3.7L Misfire

[Tonali]

We have been working on my brother's friend's 2002 Ram 1500 equipped with a 3.7L V6. The truck is experiencing a misfire in #3 cylinder. She previously had all the plugs changed and all but 1 ignition coil pack changed. We did some troubleshooting and found a misfire in #3 cylinder. We thought it may have been #5 or #6, as these can be difficult to get to and service. However, The PCM threw a P0303 code. So we replaced #3 coil pack. Truck ran okay for a few miles and began to misfire again. Figured we had a bad coil pack. Replaced with another, same problem. so we exchanged the second new one with a unit from another cylinder. Still #3 misfire. Checked spark plug. Porcelain intact. Plug has darkish tan color and slight odor of fuel. So the plug was firing okay at some point before whatever unit failed.

At this point, we believe it may be a wiring harness problem or a PCM problem. It could be mechanical (i.e., valve not seating correctly, bad rings, etc) but there are no mechanical sounds to indicate a valve and there is no oil smoke or coolant smoke.

I have to admit, I'm stumped.

 

[JAMMAN]

Are you losing coolant and have had to replace some in your reservoir?

Have your coolant checked for exhaust fumes I'm serious there is a kit for it.

If no exhaust in coolant and you are not losing coolant do a leak down test on the cylinders.

This will rule out head gasket, valves, rings (doubtful it is rings you would be smoking like crazy).

It will not rule out however oil supply to the valve lash adjusters. Compression test might show a weak #3.

How many miles does this have on it?

 

[Tonali]

About 162,000, I believe. There is no coolant loss. We can test for exhaust in the coolant. I have a feeling it's not a head gasket. I think there's a valve not seating. I am thinking about misting water into the intake. To kind of blast any deposits off the valve. The only thing else is maybe a harness. These high discharge ignition systems can be tough to troubleshoot