01/06/2017
For owners of classic Mercedes-Benz models, particularly the early R129 500SL with its robust M119.960 engine and the sophisticated KE-Jetronic Continuous Injection System (CIS), understanding the nuances of its fuel management is key to optimal performance. A common point of confusion, and indeed a specific query for many, revolves around the absence of a lambda sensor on these earlier models, especially when troubleshooting issues like high hydrocarbon emissions. This article delves into the intricacies of your 1990 R129 500SL's fuel system, explaining how it manages its fuel mixture without a closed-loop feedback system, addressing common problems like elevated hydrocarbons, and providing a comprehensive guide to servicing and replacing your fuel injectors.
Understanding Your R129's KE-Jetronic System
You're right to note the absence of a lambda sensor on your 1990 R129 500SL, as indicated by the Mercedes label under the bonnet and the lack of catalysts. This is characteristic of earlier KE-Jetronic systems, which primarily operate in an open-loop configuration for mixture control. Unlike modern electronic fuel injection systems that use a lambda (oxygen) sensor to continuously monitor exhaust gases and make real-time, closed-loop adjustments to the fuel-air mixture, your vehicle's system relies on a different principle.
How the EHA Valve Manages Mixture Without Lambda Feedback
The Electronic Hydraulic Actuator (EHA) valve, that black rectangular box on the side of your fuel distributor, is indeed central to minute mixture adjustments. However, without a lambda sensor, it doesn't receive direct feedback from the exhaust gas composition. Instead, the EHA valve is controlled by the Engine Control Unit (ECU) based on a pre-programmed 'map' or set of parameters. The ECU gathers information from various sensors around the engine to estimate the optimal fuel-air mixture. These sensors include:
- Engine Coolant Temperature (ECT) sensor: Informs the ECU about the engine's operating temperature, crucial for cold starting and warm-up enrichment.
- Air Flow Sensor Plate (within the air flow metre): Measures the volume of air entering the engine, directly influencing the base fuel delivery.
- Throttle Position Sensor (TPS): Indicates engine load and driver demand.
- Engine Speed Sensor (RPM): Provides vital information about engine operation.
- Intake Air Temperature Sensor: Adjusts for air density variations.
The ECU processes these inputs and sends a precise current to the EHA valve, which then subtly alters the fuel pressure differential across the fuel distributor's metering slits. This adjustment fine-tunes the amount of fuel delivered to the injectors. It's an intricate dance, but one performed without the real-time exhaust gas analysis that a lambda sensor provides. The system relies on its initial calibration and the accurate signals from its various sensors to maintain what it believes is the correct mixture for prevailing conditions.
Diagnosing High Hydrocarbons and Rough Running
Your observation of high hydrocarbons (HC) and difficulty getting CO below 3% without rough running is a classic symptom of an overly rich fuel mixture or incomplete combustion. Given your extensive list of replaced components – spark plugs, HT leads, distributor caps, rotor arms, vacuum pipes, and new injectors – you've addressed many common ignition and air leak issues. Your car's 106,000 miles is certainly a respectable figure, and at this mileage, various components can begin to show wear.
Potential Causes and Further Checks for High HC
Since you've already tackled many ignition and basic fuel system components, let's explore other avenues critical to the KE-Jetronic system:
1. Fuel Pressure Regulation
You're right to suspect fuel pressure. In a KE-Jetronic system, both system pressure and differential pressure are paramount. High hydrocarbons can easily result from incorrect fuel pressures leading to an overly rich mixture. You'll need a specialised fuel pressure gauge kit for CIS systems. Check:
- System Pressure: This is the main pressure supplied by the fuel pump. If too high, it will force too much fuel through the system.
- Differential Pressure: This is the pressure difference across the fuel distributor. The EHA valve controls this, and if it's out of specification, the fuel delivery to the injectors will be incorrect. A faulty EHA valve itself can cause this, even if it's receiving the correct signal from the ECU.
2. Ignition Coils
Your plan to replace the 25-year-old ignition coils is sound. Even if they haven't completely failed, old coils can weaken, leading to a less powerful spark. A weak spark can result in incomplete combustion, leaving unburnt fuel (hydrocarbons) in the exhaust. This is particularly noticeable under load or at higher RPMs.
3. Fuel Distributor Diaphragm
Your suspicion about the diaphragm in the fuel distributor is very pertinent. This is a common failure point on older CIS systems. The diaphragm separates the upper and lower chambers of the fuel distributor. If it develops a tear or becomes stiff, it can cause erratic fuel distribution, leading to a rich mixture, poor idling, and increased emissions. A leaking diaphragm can also allow fuel to enter the vacuum system, which is a significant problem. While you've replaced injectors, the distributor itself is the heart of the fuel metering process.
