W108 Fuel Injection: Fuel in Sump Oil Mystery

Embarking on a full restoration of a classic Mercedes-Benz W108 250 SE is a labour of love, and getting the engine running smoothly after such a meticulous rebuild is the crowning glory. However, as you've discovered with your neighbour's beautiful 1968 automatic sedan, the intricate world of classic fuel injection systems can present some formidable challenges. The symptoms you describe – rough running, copious black smoke, sooty spark plugs, and most alarmingly, fuel in the sump oil – point directly to a severe over-fuelling issue, and your suspicion regarding the Bosch injector pump is well-founded.

The Bosch PESAKL 708120R21 is a mechanical fuel injection (MFI) pump, a marvel of engineering for its time, but also a complex piece of equipment that requires precise calibration and healthy components to function correctly. Unlike modern electronic fuel injection, these systems rely on mechanical linkages, precise internal clearances, and the pump's own intricate design to deliver fuel. Diagnosing issues without the specialist tools and experience for MFI systems, particularly in a country setting like rural Australia, is indeed like working in the dark.

Decoding the Symptoms: What Your W108 is Telling You

Let's break down the symptoms you're experiencing, as they collectively paint a clear picture of an engine drowning in fuel:

• Rough Running and No Idle: This is a classic sign of an excessively rich fuel mixture. The engine struggles to maintain a consistent burn, leading to misfires and an inability to settle into a stable idle speed.
• Copious Black Smoke from the Exhaust: Black smoke is unburnt fuel. When the air-fuel mixture is too rich, there isn't enough oxygen to completely combust all the fuel, resulting in soot particles being expelled through the exhaust. This is a definitive indicator of over-fuelling.
• Sooty Spark Plugs: Pulling the spark plugs and finding them covered in black, powdery soot confirms the rich mixture. Ideal combustion leaves plugs with a light tan or greyish-white deposit. Sooty plugs indicate incomplete combustion due to too much fuel.
• Fuel in the Sump Oil: This is the most critical and potentially damaging symptom. Engine oil is designed to lubricate and protect internal components, not to be diluted by petrol. Fuel dilution significantly reduces the oil's viscosity and lubricating properties, leading to excessive wear on bearings, camshafts, and other critical engine parts. It can rapidly destroy a freshly rebuilt engine if not addressed promptly.

The Alarming Sign: Fuel in the Sump Oil

You've correctly identified fuel in the sump oil as a major concern. The question is, how is it getting there? You've noted no external leaks from the pump to the main oil system, which narrows down the possibilities but doesn't eliminate the pump as the culprit. There are primarily two mechanisms by which fuel can enter the engine oil in this scenario:

Mechanism 1: Internal Injector Pump Leakage (The Prime Suspect)

The Bosch PESAKL pump, like many mechanical fuel injection pumps of its era, is lubricated by engine oil. This means that if internal seals or components within the pump wear, fuel from the high-pressure side of the pump can leak directly into the pump's lubrication gallery, which is directly connected to the engine's oil sump. This is a very common failure mode for these vintage MFI pumps and would explain the fuel in the sump oil without any visible external leaks.

Specifically, the pump contains precision-machined plungers and barrels that pressurise the fuel. Surrounding these plungers are seals designed to prevent fuel leakage into the pump's oil-filled housing. Over decades of operation, these plunger seals and other internal seals (such as those on the drive shaft) can harden, crack, or simply wear out. When this happens, fuel under pressure can bypass these seals and mix with the oil that lubricates the pump's internal mechanisms. Since the pump's oil supply is often integrated with, or drains back into, the engine's sump, this contaminated oil then dilutes your engine's lubricant.

The fact that you've already checked linkages and installed new injectors, yet the problem persists, strongly points towards an issue within the pump itself.

Mechanism 2: Excessive Fuel Wash-Down from Cylinders

While less likely to be the primary cause of *significant* fuel in the oil compared to an internal pump leak, it's still a contributing factor if the engine is severely over-fuelling. When the engine receives far too much fuel, not all of it will combust. The unburnt fuel can then wash down the cylinder walls, past the piston rings, and into the crankcase, where it mixes with the engine oil. This process is exacerbated by rough running and misfires, as combustion is even less efficient.

