I am hunting down a misfire. I haven't pulled the codes yet, and was hoping for a little backyard mechanic action before I take it to a shop.

I have been told that it is likely the ECU because it seems like a fuel injector is not firing. While the engine is running you can unplug the connector from each injector, and the engine idles differently except when you unplug the #3 fuel injector.

What are some other tests that I could run so I can be a bit more informed when I get feedback from the mechanic?

I am in Doha, Qatar, and want to have a basic understanding of what it could be just in case a mechanic here tells me it is something completely different. I am still new to Jeeps, and computer controlled fuel injection in general.

Thanks in advance!

 

Is this a 2001? 6 Cylinder 4.0L?

Could be a fuel or spark issue.

Pull the #3 spark plug. What does it the tip look like? Is it wet? Is it black? Is there a proper gap?

Each injector gets 12V on the Dark Green/Orange wire. With the key on RUN, you can check for 12V at each injector. The 2nd wire on each injector is grounded through the PCM (all have different colors). Do not check for ground.

You can also get (or make) a "noid light" that connects to the injector connector. When the jeep is started (and running) the light should flicker when the injector fires. Doesn't tell you the injector is bad, it only says it's getting 12V and a good ground when the PCM says so.

You can also take the connector off the PCM and check for corrosion, arcing, etc. Give it a good cleaning if needed.

 

Yes, this is a 2001 4.0L.

I'll see what I can do about checking the plugs. I had a friend check it out because I was at work, and he said the coil was good and he cleaned the plugs.

Just so I understand what I am checking... Does the injector have constant power, and it is the ground that is switched on and off to fire the injector? If there is power at the injector, does that likely mean injector?

 

Yes, injectors get 12v and the PCM grounds them when it's time to fire. If there's power at the injector, it just means the 12v wiring to the injector is OK. You would also need to verify the ground side and signal from the PCM. That's where the noid light works best. It tell you that you have power, ground and signal from the PCM (or not). If the noid light flickers and you've determined it's not a spark or compression issue, then the fuel injector is the prime suspect. If the noid light doesn't flicker and you verify 12V, then it's likely the ground wire (wire, connectors, splices), the connector at the PCM or even the PCM itself.

Another quick test is to disconnect the fuel injector connector and using a 12v source jump to the pins on the injector (engine off for this test). You should hear a faint click each time you connect and disconnect. Again, not definitive, the injector may still be clogged but no clicking is a strong sign it's failed.

 

PCM is not something that should be considered just yet. Is it a steady misfire you can feel?
Reading the codes would be a great idea.