4. Air Flow Meter (AFM) Potentiometer
Inside the air flow metre, there's a potentiometer that senses the position of the air flow sensor plate. Over time, the carbon track inside this potentiometer can wear, creating 'dead spots' or sending incorrect signals to the ECU. The ECU then receives inaccurate information about the incoming air volume, leading it to miscalculate the fuel delivery via the EHA, often resulting in a rich condition.
5. Temperature Sensors
The Engine Coolant Temperature (ECT) sensor is crucial for mixture enrichment, especially during cold starts and warm-up. If this sensor reads incorrectly (e.g., always indicating a cold engine), the ECU will continuously command a rich mixture, leading to high CO and HC, even when the engine is warm.
6. Vacuum Leaks (Beyond Hoses)
While you've replaced vacuum pipes and rubber hoses to the injectors, ensure there are no other subtle vacuum leaks. Even a small leak after the air flow metre can introduce unmetered air, causing the system to run rich as the ECU tries to compensate for what it perceives as too little air.
It's important to approach these checks systematically. Starting with fuel pressures (system and differential) would be a logical next step, followed by the coils and then a closer inspection of the fuel distributor and AFM potentiometer.
Fuel Injector Removal and Servicing
The fuel injectors on your R129 500SL are responsible for spraying the fuel into the intake manifold. Even though you've replaced them, understanding their removal process is vital for future maintenance or troubleshooting. If injectors become clogged, they may not put out the correct amount of fuel, causing rough running, or if their injector seals fail, you might experience fuel smells and leaks. In 1994, Mercedes-Benz switched from CIS to electronic port fuel injection, where an engine control unit electronically grounds individual injectors for precise fuel delivery. While your system is different, the principle of clean, evenly flowing injectors remains crucial.
Project Details: Removing and Rebuilding the Fuel Rail/Injectors
Project Time: Expect this project to take approximately 3 hours, factoring in careful work and proper depressurisation.
Estimated Cost: The labour cost for a professional would be around $25 (or £25), but doing it yourself saves on talent charges.
Tools Required:
- 10mm socket with ratchet and extension
- 17mm wrench
- 15mm wrench
- Small pry bar
- Pick tool
Parts Required:
- New injector seals (always replace these!)
- Crankcase breather hoses (if replacing as a complementary measure)
Performance Gain: Clean and evenly flowing fuel injectors will restore optimal fuel atomisation, leading to smoother engine operation, potentially improved fuel economy, and reduced emissions.
Step-by-Step Removal Process
Before you begin, ensure the engine is cool to the touch.
Step 1: Depressurise the Fuel System
This is a critical safety step to prevent fuel spray and fire hazards. Locate your fuel pump fuse (refer to your owner's manual or fuse diagram – it's typically in the main fuse box under the bonnet or in the boot). With the engine running, carefully pull the fuel pump fuse. Allow the engine to run until it completely stalls. This will relieve the pressure in the fuel rail and lines. Once stalled, turn the ignition off and disconnect the battery's negative terminal for added safety.
Step 2: Remove Engine Cover / Air Filter Housing
Access to the fuel rail and injectors requires removing the engine cover, which often incorporates the air filter housing on these models. This usually involves unclipping or unscrewing retaining clips and removing any associated intake ducting. Refer to a specific tech article or workshop manual for your R129 for precise instructions on engine cover removal, as variations can exist.
Step 3: Disconnect Fuel Lines and Vacuum Hoses
- Identify the fuel lines connected to the fuel distributor and the fuel rail. Use your 17mm and 15mm wrenches to carefully loosen and disconnect these lines. Be prepared for a small amount of residual fuel to leak out – have rags ready.
- Disconnect any vacuum lines or electrical connectors attached to the fuel distributor or components near the injectors. Label them if necessary to aid reassembly.
Step 4: Remove the Fuel Rail Retaining Bolts
The fuel rail, which holds the injectors in place, is typically secured by several bolts. Use your 10mm socket and ratchet to carefully unbolt the fuel rail from the intake manifold. Keep track of all bolts and washers.
Step 5: Carefully Pry Out the Fuel Rail and Injectors
Once the bolts are removed, the fuel rail and injectors may still be seated firmly due to the injector seals. Use your small pry bar carefully to gently lift the fuel rail assembly away from the intake manifold. Work slowly and evenly to avoid bending the fuel rail or damaging the injector bores. The injectors will likely come out with the rail.
Step 6: Remove Injectors from Fuel Rail and Replace Seals
Once the fuel rail assembly is out, you can carefully pull the individual injectors from the rail. Note the orientation of each injector. The old injector seals (O-rings) will be on both ends of the injector – where it seats into the rail and where it seats into the manifold. Use your pick tool to carefully remove the old, hardened seals. Clean the injector bodies thoroughly. Install new injector seals, ensuring they are correctly seated and lightly lubricated with a drop of clean engine oil or petroleum jelly to aid reassembly and prevent tearing.
