TEST 2: Checking Fuel Pressure With A Fuel Pressure Gauge
The fuel pressure specification, from the Jeep repair manual, is 30 to 35 PSI. The only way this specification can be checked is by using a fuel pressure gauge.
Checking the fuel pump with a fuel pressure gauge is the most accurate and professional way of checking the state of the fuel pump (by the way, it's how it's done at any professional repair shop).
We can connect a fuel pressure gauge to the fuel system using the Schrader valve located on the fuel injector rail (see photo above).
NOTE: If you don't have a fuel pressure gauge, take a look at the section: Where To Buy A Fuel Pressure Test Gauge.
OK, let's get started with this test:
Place a shop towel around the Schrader valve. The shop towel's job is to absorb any fuel that may leak when doing step 2.
Connect the fuel pressure gauge to the Schrader valve on your 4.0L Jeep's fuel injector rail.
When ready, ask your helper to cycle the key on and off but don't crank the engine while you observe the fuel pressure tester's gauge.
Check the connection at the Schrader valve for fuel leaks and if any tighten the fuel pressure a bit more (by hand only) to eliminate them.
Your fuel pressure gauge should register: 30 to 45 PSI with the Key On Engine Off (KOEO) if the fuel pump is OK.
Let's take a look at what your results mean:
CASE 1: If the fuel pressure gauge registered 0 PSI This confirms that the cause of your 4.0L engine's no start problem is caused by a lack of fuel.
Now, I usually take one more precaution, before condemning the fuel pump as BAD and this is to check that the fuel pump is getting 12 Volts as the engine is cranking. This is just to make sure that the fuel pump relay and fuse are OK and doing their job.
What you'll have to do to accomplish this is to attach a multimeter in Volts DC mode to the wire that supplies this voltage to the fuel pump and while a helper cranks the engine... verify that the fuel pump is getting this power. If the 12 Volts are being supplied to the fuel pump... you have now 100% verified that the fuel pump is fried and needs to be replaced.
CASE 2: If the fuel pressure gauge registered 30 to 45 PSI: This fuel pressure gauge result let's you know that the fuel pump is working and delivering enough fuel to the fuel injectors. The reason your 4.0L equipped Jeep is not starting is due to another reason. The fuel pump is OK.
Where to Buy a Fuel Pressure Test Gauge
You can buy a fuel pressure test gauge just about anywhere and is one of the most important tools any serious DIY'er should have in his/her tool box.
The following fuel pressure test gauge kits are pretty good deals and include the adapter to tests all Jeep with a Jeep-type Schrader valve:
All of the fuel pump pressure test kits above have the fitting that will connect to your Jeep's Schrader valve.
Where To Buy The Fuel Pump And Save
The fuel pump in your 4.0L Jeep is located inside the fuel tank. The fuel pump can be bought in any auto parts store, but I think you'll find the better price online. The following fuel pumps are pretty good deals:
1997 and Newer 4.0L Jeeps ⇈
1996 and Older 4.0L Jeeps ⇈
Not sure if the above fuel pumps fit your particular 4.0L Jeep? Don't worry, once you get to the site, they'll make sure it fits by asking you the specifics of your vehicle. If it doesn't fit, they'll find you the right one.
More Jeep 4.0L Test Tutorials
If this tutorial was helpful/informative, you can find a complete list of tutorials here: Jeep 4.0L Index of Articles.
Here's a sample of the tutorials you'll find there:
- Rear O2 Sensor Heater Test -P0141 (1996-1998 4.0L Grand Cherokee).
- How To Test The MAP Sensor (1997-2003 Jeep 4.0L).
- How To Do A Cylinder Balance Test (Jeep 4.0L).
- Jeep PWM Fan Relay Test Troubleshooting An Overheating Condition.
- How To Test The Crank Sensor (Jeep 4.0L -1997-2004).