That's what I thought.... So that pretty much has to mean the P0093 refers to a leak between the pump and the rail, because that's the only section of the system where quantity can be measured? A leak there would make cold starts problematic, since the fuel pressure and injector timing are tightly controlled anyway, but especially on cold start.

Nowhere in the fuel system is the quantity measured.
Quantity is just a guess based on fuel rail pressure.
If rail pressure is sensed to be too low, there must not be enough fuel supplied, and therefore there must be a leak somewhere.
Such a leak could be anywhere between the fuel tank and the fuel rail pressure sensor.

And because the fuel line from CP3 on back to the tank is under suction, a leak there would suck air into the line instead of fuel dripping out.

_________________
U.S. Army Retired

The point is that the only two points of measurement are the FCA and the fuel pressure sensor. The only thing that could generate a "large leak" code is that feedback loop. Everything before the pump is a mystery. Thus, the code has to be a leak between those two points, does it not?

What I think Flash is saying, and he is correct, if the CP3 cannot for any reason produce the desired high pressure rail pressure, the ECM would generate this Fuel System Leak code. It is probably a combination of sensors suppling information to the ECM that triggers the P0093 code.

It could be caused by several different components or a combination of problems from the injection pump to the injectors.
A bad fuel injector, leaking, high pressure rail fitting, a bad or weak CP3 injection pump, a malfunctioning Fuel Quality Solenoid, a malfunctioning Fuel Cascade/Overflow Valve, or one of the fuel sensors on the fuel rail giving an erroneous signal, i.e. Fuel Rail Pressure Sensor or Fuel Pressure Solenoid.

The suction or fuel feed to the injection pump can certainly affect the HP pressure output of the CP3 pump. (fuel starvation)
A bad or leaking fuel filter, leaking fuel filter head, leaking fuel line, or leaking fuel line fitting which would include all fuel lines and fittings from the back of the CP3 to the top of the fuel tank and even the suction line inside the fuel tank.
If the CP3 cannot get enough of the necessary volume of fuel for any reason on its suction side, it cannot pump or supply the required high pressure fuel needed by the injectors. This is why many install an intank or inline supply/lift fuel pump to ensure the CP3 never suffers from a lack of fuel supply volume (fuel quantity).

_________________
Supporting Vendor and Moderator of LOST
05 Jeep Liberty CRD Limited
Ironman Springs/Bilstein/Shocks
Yeti StgIV Hot Tune
Week's BatteryTray
No FCV/EGR
Samcos/ProVent
SunCoast/Transgo
Carter Intank-pmp
2mic.Sec.Fuel Filter
Flowmaster/NO CAT
V6Airbox/noVH
GM11 Bld.fan/HDClutch
IronrockArms/wwdieselMount
98 Dodge Cummins 24 Valve

Sorry, when "Pumping" the plunger, yes, it gets very firm after 4-5 pumps. I mean I pumped it and just kept pumping even after it was firm, as much as I could just to make sure. I'd crank it, and check the plunger, which was no longer firm, hit it again till it was very firm, and crank it again, with no start. Starting fluid is the only thing that got it going, but driving it now, and sits all night, starts right up the next day just fine.

I have not yet checked to see if the old washer is on the new filter (but it did the no start thing prior to the filter change). Jeep has run very nicely when it does, after the filter change for several weeks, and is currently starting and running normally right today. So it's just when it sits for a lengthy period that it no longer will fire, only crank. That's why I was wondering if it "drained back" for some reason and the injector pump just wasn't able to suck the fuel enough to start it, but it just hasn't drained back enough after a day, where it will still start and run.

Would the Torque Pro app on the fuel pressure be enough data to suggest not pulling enough, or should I install a fuel pressure gauge after the filter somewhere to check?

No Fuel leaks or any leaks of any fluids at this time. It's parked in the snow, and it's always nice and white still underneath when it's moved.

No P0093 codes. No other codes, other than the MAF, since I did the Weeks stage 1+2 upgrade.

...and it has 230,000 + miles on it.

After pumping, you need to open the bleeder valve to release any trapped air.... then pump & release again. Repeat until only fuel comes out of the bleeder.

Since you don't have a lift pump or intank fuel pump installed, everything in the fuel system from the back of the CP3 to the fuel pickup inside the fuel tank is under a vacuum (negative pressure) when the engine is running. The CP3 has to "SUCK" the diesel fuel from inside the fuel tank through all the fuel lines and through the fuel filter head and fuel filter to the injection pump. So any tiny leak anywhere on the system will allow air to be sucked into the fuel system and create problems.

When you spray starting fluid into the intake, (which is a very bad thing on diesel engines) it is telling you there is a fuel starvation issue!

This is the reason why most install an intank fuel pump to put the entire fuel system under positive pressure from inside the fuel tank all the way to the back of the CP3 injection pump. Putting the system under pressure prevents any air from entering the fuel system and any leaks anywhere on the system will be self evident very quickly as you will see fuel leaking out. Installing an in-line fuel lift pump will work, but it can have the same problem on its suction side back to the fuel tank. An intank fuel pump is the best option to eliminate all air infusion into the fuel forever.

As to your questions;
if you have a leak on the suction side of the CP3 with no lift pump installed, you will not see fuel leaking out, it will only allow air to be sucked in when the system is under negative pressure (vacuum). When the vehicle sets for long periods of time and you have a small leak somewhere on the suction side, it can allow the fuel to drain back into the fuel tank thus emptying out the supply lines of fuel and making starting very difficult if not impossible.
If you install a gauge between the fuel filter and the CP3 injection pump and you don't have a lift pump, it would have to be a vacuum gauge and it really would tell you nothing and would be of no value. No scanner data will help in this situation as there are NO sensors on the suction side of the fuel system.

Another thing to consider, the OEM push-on fuel line connections used at both ends of the flexible fuel lines back at the fuel tank are designed to seal against pressure, NOT vacuum. They do a really good job of sealing if the fuel system is under positive pressure, but do a horrible job sealing under vacuum. Many owners simply cut them off and attach or replace the rubber hoses using conventional screw type or crimp on clamps.

Bottom line, if you want to prevent all future air in fuel issues, install an intank fuel pump, your engine will actually idle smoother when there is positive pressure feeding the CP3 injection pump.

_________________
Supporting Vendor and Moderator of LOST
05 Jeep Liberty CRD Limited
Ironman Springs/Bilstein/Shocks
Yeti StgIV Hot Tune
Week's BatteryTray
No FCV/EGR
Samcos/ProVent
SunCoast/Transgo
Carter Intank-pmp
2mic.Sec.Fuel Filter
Flowmaster/NO CAT
V6Airbox/noVH
GM11 Bld.fan/HDClutch
IronrockArms/wwdieselMount
98 Dodge Cummins 24 Valve