^truth be told, I did that even before it started acting up because I'm used to a diesel (I've always turned the key to on and waited, then turned over the ignition and waited for oil pressure to stabilize before putting it in gear). For the cherokee, it still takes two run-throughs of 'on, wait, ignite'.

Hopefully someone will list the numbers for acceptable fuel pressure after cool down.

 

Yep your 2001 should hold pressure for a looong time. The spec is 49 PSI (+/- 1.5 PSI while running). Exactly how long it should hold it I'm not sure but 39 PSI after 35 minites is totally unaccetable.

If one spark plug doesn't look rich (or check for leaking injector by another means) you have a bad check valve. This is part of the fuel pump assembly inside the tank. If you don't want to drop the tank just keep "priming" it by cycling the key on and off before starting to give the pump a chance to build pressure.

 

Been trying to find what leakdown values should be like, but haven't found anything yet...

 

Hmmm. Here's what I found in my 1999 FSM

FUEL PRESSURE LEAK DOWN TEST

Use this test in conjunction with the Fuel Pump Pressure Test and Fuel Pump Capacity Test.

Check Valve Operation: The electric fuel pump outlet contains a one-way check valve to prevent fuel flow back into the tank and to maintain fuel supply line pressure (engine warm) when pump is not operational. It is also used to keep the fuel supply line full of gasoline when pump is not operational. After the vehicle has cooled down, fuel pressure may drop
to 0 psi (cold fluid contracts), but liquid gasoline will remain in fuel supply line between the check valve and fuel injectors. Fuel pressure that has
dropped to 0 psi on a cooled down vehicle (engine off) is a normal condition. When the electric fuel pump is activated, fuel pressure should
immediately (1–2 seconds) rise to specification.

Abnormally long periods of cranking to restart a hot engine that has been shut down for a short period of time may be caused by:

• Fuel pressure bleeding past a fuel injector(s).
• Fuel pressure bleeding past the check valve in the fuel pump module.

(1) Disconnect the fuel inlet line at fuel rail. Refer to Fuel Tubes/Lines/Hoses and Clamps in this section of the group for procedures. On some engines, air cleaner housing removal may be necessary before fuel line disconnection.

(2) Obtain correct Fuel Line Pressure Test Adapter Tool Hose. Tool number 6539 is used for 5/16” fuel lines and tool number 6631 is used for 3/8” fuel lines.

(3) Connect correct Fuel Line Pressure Test Adapter Tool Hose between disconnected fuel line and fuel rail (Fig. 8).

(4) Connect the 0-414 kPa (0-60 psi) fuel pressure test gauge (from Gauge Set 5069) to the test port on the appropriate Adaptor Tool. The fittings on both tools must be in good condition and free from any small leaks before performing the proceeding test.

(5) Start engine and bring to normal operating temperature.

(6) Observe test gauge. Normal operating pressure should be 339 kPa - 634 kPa (49.2 psi - 65 psi).

(7) Shut engine off.

(8) Pressure should not fall below 30 psi for five minutes.

(9) If pressure falls below 30 psi, it must be determined if a fuel injector, the check valve within the fuel pump module, or a fuel tube/line is leaking.

(10) Again, start engine and bring to normal operating temperature.

(11) Shut engine off.

(12) Testing for fuel injector or fuel rail leakage: Clamp off the rubber hose portion of Adaptor Tool between the fuel rail and the test port “T” on
Adapter Tool. If pressure now holds at or above 30 psi, a fuel injector or the fuel rail is leaking.

(13) Testing for fuel pump check valve, filter/regulator check valve or fuel tube/line leakage: Clamp off the rubber hose portion of Adaptor Tool between the vehicle fuel line and test port “T” on Adapter Tool. If pressure now holds at or above 30 psi, a leak may be found at a fuel tube/line. If no leaks are found at fuel tubes or lines, one of the check valves in either the electric fuel pump or filter/regulator may be leaking.

Note: A quick loss of pressure usually indicates a defective check valve in the filter/regulator. A slow loss of pressure usually indicates a defective check valve in the electric fuel pump.

The electric fuel pump is not serviced separately. Replace the fuel pump module assembly. The filter/regulator may be replaced separately on certain applications. Refer to Fuel Filter/Fuel Pressure Regulator Removal/Installation for additional information.

 

Thanks, my FSM in PDF kept shutting down, so no access.
Did you see that: " should not drop below 30 PSI after 5 minutes" (item 8).
Thats what I had vaguely in mind as well. The drop to 39 PSI after 30 minutes as the OP stated, isn't actually that bad? Or am I reading this wrong?

 

No that's what I gathered as well. Technically the OP's results are within spec, what matters according to the FSM is if there's a fast drop off at the beginning.

Although Pelican's results I think are worth something.

 

Thank you for posting the information from the FSM. In thinking back on the 'equalizing sizzle' sound I'd heard when I did the leak test (turning epower on while injectors are out) I was hoping the loss of pressure was from an o-ring. While taking off the fuel rail to check I pressed the valve stem to release pressure and no fuel came out. However, after removing the fuel rail and inverting it there was definitely still fuel. So it is still holding at least some fuel. And, one of the o-rings was in fact scruffed up.