fine after getting it started, does not struggle under load. This should indicate my cp4 pump is functioning properly (unless I'm mistaken)
I do not have access to a scan tool.
I have strong mechanical capabilities, a pretty well equipped garage, and I have a firm understanding of how this system operates. Have been driving and servicing duramax diesels since 2010...
Any advice on how to proceed with getting my truck back on the road?

 

[2004LB7]

Welcome

Vacuum test your filter housing. If you've replaced all the seals I bet it's cracked and leaking there or the water sensor

 

[captain mike]

I did vacuum test it. Blocked off the input and applied 10inHG vacuum to the output (using the rubber hoses off the truck to also rule them out as air intrusionpoints), with filter on. Left sit for an hour, no loss of vacuum. After reinstalling the filter housing to the truck and connecting the inlet hose, I blocked the fuel line exiting the tank at the blue connector fitting and applied 10 inHG to the filter housing outlet hose. Left sit one hour, did not lose vacuum. Reconnected to the hp pump and have air intrusion.

 

[captain mike]

Again this issue was non existent prior to doing a head gasket change.
Also worth noting that there is no visible fuel leaking from the positive pressure side from the filter

 

[Ron Nielson]

Mike - Remember that you can have a vacuum leak that will let air in, but will not let liquids out. I once had a filter on the inlet side of my fresh water pump in my 5th wheel and it never leaked a drop but when the pump ran, you could see air bubbles leaking in.

Our trucks are notorious for what we all think are vacuum leaks and eventually the problem is solved, but not many owners post/respond with their fix. The only exception posted is to replace the fuel filter head with a new one.

You've done an admirable job in vacuum testing your system. I don't think I have ever read a post by someone who went that far.

 

[captain mike]

Thank you. I certainly try to exhaust every option before turning to the ol interwebs for answers. I'm fairly certain my problem is not in my filter assembly, nor any of the points from the tank to the filter... I've isolated each section and vacuum tested each, as well as the entire run from the tank to the outlet of the filter assembly, and it holds 10inHG no problem. Once I hook back up to the inlet fitting to the cp4 I start losing suction.

 

[JoshH]

When you vacuum test the system with the CP4 hooked up, are you blocking off the quick connect on the return hose where it goes back down to the tank?

 

[captain mike]

I have not vacuum tested it with the cp4 hooked up, only hand prime with the cp4 connected. I can't connect my vac pump to anywhere on the outlet side of the cp4

 

[captain mike]

But, no. I did not block the return when hand priming the system with the cp4 hooked up

 

[JoshH]

Once I hook back up to the inlet fitting to the cp4 I start losing suction.

This made it sound like you were vacuum testing the system with the CP4 hooked up. If you are talking about losing pressure on the hand primer with the CP4 hooked up, that is 100% normal. They do not hold pressure when the system is hooked up properly because there is some internal return through the CP4 pump. If you suspect there is a leak somewhere after the CP4, you could block off the return from the engine and either pressurize the system with the hand pump while looking for a fuel leak or pull a vacuum and see if it holds. If the engine hasn't been driven since it was put back together, there may still be air in the rails causing extended cranking.

 

[2004LB7]

As Josh said, there is a bleed point in the CP4 that can let pressure through the pump back to the tank. Or in your testing, from the return

 

[captain mike]

This made it sound like you were vacuum testing the system with the CP4 hooked up. If you are talking about losing pressure on the hand primer with the CP4 hooked up, that is 100% normal. They do not hold pressure when the system is hooked up properly because there is some internal return through the CP4 pump. If you suspect there is a leak somewhere after the CP4, you could block off the return from the engine and either pressurize the system with the hand pump while looking for a fuel leak or pull a vacuum and see if it holds. If the engine hasn't been driven since it was put back together, there may still be air in the rails causing extended cranking.

