Rick's Free Auto Repair Advice

P0300 Rough idle Dodge Ram

Diagnose and fix P0300 Rough idle Dodge Ram

The Dodge Ram 4.7L engine is known for developing carbon buildup on the valves that can cause a P0300 rough idle Dodge Ram issue. P0300 is a random misfire, meaning that it’s occurring in all cylinders in no specific pattern.

First things to check for P0300 Rough idle Dodge Ram

Spark plugs and wires

Obviously, worn spark plugs can cause a P0300 condition. So check those first. If the gap is too large, you’ll still get a spark but it will be weaker and shorter, causing a random misfire. A shop would put this on a scope to find the value for the firing line and spark duration. If you’re not sure, replace all the spark plugs and wires.

Vacuum leak

Next, check for a vacuum leak. A vacuum leak allows extra air into

vacuum gague readings

Attach a vacuum gauge to any port on the intake manifold and compare to this chart

the air/fuel mixture causing a lean misfire. If you don’t find any leaks, slap a vacuum gauge on the intake and look for 18-19 in. of manifold vacuum. The needle should be steady. A bouncing needle can be a sign of carbon buildup on the valves, or worn valve guides.

Crankshaft position sensor and camshaft position sensor

It’s really hard for a DIYer to check the signal from these sensors because that requires a scope. However, if you’ve replace either of these sensors and the problem showed up afterwards, suspect either a bad sensor or improper alignment of the sensor.


You really should have a scan tool with short term fuel trim and long term fuel trim readings. These readings tell you what the computer is doing to compensate for a worn engine, vacuum leak or bad sensor readings. If the computer is adding fuel, suspect a vacuum leak, plugged injector or fuel pressure or fuel pump problem. If it’s subtracting fuel, suspect a leaking fuel injector or fuel pressure issue.

short term fuel trim

Scan tool fuel trim readings

Carbon deposits

This is really hard to find if you’re a DIYer. But if you’ve checked everything else, you can just assume you’ve got a carbon buildup issue and perform an engine decarbon procedure. See this post for step by step instructions

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Posted on by Rick Muscoplat


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