I have a 99 Dakota with the 5.2L magnum (250k miles)that I just picked up a couple months ago. First thing I did was install new plugs(Autolite 5224) serpentine belt and air filter. Ran great with no codes. Recently it threw a P0303 so I installed new plug wires, then new cap and rotor. I swapped #1 and #3 injectors to see if it was the injector but I still get a P0303 code. #3 cylinder has 120 psi and I have no vacuum in the crankcase so it can't be the valley pan gasket. There is a noticeable mis coming from the exhaust pipe and at idle and the spark plug is grey ashen which tells me it's running Lean.

Any suggestions where to go next?
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Broke this out, and moved it to the correct section. Don't hijack someone elses thread.

 

Got it. Thanks.

 

Did you clear all codes/reset the computer when you swapped injectors?

Have you put a noid light on the injector to verify that the injector is being commanded to fire?

 

Codes were cleared. All injectors are firing. My first thought was it may be a small valley pan leak directly over #3 but there is no oil fouling on the plug.

 

If it was an issue with the crank sensor, oxygen sensor, or a cylinder head ground strap, it would be multiple cylinders affected.

1-8-4-3-6-5-7-2 the same hole in the crank sensor fires cylinders 3 and 2.

The driver-side upstream oxygen sensor detects misfires for 1,3,5,7 (there are no knock sensors - misfires are 100% done by oxygen sensors)

A head ground issue would also affect 1,3,5,7 the same.

The only thing that would affect #3 only would be fuel or air. Since it's a common rail system & you already swapped injectors, I'd be inclined to think it's an issue between the #3 driver in the PCM and the #3 injector plug causing a lean spray, or an issue with the valvetrain on that cylinder preventing proper intake/exhaust flow. Again, the oxygen sensor is what detects misfires, so an issue with exhaust delivery will get marked as a misfire.

 

When I get a chance I'll do a leak down check. I usually don't bother if it has good compression but I'm running out of ideas. If there's an issue with the ECU not sending enough fuel I doubt I can find anyone who can fix that around here.