How to Fix a Misfire P0300 on a Dodge 5.7L V8 Hemi

How to troubleshoot a misfire on a Dodge Ram equipped with the 5.7 L V8 Hemi. The first thing I would recommend doing is using your OBD2 scan tool to scan for any fault codes. If there is a misfire, it should be followed by a check engine light being illuminated. #dodge #hemi #emanualonline

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Tools/Supplies Needed:
-ratchet and socket set
-multimeter
-torch set

Procedure:
As you can see we have the common P0300 multi cylinder misfire. This is also followed by some other fall codes such as the O2 sensors on both banks and the fuel trim being lean on both banks. The next feature with the scan tool wise scanning the misfire data.

I kept the scan tool hooked up to my truck as I was driving it. My misfire Wasn’t constant, it would just show up at random times. With the live misfire log activated on the scan tool, as I was driving, this would show which cylinders were misfiring. This would allow me to determine if it was cylinder specific, bank specific, or throughout the whole engine. During the data log, it was shown that all cylinders were having a misfire issue. Therefore my next step was to check something that was common with all cylinders, this would be the spark plugs.

I did replace spark plugs in this truck about 20,000 km ago, however there can be issues with different brands of spark plugs depending on the vehicle.

First was disconnecting the electrical connectors for each of the coil packs. The assistance of a standard screwdriver can be used here to remove that electrical plug.

Remove the coil packs, a 10 mm socket, and ratchet were used for this. Then remove the spark plugs. 

After all the spark plugs were replaced, here you can see what they look like. I did notice during the replacement that they were burned up a bit, but the biggest noticeable issue here was one bank was running rich. This is shown by the excessively black plugs.

As for the coil packs, these two can also cause a problem. Typically only one or two coil packs may fail, you won’t usually see all of them fail at once. These can be checked using a multimeter.

Looking at the specific values in the repair manual for the Dodge Ram, the 5.7 L Hemi has a primary coil resistance of 0.558 to 0.682 ohms. It’s always important to check the ohms reading on your meter first by touching the probes together, then minus this value from your final reading. All of these coils appear to be checked out within the appropriate specifications.

Reinstall all the coil packs. After everything was back together I did find the truck ran much better. However the misfire was still popping up.

I noticed I also had O2 sensor fault codes, this is the O2 sensors before the catalytic converters. So these would be known as the upstream sensors. These were adjusted your air fuel ratio, the ones after the catalytic converter are monitoring the performance of your admission system.

If you have one sensor faulty, most likely the other one won’t be far behind so it’s best to replace them as a pair. This involved using a torch to heat up the bung and then remove the sensor.

The new sensor was installed. Being that one bank was running more rich than compared to the other, a faulty O2 sensor will cause this. After that my truck was running properly.

Other areas to look for on a misfiring 5.7 L hemi would be a faulty injector. The electro connector can be probed at the injector to ensure you are getting a proper voltage value. If you find it specific to one cylinder, you can move this injector around to see if it’s traveling to each cylinder.

If it doesn’t move from that cylinder, these engines are also known for having valve train issues. You may have a worn camshaft, lifter issue, or valve spring issue. Issues like this can be determined using a bar scope inside your combustion chamber to inspect the valve movement, a compression test can be done as well as a leak-downn test. After that would be disassembling the engine for further inspection. This is especially a problem on these engines with the multi cylinder deactivation. Thankfully this is disabled on my truck.

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