This mechanism is more about what happens *after* the fuel leaves the injectors, whereas an internal pump leak is about fuel bypassing the intended path *before* it even reaches the injectors effectively, or directly from the pump's pumping elements.

Comprehensive Troubleshooting Steps and Considerations

Given the complexity and the lack of local MFI specialists, here's a detailed approach to further diagnose the problem, building on what you've already tried:

1. Confirm Fuel Dilution

• Smell Test: A strong smell of petrol from the oil dipstick is a clear indicator.
• Visual Check: Rub a small amount of oil between your fingers. If it feels unusually thin or watery, or if you can see a visible sheen of fuel on the surface when left to settle, it's diluted.
• Professional Oil Analysis: If possible, send a sample of your sump oil to a laboratory for analysis. They can accurately determine the percentage of fuel dilution and provide a definitive answer. This is highly recommended for a recently rebuilt engine.

2. Injector Inspection (Beyond Just New Installation)

Even new injectors can be faulty or installed incorrectly. A proper injector test is crucial:

• Drip Test: With the injectors removed from the cylinder head but connected to the fuel lines, carefully observe if they drip fuel when the fuel pump is pressurised (but the engine isn't running). They should hold pressure without dripping.
• Spray Pattern Test: While cranking the engine (or using a specific test rig if available), observe the spray pattern. It should be a fine, conical mist, not a solid stream or erratic dribbles. A poor spray pattern means inefficient atomisation and potentially more fuel washing down.

3. Cold Start Valve (CSV) / Auxiliary Air Valve (AAV)

These components are designed to enrich the mixture and increase idle speed during cold starts. If either is stuck open, it will continuously add extra fuel and air, leading to a rich mixture even when the engine is warm. While less likely to be the direct cause of fuel in the oil, they contribute to over-fuelling:

• CSV: Check its electrical connection and ensure it's not constantly energised. Disconnect it temporarily once the engine warms up to see if it makes a difference.
• AAV: This is a thermostatic valve. Ensure it closes fully as the engine warms up.

4. Ignition System and Timing

While you suspect a fuel issue, poor ignition can exacerbate the problem. If the spark is weak or the timing is off, combustion will be incomplete, leaving unburnt fuel to exit as black smoke or wash down the cylinder walls. Double-check:

• Spark Plugs: Ensure they are the correct heat range and gapped properly.
• Ignition Leads: Check for resistance and integrity.
• Distributor Cap and Rotor: Look for cracks, carbon tracking, or excessive wear.
• Ignition Timing: Confirm the base timing is set precisely according to the workshop manual using a timing light. Incorrect timing can significantly affect combustion efficiency.

5. Fuel System Pressure and Regulator

The MFI system relies on precise fuel pressure. The system pressure regulator (often located near the fuel distributor) maintains the correct pressure. If it's faulty and allowing pressure to be too high, the injectors will deliver more fuel than intended.

• Pressure Test: If you have access to a fuel pressure gauge that can connect to the MFI system, check the system pressure against the manufacturer's specifications.

6. Compression Test

While not a cause of over-fuelling, poor engine compression can lead to incomplete combustion and make the engine run very roughly, mimicking a rich condition. It's a good general health check for a rebuilt engine. Low compression can also make the engine more susceptible to fuel wash-down if over-fuelling occurs.

7. Isolating the Injection Pump as the Source of Fuel in Oil

This is the tricky part without specialist equipment. Since the PESAKL pump is lubricated by engine oil, you can't easily isolate its oil supply to check for fuel. However, the presence of significant fuel in the sump oil, combined with the extreme over-fuelling symptoms, makes an internal pump leak highly probable.

• Observational Test (with caution): If the engine is running, carefully observe the area around the injection pump for any signs of external leakage, however minor. While you've seen none to the main oil system, sometimes a tiny weep can occur at seals.
• Pump Disassembly (Last Resort for DIY): Unless you have specific MFI experience and tools, attempting to disassemble the pump is not recommended. These are incredibly precise instruments that require specialist knowledge for repair and calibration.

