NO START 1: Checking For Spark
Whether your specific 4.0L equipped Jeep has a distributor equipped ignition system or a Coil-On-Plug (COP) ignition system, the very first you should do is check for spark.
Why? Because it's usually the easiest of all the tests you may have to do and also because the ignition system is usually the culprit behind most no-start conditions.
Testing for spark can very easily and safely be done with a dedicated spark tester.
If the ignition system is the cause of the cranks but does not start condition, you're not gonna' see spark at any of the 6 spark plug wires or 6 COP ignition coils. If there is spark being fed to all cylinders, then you can eliminate the ignition system as the cause of the no-start condition and can move on to other tests (like testing fuel pressure).
Remember, the idea behind checking for spark is to see if all of the engine cylinders are getting spark. Here are the most common results from a spark test:
CASE 1: Spark was present in all cylinders. This result tells you several important things:
- The crankshaft position sensor is functioning correctly.
- The ignition coil is good (distributor equipped system).
- The COP ignition coils are (distributor-less ignition system).
Since you're getting spark on all 6 cylinders, your next step is to verify fuel that the fuel pump is working and supplying fuel to the fuel injectors. Go to: NO START 2: Checking Fuel.
CASE 2: Spark was NOT present in all of the cylinders (DISTRIBUTOR TYPE). This test result tells you without a doubt that the no-start condition of your 4.0L equipped Jeep is due to a malfunction in the ignition system.
Now, with no spark in any of the engine cylinders, this what I would suggest:
- Check for spark directly on the ignition coil tower using a spark tester (if distributor equipped).
- This is the best way to test the distributor cap. The distributor cap and rotor are infamous for causing no spark no-start conditions.
- If you do get spark coming out of the ignition coil tower, you now know beyond a shadow of a doubt the distributor cap and rotor must be replaced.
- If you don't get spark from the ignition coil, then there's a good chance that the crank sensor is fried and should be tested.
- The following tutorial will show you how to test the ignition coil:
CASE 3: Spark was NOT present in all of the cylinders (COP IGNITION COIL TYPE) This test result lets you know that the lack of spark is the culprit behind the cranks but does not start condition of your 4.0L Jeep.
It's almost impossible for all 6 COP ignition coils to fail at the exact same time, so the most likely cause for this no spark condition is a bad crankshaft position sensor.
But before you go out and replace the crank sensor, you need to test it (to make sure it's fried). But before you go out and replace the cranks sensor, you need to test it (to make sure it's fried).
NO START 2: Checking For Fuel
Checking that the fuel pump is delivering fuel to the fuel injectors is not that hard on your 4.0L equipped Jeep since the fuel injector rail comes equipped with a Schrader valve (this is the valve to which you can connect a fuel pressure gauge).
Testing the fuel pressure with a fuel pressure gauge is the absolute best way to test the fuel pump. Any other method of testing the fuel pump is not as accurate.
Still, the other method that I've used to test for a lack of fuel is spraying starting fluid into the throttle body. And then having a helper crank the engine.
If the engine starts, then I now know that I need to take a closer look at the fuel pump to see if it's fried or not.
You can find an in-depth tutorial on testing the fuel pump here: How To Test The Fuel Pump In 2 Tests (4.0L Jeep). Now, when testing the fuel pump (with a fuel pressure gauge), you'll usually see one of two results:
CASE 1: Fuel pressure is at specification. Not only does this result tell you that the fuel pump is OK but that the following components, that supply the fuel pump with power, are OK too:
- Fuel pump fuse.
- Fuel pump relay.
And so, there's no need to spend time testing them or money replacing them.
CASE 2: Fuel pressure is not present. This usually means that the fuel pump has failed, but not always. I would recommend testing/checking the following before condemning the fuel pump:
- After verifying that no fuel pressure exists, check that the fuel pump is getting power by tapping into the power circuit that feeds the pump with 12 Volts with a multimeter.
- Once you're tapped in, have a helper crank the engine while you observe your multimeter in Volts DC mode. If voltage is present (12 Volts), then you have confirmed that the fuel pump fuse and fuel pump relay are working perfectly.
- Confirming power to the fuel pump (with a multimeter) also verifies that the fuel pump has failed and needs to be replaced.
- If no voltage is present, as your helper cranks the engine, then the cause of lack of fuel condition is due to either a bad fuse, fuel pump relay.
