If your fuel injected Jeep is running rough like it has a miss on one cylinder, the trouble may be a bad fuel injector. Fuel injection is great when everything is working correctly but sometimes trouble shooting the system can be like shooting in the dark. You can’t just remove the air filter and look inside like you can with a carburetor.
I have been working with a 1989 Jeep Cherokee six cylinder recently. The truck did not run when it came to me and I did not know its history.
One I got it running, it had a distinct miss of a dead cylinder. I first checked to make sure it was getting spark to all the plugs. I did this first by using my in line spark tester. This tool flashes when the coil sends fire to the plug. All six were flashing. I then pulled the spark plugs out one at a time and visually inspected them. They all looked fine and all looked the same giving me no clue which one was the trouble cylinder.
On older engines, you could simply remove the plug wires one at a time to see which one did not make any difference in the way the engine runs. However, I have found that you can’t do that on modern electronic ignition systems as the power in the spark plug wire will give you a very uncomfortable zap if you try to remove it while the engine is running. So I don’t use that method on modern engines.
Next I checked the fuel pressure at the fuel rail. The pressure was within specifications so I moved on to testing individual injectors.
To check the injectors, I used a mechanic’s stethoscope. I placed the tip of the stethoscope on each injector body and listened at idle. On the injectors that worked correctly, there was a very distinct ticking sound. On the injector that was giving trouble, there was no ticking sound. I moved the stethoscope back and forth between injectors a few times to get a clear image of the sound that it was supposed to make when it was working.
Before removing the fuel rail to change the injector, I tested the electrical signal to the injector. I made a simple test lamp using a parking light socket and bulb and plugged it into the wiring harness. Since the light blinked, I knew the injector was getting power and must be stuck or bad. You can also purchase a noid light that will plug into the harness, but for a quick check you can make a simple test lamp like I did.
To test the fuel injector firing coil you can use an ohm meter. Simply disconnect the wiring harness form the injector and place the probes across the two contacts of the injector. An intact coil will read around 15 ohms. However, as seen on this Jeep, an intact coil does not mean the injector works.
Once you have located the injector giving trouble you will have to remove the fuel rail to replace it. See my other articles for how to replace an injector or repair a leaking injector.