Apologies if my quote is misleading. You are correct that after hooking the line back to the cp4 inlet hose, I start to lose pressure from the hand primer. I've read that with a properly operating system, I should be able to pump the primer til firm, and it should hold and remain firm even overnight. Is this incorrect?
I drove it about 800 miles after doing the head gaskets. At first, the truck would die 10 seconds after starting, and I'd have to re-prime and bleed off air to get it to start... this was at random, only happened occasionally. After a while it became more frequent, and starting it became harder and harder. Then one night after it was parked for an hour, it simply wouldn't start back up.

 

[2004LB7]

Are you sure the return pressure regulator is working properly?

 

[captain mike]

The rail pressure regulator seems to be fine, there is no fuel coming from the fitting when cranking. I'm not sure about return pressure regulator, where is it located?

 

[JoshH]

You are correct that after hooking the line back to the cp4 inlet hose, I start to lose pressure from the hand primer. I've read that with a properly operating system, I should be able to pump the primer til firm, and it should hold and remain firm even overnight. Is this incorrect?

That is incorrect. They will not hold pressure overnight, but as I suggested earlier, if you make the engine a closed system by capping off both quick connect fittings, you can either pressurize or vacuum the entire system on the engine. With the two hoses coming up from the frame being disconnected and the ports being blocked off, it should maintain pressure or vacuum for a very long time. If you find it doesn't, you can start removing segments of the system until you isolate exactly where the leak is coming from.

 

[captain mike]

That is a solid idea, thank you. I need an outside opinion, as I've just been bashing my head on a wall... I agree that with the ends blocked off I can vac or pressurize the system... the thought of doing it on the feed/return lines at the engine hadn't yet occurred to me I guess... seems pretty simple and straightforward. I'll try that tonight and see where it leads me. At this point I'm simply praying to find a leak at all, just so I can fix it. My vac side is sealed and holding, my pressure side has no fuel leaks that I've been able to find... injectors aren't returning excessive fuel... so very stumped!!

 

[2004LB7]

The rail pressure regulator seems to be fine, there is no fuel coming from the fitting when cranking. I'm not sure about return pressure regulator, where is it located?

The injector return regulator.is connected to the return lines coming off the injectors. Each bank comes together at the regulator before connecting to the fuel return back to the tank. It's supposed to hold something like 58-160 psi on the injector fuel return in order for the injectors to be able to open properly. With a helper, you can get a pair of pliers and pinch off the return at the regulator while cranking and if the regulator is bad it might allow enough pressure to start the engine. If it does start after pinching the line then you'll have a good idea what to replace

If I'm not mistaken, the regulator sits right around the thermostat location

 

[captain mike]

Ok, I'm familiar with that part. I thought that might be the regulator, thank you for the info. I'm headed home soon to check a few more things, this included

 

[captain mike]

Update:
I plugged the return line from from the engine to the tank, I was able to maintain much better pump pressure at the hand primer. I found a damaged return line o-ring, replaced it. Could have been weeping, but it certainly did not appear to have been leaking, replaced it anyway. I put a clear hose from the filter output to the hp input... I pumped the primer with bleed screw open, until fuel was flowing with no air, and tightened the bleed screw, pumped the plunger til firm. No air bubbles to speak of. Cranked engine, came back to find the clear hose was now empty. Pumped primer til firm, but no fuel flowed out of the outlet. I opened the bleed screw, and there was steady flow of fuel, under light pressure, with no air, yet the output hose (clear) was still empty. So I pressed the valve in the test port above the cp4, it purged all the air from the clear hose and the hose immediately filled with fuel ( no bubbles from the filter side.) I cranked the engine again and it fired right up, idled fine. I checked the clear hose while idling, and there are bubbles coming from the cp4 side towards the filter. After shutting it down, the clear hose is full of fuel, but primer pump is not firm.

 

[2004LB7]

Primer bulb won't stay firm after cranking or running the engine. Primer is only hard when you have positive pressure on the output of the filter. Once you crank or run the engine it lowers the pressure or can even go into a partial vacuum. So the primer goes soft

As long as as the lines stay full of fuel and don't drain back to the tank or otherwise fill with air then it should be fine

If you still have trouble starting when the lines are full of fuel, I'd double check the return regulator