A no-start condition, on your 4.0L Jeep, is usually caused by a lack of fuel, or a lack of spark, or a lack of air (generally engine compression).
Knowing that the cause of the problem lies in these 3 specific areas will help you figure out what tests you need to perform to find the faulty component and thus solve the problem.
In this tutorial, which is a primer on what can cause a cranks but does not start condition, I'll share some insights and tips, based from my actual experience, that will help you narrow down your troubleshooting efforts.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar Un Arranca Pero No Prende (4.0L Jeep) (at: autotecnico-online.com).
Difference Between A No-Start And A No-Crank Condition
Before I jump into the specifics of what causes a no-start condition, I want to clarify the difference between two common terms you'll see and hear as you diagnose the problem on your 4.0L equipped Jeep:
Cranks But Does Not Start Condition: In this type of condition, you turn the key and the engine in your Jeep cranks (turns over) but it doesn't start (run).
The usual suspects, that can cause this type of problem, are a bad fuel pump, bad crank sensor, bad ignition coil (if your Jeep is distributor equipped), etc.
Does Not Crank Condition: Means that the engine is not cranking when you turn the key to crank the engine. In other words, the engine doesn't turn over at all. This is usually due to a bad starter motor, bad ignition switch, bad neutral safety switch, or the engine is locked up.
If your 4.0L Jeep is experiencing a no-crank condition (instead of a cranks but does not start), the following tutorial may be of help: How To Test The Starter Motor (Jeep 4.0L).
No Start Condition Basics
As I mentioned at the beginning of this tutorial, a no-start condition is caused by a lack of one of 3 basic things. These three things are:
- Air.
- Fuel.
- Spark.
When your 4.0L equipped Jeep cranks but does not start, it's because one of these 3 things is missing from the mix when you're cranking the engine to get it started. It's as simple as this! I know, I know, I may be over-simplifying it all but knowing that only one of three things is missing really helps to put the problem into perspective!
Also, knowing this means that when your Jeep doesn't want to start, your diagnostic/troubleshooting efforts are now directed/focused at finding out which one of these 3 things is missing.
More specifically, this means that troubleshooting the problem requires that you check and confirm that:
- All 6 cylinders are getting spark (with a spark tester).
- That the fuel pump is creating enough fuel pressure.
- And if necessary, check the engine's health with a compression test.
I'll go into more specific details in the following headings:
1.) Ignition System
- The ignition system is the one tasked with creating and delivering spark to each of the 6 cylinders. Without spark, the engine will crank but not start.
- The ignition system of the Jeep vehicles covered by this tutorial use either a distributor type system or a COP (Coil-On-Plug) ignition coil system.
- In my experience, the most common component failures, of the ignition system that cause a 'no-start no-spark' condition are:
- A bad ignition coil.
- A bad distributor cap (if distributor equipped).
- A bad distributor rotor (if distributor equipped).
- A bad crankshaft position sensor.
- All of the above ignition system components can be tested in a methodical way to find out exactly what has failed (if indeed something has).
2.) Fuel System
- The fuel system is the one responsible with supplying the engine with fuel.
- The fuel system component that causes the majority of no-start no-fuel problems:
- A bad fuel pump relay.
- A bad fuel pump.
- The fuel pump can be tested to make sure it has really fried using a fuel pressure gauge.
3.) Engine Mechanical System
- The components that are responsible for drawing in the air the engine needs are the: engine pistons, cylinder head valves, and all the other related components like: timing chain, etc.
- Although rare, internal engine mechanical problems can and do cause no-start conditions.
- Possible internal/external engine problems are:
- A blown head gasket.
- A blown engine.
- A busted timing belt.
OK, the list of possible things that can go wrong looks pretty long but it is rare to see (or have) two different components go bad from two separate systems at the same time.
The cool thing is, is that there is a diagnostic strategy that you can use to figure out exactly what's wrong with your particular no-start problem. Let's find out more about it in the next subheading.
