Misfire at high RPM +Black Spark Plugs After 3 Days of use … Here’s What I Found…

[Aivars]

After a long and cold winter (haven't seen such winter at least last 10 years) where the engine spent a lot of time idling (warming up, working on installs, etc.), I started experiencing an issue in spring.

Under hard acceleration (around ~5800 RPM), I would get a check engine light with a misfire on cylinder 4. The misfire lasted for about 1–2 minutes, then disappeared, and the engine returned to running smoothly on all 6 cylinders. During the event, it was clearly noticeable that one cylinder was not working.

Like most of us would do, I started with the basics:

• Fuel system flush
• New spark plugs (OEM Mopar)
• Swapped ignition coils between cylinders
• Swapped fuel injectors

None of that solved the issue — misfire stayed on cylinder 4.

Additionally, the brand new spark plugs turned completely black after just 3 days of driving, which is not typical for a gasoline engine and clearly indicated a serious combustion issue (rich condition / incomplete burn).

Before moving to deeper diagnostics, I performed a compression test. 2nd and 4th cylinder showed compression under 10 bars even closer to 9. This was the first clear sign that something was not right internally and confirmed that further investigation was necessary.

Next step — deeper diagnostics.

We used a borescope camera inside the cylinder. At first, the situation looked quite bad — it seemed like the valves might be damaged. Based on that, the decision was made to remove the both cylinder heads.

After disassembly, it turned out:

• All valves were actually intact
• The “damage” seen on camera was carbon buildup and deposits from unburnt fuel
• Visually misleading, but not mechanical failure

However, after pressure testing the head, the real issue was found:
The head was not sealing properly (leakage at one or more valve seats)

This explains everything:

• Loss of compression under load
• Misfire at high RPM
• ECU cutting fuel to that cylinder

The head was then fully rebuilt:

• Valve seats restored
• Valves lapped
• Full sealing restored

Now the head is in perfect condition, as shown in the photos.

During the process, we also found that the water pump needed attention. The existing pump is still functioning, but when rotating it by hand, it produces a slight whining noise. This indicates that it is no longer operating as it should and could potentially fail in the near future.

In Latvia, this part is extremely difficult to get quickly. OEM Mopar would take ~3 weeks to arrive, so I decided to go with a Gates aftermarket water pump available in Europe.

Current status:

• Cylinder head fully rebuilt
• Waiting for water pump
• Reassembly coming next

Hopefully this will bring the engine back to full power and smooth operation.

I’ll update once everything is back together and tested.

Photos attached for reference.

 

[Aivars]

Before installation, the rebuilt head looks absolutely mint. Freshly machined surface, restored valve seats, and everything cleaned up properly — honestly looks almost brand new and ready to go back on the engine.

 

[John Kloehr]

Good troubleshooting. As soon as I came across your compression report in bars, I started looking for where in the world you were.

I think a leak down test might have identified the valve seats before teardown, but you found the issue anyway. There had to be a reason for low compression.

The failure could have been caused by excessive idling in cold weather, the 3.6L may not reach full operating temperature and then incomplete combustion leads to carbon buildup. An overheat can also cause this failure, but you did find carbon.

 

[Aivars]

Good troubleshooting. As soon as I came across your compression report in bars, I started looking for where in the world you were.

I think a leak down test might have identified the valve seats before teardown, but you found the issue anyway. There had to be a reason for low compression.

The failure could have been caused by excessive idling in cold weather, the 3.6L may not reach full operating temperature and then incomplete combustion leads to carbon buildup. An overheat can also cause this failure, but you did find carbon.

Yeah, I had already suspected for a while that excessive idling on a gasoline engine isn’t ideal. This winter the temperatures were quite low, so the truck spent a lot of time idling while warming up for comfort.

In reality, that likely meant the engine wasn’t consistently reaching full operating temperature under load, which over time led to incomplete combustion and noticeable carbon buildup — exactly what I found during teardown.

At this point the root cause is clear. Now it’s just a matter of putting everything back together and getting it back on the road. Definitely not making the same mistake next winter.

 

[A-TEAM]

I don't have the 3.6 in my ram but I do have a 3.6 in a wk2 grand cherokee and my water pump was leaking so I replaced it with an OEM and it failed again within 2k miles. Replaced again with a gates pump and its been good to go since. So I think you made the right choice on the pump!

 

[Aivars]

The water pump arrived in two days, so that part is now ready to go in. This is how the engine looked this morning while everything was being reassembled, and I also snapped this shot of one of the cams and valvetrain components laid out before going back together. . If everything continues as planned, it should be back on the road Monday.

 

[Quint]

Are there many other Rams being driven in Latvia?

 

[Aivars]

Are there many other Rams being driven in Latvia?

Compared with Audi, BMW, VW, or Toyota, no — RAMs are not common in Latvia. But compared with other full-size American pickups, RAM is one of the more noticeable ones here. Most people over here mainly drive small or mid-size diesel cars, so a truck like a Ram naturally stands out in everyday traffic. It is no longer something people are completely unfamiliar with, but it still feels like an uncommon yet already established truck with a seriously impressive size and presence.

 

[PhillyBoiP]

Great read and even better troubleshooting, props to you for sure on this!

 

[Aivars]

Great read and even better troubleshooting, props to you for sure on this!

Thanks man, appreciate it! I wanted to document the whole troubleshooting process properly in case it helps someone else too.