Step 7: Reassembly
Reassembly is the reverse of removal. Ensure new seals are used on all injectors. Carefully push the injectors back into the fuel rail. Guide the entire fuel rail assembly back into position, ensuring each injector seats correctly into its bore in the intake manifold. Install and hand-tighten the retaining bolts, then torque them to the manufacturer's specifications. Reconnect all fuel lines, vacuum hoses, and electrical connectors. Finally, reinstall the engine cover/air filter housing. Reconnect the battery and replace the fuel pump fuse. Before starting the engine, cycle the ignition key a few times (without cranking) to allow the fuel pump to re-pressurise the system. Check for any leaks before starting and test driving.
Comparative Fuel System Overview
To further clarify the context of your 1990 R129, here's a brief comparison of the fuel systems mentioned:
| Feature | Early KE-Jetronic (e.g., 1990 R129 M119.960) | Later Electronic Port Injection (e.g., 1994+ M119) |
|---|---|---|
| Fuel Delivery Method | Continuous Injection System (CIS) - mechanical/hydraulic with electronic control (EHA) | Electronic Port Fuel Injection (EFI) - fully electronic, individual injector control |
| Mixture Control Principle | Open-loop (ECU relies on pre-programmed map and sensor inputs) | Closed-loop (ECU uses lambda sensor feedback for real-time adjustments) |
| Lambda (Oxygen) Sensor | Typically not fitted, or if fitted, for very basic feedback on some variants, but not primary mixture control | Standard equipment for precise emissions control |
| Catalytic Converter | Often not fitted (or basic non-regulated type) | Standard equipment (three-way catalytic converter for emissions reduction) |
| Common Issues | Fuel distributor diaphragm, EHA valve, air flow metre potentiometer, fuel pressure regulators, vacuum leaks | Clogged injectors, faulty sensors, ECU issues, wiring problems, O2 sensor failures |
Frequently Asked Questions (FAQs)
What is KE-Jetronic Injection?
KE-Jetronic is a type of mechanical fuel injection system developed by Bosch. It's an evolution of K-Jetronic, adding electronic control (the 'E' for electronic) primarily via the EHA valve to fine-tune the fuel mixture based on various engine sensors. Unlike modern systems, it continuously sprays fuel into the intake ports rather than pulsing it.
Why does my 1990 500SL not have a lambda sensor?
In 1990, emissions regulations in many markets were less stringent than today. Mercedes-Benz, like many manufacturers, designed their KE-Jetronic systems to meet these regulations without the need for a closed-loop lambda sensor feedback. These systems relied on precise mechanical and electronic calibration, along with accurate sensor inputs, to maintain the correct mixture within acceptable limits for the time.
Can I convert my KE-Jetronic system to use a lambda sensor?
While technically possible, converting a KE-Jetronic system to a fully closed-loop electronic fuel injection system with a lambda sensor is an extremely complex and costly undertaking. It would involve replacing virtually the entire fuel management system, including the fuel distributor, injectors, ECU, and potentially modifying the exhaust. For most owners, maintaining the original system as intended is the most practical and cost-effective approach.
What are the primary causes of high hydrocarbons in a KE-Jetronic engine?
High hydrocarbons (unburnt fuel) in a KE-Jetronic engine primarily point to an overly rich fuel mixture or incomplete combustion. Common causes include incorrect fuel pressures (system or differential), a faulty EHA valve, a worn air flow metre potentiometer, incorrect signals from temperature sensors, ignition system weaknesses (weak coils, faulty plugs/leads), or internal leaks within the fuel distributor (e.g., diaphragm).
How do I know if my fuel injectors are faulty or leaking?
Symptoms of faulty or leaking injectors include a strong smell of petrol under the bonnet, visible fuel leaks around the injector seats, rough idling, misfires, reduced fuel economy, and potentially high CO or HC emissions. A common diagnostic is to perform a fuel pressure leak-down test or visually inspect the injectors once removed for spray pattern issues or drips.
Conclusion
Your 1990 R129 500SL, with its KE-Jetronic system, is a testament to Mercedes-Benz engineering of its era. While it may lack the lambda sensor feedback of more modern vehicles, its sophisticated open-loop system is capable of delivering excellent performance when properly maintained. Tackling your high hydrocarbon issue requires a methodical approach, starting with thorough fuel pressure diagnostics and a close inspection of the fuel distributor and AFM potentiometer, in addition to your planned coil replacement. By understanding how your system operates and systematically troubleshooting potential culprits, you can restore your classic SL to its smooth-running glory and enjoy many more miles on the road.
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