We've done a lot trying to figure out one problem. Misfire on cylinder 4. All that can be replaced has been. Now, we took off the fuel rail to see the injector has oil mixed in. It's the only one. Where do I trace that back to?

 

If you’re seeing oil mixed in with fuel at the injector on cylinder 4 in your Porsche Cayenne 955, that’s not normal—and it usually points to a fairly specific set of problems. Let’s narrow it down logically.

🚨 First: Clarify what you’re actually seeing

Before chasing ghosts, confirm:

• Is it engine oil (thick, brown/black, smells like oil)?
  
• Or fuel residue mixed with carbon (can look oily but isn’t engine oil)?
  

If it’s truly engine oil inside the injector port or rail, it should NOT be there at all.

🔧 Most Likely Causes

1. Leaking injector seal / cracked injector body

• Each injector has O-rings sealing fuel and air paths
  
• If one fails:
  
  • Oil vapor from intake can contaminate the injector tip area
    
  • Misfire on that cylinder (your cyl 4)
    

👉 But this usually doesn’t put oil inside the fuel rail—just around the injector tip.

2. Crankcase ventilation (PCV) system failure (VERY common)

On the Cayenne 955, the PCV / crankcase ventilation system is a big suspect.

• When it fails:
  
  • Oil vapor gets sucked into the intake excessively
    
  • Oil can pool in intake runners
    
  • Ends up around injectors → misfire
    

👉 This is one of the MOST common causes on these engines

3. Intake valve / valve guide seal leak

• Oil leaks past valve seals into intake port
  
• Fouls:
  
  • Spark plug
    
  • Injector tip area
    
• Causes persistent single-cylinder misfire
  

👉 Especially if:

• Plug is oily
  
• Compression is still OK
  

4. Head gasket issue (less likely but serious)

• If oil is somehow entering passages abnormally
  
• Would usually come with:
  
  • Coolant issues
    
  • Multiple cylinder symptoms
    

👉 Not the first place to look unless other symptoms exist

5. Fuel rail contamination (RARE but possible)

• If oil is actually inside the fuel rail
  
• That means contamination came from:
  
  • Bad fuel (very rare)
    
  • Someone introduced oil accidentally
    
  • Internal failure (extremely unlikely)
    

🔍 What You Should Do Next (Step-by-Step)

1. Inspect cylinder 4 spark plug

• Is it:
  
  • Wet with oil? → internal oil intrusion
    
  • Dry but carbon fouled? → fuel/combustion issue
    

2. Check inside intake runner (cylinder 4)

• Use a borescope if possible
  
• Look for:
  
  • Oil pooling
    
  • Wet intake walls
    

3. Inspect PCV system (HIGH PRIORITY)

• Check:
  
  • Breather hoses
    
  • Oil separator
    
• Signs:
  
  • Oil in intake tubing
    
  • Excess vacuum at oil cap
    

4. Compression / leakdown test

• Confirms:
  
  • Valve seal vs ring issue
    

5. Swap injector (if not already done)

• Move cyl 4 injector to another cylinder
  
• See if misfire follows
  

⚠️ Key Insight

If oil is around the injector tip → intake/valve/PCV issue
If oil is inside the fuel rail → something is very wrong/unusual

💡 My Read Based on Your Clue

Since you said:
“oil mixed in at the injector”
The most likely root cause on a Cayenne 955 is:
👉 Failed PCV/oil separator system feeding oil into the intake
If you want, tell me:

• Engine type (V6 or V8)
  
• What parts you’ve already replaced
  
• What the plug from cyl 4 looks like
  

…and I can narrow this down to almost exact failure point.