Weird problem
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3rd Gear
Joined: Mar 2025
Posts: 3
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Weird problem
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?
Three Wheelin'
Joined: Aug 2016
Posts: 1,600
Likes: 273
From: Southern AZ
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.
On the Cayenne 955, the PCV / crankcase ventilation system is a big suspect.
If oil is inside the fuel rail → something is very wrong/unusual
“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:
🚨 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)?
🔧 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)
- Oil vapor from intake can contaminate the injector tip area
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
- Oil vapor gets sucked into the intake excessively
3. Intake valve / valve guide seal leak
- Oil leaks past valve seals into intake port
- Fouls:
- Spark plug
- Injector tip area
- Spark plug
- Causes persistent single-cylinder misfire
- 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
- Coolant issues
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)
- Bad fuel (very rare)
🔍 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
- Wet with oil? → internal oil intrusion
2. Check inside intake runner (cylinder 4)
- Use a borescope if possible
- Look for:
- Oil pooling
- Wet intake walls
- Oil pooling
3. Inspect PCV system (HIGH PRIORITY)
- Check:
- Breather hoses
- Oil separator
- Breather hoses
- Signs:
- Oil in intake tubing
- Excess vacuum at oil cap
- Oil in intake tubing
4. Compression / leakdown test
- Confirms:
- Valve seal vs ring issue
- 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 issueIf 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