The Challenge of Classic MFI and Seeking Expert Help

Your experience highlights the significant challenge of maintaining and repairing classic mechanical fuel injection systems. The MFI pump is not a DIY repair item for most mechanics, let alone enthusiasts, without specialised training, tools, and test benches.

Since local expertise is scarce, consider:

• Specialist MFI Repair Shops: Even if they are interstate or overseas, sending the pump away to a reputable MFI specialist (there are a few world-renowned ones for Bosch MFI) is often the most cost-effective and reliable solution in the long run. They have the necessary test benches to simulate engine conditions, diagnose internal leaks, replace worn plunger seals, and calibrate the pump precisely.
• Online Forums and Communities: While you've already found some valuable insights from 'bostosh' and '124wagon_boy', these forums are also excellent places to ask for recommendations for MFI specialists who might be able to help, even remotely.

Comparative Table: Symptoms vs. Potential Causes

Frequently Asked Questions (FAQs)

Q1: Can a faulty injector pump cause fuel to get into the engine oil?

A1: Absolutely, and it's a very common failure mode for mechanical fuel injection (MFI) pumps like your Bosch PESAKL. These pumps are lubricated by engine oil, and if internal seals (especially plunger seals or drive shaft seals) wear out, fuel under pressure can leak past them and mix directly with the engine oil, leading to fuel dilution in the sump.

Q2: How dangerous is fuel in the sump oil?

A2: It's extremely dangerous for your engine. Fuel significantly reduces the oil's viscosity, meaning it becomes much thinner and loses its ability to properly lubricate critical engine components like bearings, camshafts, and piston rings. This leads to accelerated wear, increased friction, and can quickly result in catastrophic engine failure, especially in a freshly rebuilt engine.

Q3: What's the difference between fuel in oil from an injector pump leak versus wash-down from cylinders?

A3: An internal injector pump leak means fuel is bypassing internal seals within the pump itself and mixing with the oil that lubricates the pump, which then flows into the engine sump. Fuel wash-down, on the other hand, occurs when excessive unburnt fuel in the cylinders passes the piston rings and drips into the sump. Both result in fuel in the oil, but the pump leak is often more direct and can introduce a larger volume of fuel quickly.

Q4: Can I repair the Bosch PESAKL pump myself?

A4: For most individuals, specialist repair of a Bosch PESAKL pump is highly recommended. These pumps are precision mechanical devices that require specific tools, a clean environment, and extensive knowledge for disassembly, inspection, seal replacement, and, crucially, calibration on a test bench. Incorrect reassembly or calibration can lead to further engine damage or continued poor running.

Q5: What should I do immediately if I have fuel in my engine oil?

A5: Stop running the engine immediately. Continued operation with diluted oil will cause severe damage. Drain the contaminated oil, replace the oil filter, and refill with fresh, correct-grade engine oil. This is a temporary measure to prevent further damage while you diagnose and repair the root cause of the fuel leak. Do not run the engine extensively until the source of the fuel in the oil is identified and rectified.

Conclusion

Your Mercedes W108 250 SE's symptoms, particularly the presence of fuel in the sump oil, strongly indicate a serious over-fuelling issue, with an internal leak within the Bosch PESAKL injection pump being the most probable culprit. While your efforts to check linkages and replace injectors are commendable, the complexity of these mechanical fuel injection systems often necessitates expert intervention for internal pump issues.

Continue your systematic diagnosis, focusing on the injector drip and spray patterns, and ensure all ancillary components like the cold start valve are functioning correctly. However, be prepared that the ultimate solution may involve removing the injector pump and sending it to a reputable MFI specialist for thorough testing, repair, and calibration. It’s a significant investment, but it's the most reliable path to getting this beautifully restored vehicle back on the road and running as smoothly as it deserves. The patience and persistence you've shown so far will undoubtedly pay off.

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