Troubleshooting Your Mercedes 190E's Idle Control

Diagnosing Idle and Performance Issues in your Mercedes 190E

Many owners of the classic Mercedes-Benz 190E series, particularly those with the 2.3 and 2.6 litre engines, will at some point encounter a suite of frustrating running problems. These often manifest as difficult starting, particularly from cold, a rough initial idle, a lack of power, and an uneven throttle response. If your beloved 190E is exhibiting these symptoms, it's time to delve into the intricacies of its fuel management system. Two key components that frequently contribute to these issues are the Idle Control Valve (ICV), often referred to as the Auxiliary Air Valve (AAV) on these models, and the Cold Start Injector. Understanding their function and how to test them is crucial for restoring your car's optimal performance.

Understanding the Role of the Idle Control Valve (Auxiliary Air Valve)

The Auxiliary Air Valve (AAV), or Idle Control Valve, is a vital part of the K-Jetronic fuel injection system. Its primary function is to regulate the idle speed of the engine, especially during the warm-up phase. When the engine is cold, the AAV opens a bypass circuit, allowing extra air to enter the intake manifold. This extra air increases the engine's idle speed, preventing stalling and ensuring smoother operation until the engine reaches its normal operating temperature. As the engine warms up, the AAV gradually closes this bypass, returning the idle speed to its normal, lower setting.

Common Symptoms of a Faulty Auxiliary Air Valve

A malfunctioning AAV can lead to a variety of drivability issues:

• Rough Idle (Cold): If the AAV is stuck open or not opening sufficiently, the engine may idle too low when cold, leading to a rough, uneven idle and potential stalling. Conversely, if it's stuck open, you might experience an excessively high idle speed.
• Stalling: A failing AAV that doesn't provide enough extra air when cold can cause the engine to stall shortly after starting.
• Low Idle Speed (Warm): If the AAV doesn't close properly as the engine warms up, it can lead to a persistently low idle speed, which might feel unstable or rough.
• Uneven Throttle Response: While less directly linked than idle issues, a poorly functioning AAV can indirectly affect air-fuel mixture at low RPMs, contributing to hesitation or stumbling.

How to Test the Auxiliary Air Valve

Testing the AAV can be done with a few simple checks:

1. Visual Inspection: Look for any obvious signs of damage, such as cracked rubber hoses or a corroded body.
2. Temperature Sensitivity Test: With the engine cold, start it. The idle speed should be higher than normal (typically around 900-1000 RPM). As the engine warms up, the idle speed should gradually decrease to its normal setting (around 750-850 RPM when fully warm). If the idle speed remains consistently low regardless of engine temperature, the AAV may not be opening correctly. If it stays high, it might be stuck open.
3. Heated Water Test: For a more definitive test, you can remove the AAV from the car. Connect a small hose from the cold start injector line to the AAV's air inlet. Submerge the AAV in a bowl of hot (not boiling) water. You should hear a faint clicking or feel a slight movement as the internal mechanism responds to the heat, effectively closing the air bypass. If there's no discernible change, the valve may be faulty.

The Cold Start Injector: Ensuring Easy Ignition

The Cold Start Injector is another critical component for reliable starting, especially in cooler temperatures. It's essentially an electrically operated fuel injector that provides a burst of extra fuel when the engine is cold. This richer mixture helps to overcome the engine's inherent lean condition when cold, making starting much easier. It's controlled by the Thermo Time Switch, which monitors engine temperature.

What Happens if the Cold Start Injector is Leaking or Defective?

A faulty Cold Start Injector can cause a range of problems, mirroring some of the symptoms described earlier:

• Difficult Cold Starting: If the injector fails to operate, the engine will start lean and may crank for a prolonged period or not start at all.
• Rough Idle (Cold): Even if the engine starts, a non-functioning cold start injector will result in a lean mixture, leading to a rough and unstable idle.
• Leaking Cold Start Injector: This is a common issue. If the injector leaks fuel even when it's not supposed to, it can over-fuel the engine. This can lead to hard starting when warm, rough idling, poor performance, and increased fuel consumption. A leaking injector essentially acts like an extra fuel source, disrupting the carefully controlled air-fuel ratio.
• Hot Starting Issues: A leaking cold start injector can contribute to difficulty starting when the engine is warm, as the excess fuel can flood the cylinders.

How to Test the Cold Start Injector

Testing this injector involves checking both its ability to spray and its tendency to leak:

1. Injector Spray Test: Disconnect the fuel line to the cold start injector. Place the injector's outlet into a suitable container. Turn the ignition key to the 'on' position (without starting the engine) when the engine is cold. The cold start injector should spray a fine mist of fuel for a few seconds. If it dribbles, sprays erratically, or doesn't spray at all, it's likely faulty.
2. Leak Test: With the engine cold, remove the cold start injector from the intake manifold. Reconnect the fuel line. Turn the ignition key to the 'on' position. Inspect the tip of the injector. It should not drip or leak any fuel. If it does, the injector is leaking and needs to be replaced.
3. Thermo Time Switch Check: Since the cold start injector relies on the Thermo Time Switch, it's essential to ensure the switch is functioning. A defective Thermo Time Switch will prevent the cold start injector from activating, even if the injector itself is fine. Testing the Thermo Time Switch usually involves checking its resistance at different temperatures or checking for continuity when the engine is cold and the ignition is on. This often requires a multimeter and specific resistance values from a service manual.

Other Potential Culprits and Troubleshooting Steps

While the AAV and Cold Start Injector are common causes, other factors can contribute to similar symptoms. It's important to consider these as well:

Fuel Pressure and Delivery

A lack of fuel pressure or insufficient fuel volume can lead to poor performance and starting issues. This can be caused by a weak fuel pump, a clogged fuel filter, or a faulty fuel pressure regulator.

Airflow Sensor Plate

The airflow sensor plate is a crucial component that measures the amount of air entering the engine. If this plate is sticking, binding, or its adjustment is incorrect, it can significantly disrupt the air-fuel mixture, leading to hesitation, poor power, and rough idling.

Table: Common Symptoms and Potential Causes

Vacuum Leaks

Anywhere that unmetered air can enter the intake system after the airflow sensor can cause a lean condition, leading to rough idling, hesitation, and poor performance. Common culprits include cracked vacuum hoses, a faulty brake booster, or a leaking intake manifold gasket.

Ignition System

A weak or intermittent spark can also mimic fuel delivery problems. Ensure your spark plugs, distributor cap, rotor, and ignition leads are in good condition and properly gapped.

OVP Relay (Overvoltage Protection Relay)

While less common for the specific symptoms described, a faulty OVP relay can affect various electronic components in Mercedes-Benz vehicles of this era, potentially leading to erratic behaviour. However, you typically wouldn't expect unlit dash warning lights if the OVP was the primary issue, unless it's a very specific failure mode. Testing the OVP relay usually involves checking for continuity and voltage across its pins, which can be a bit more involved and often requires specific knowledge of its internal workings.

Conclusion

Addressing starting and idling issues in your Mercedes 190E often involves a systematic approach. By focusing on the Auxiliary Air Valve and the Cold Start Injector, and by performing the outlined tests, you can often pinpoint the source of the problem. Remember to also consider other vital systems like fuel pressure, the airflow sensor, and the ignition system. With careful diagnosis and the right parts, you can restore your 190E to its former smooth-running glory.

Frequently Asked Questions

Q1: My 190E starts fine when warm but is difficult to start when cold. What could be the issue?

A1: This strongly suggests a problem with the cold start system. Check the Cold Start Injector for proper operation and leaks, and ensure the Thermo Time Switch is functioning correctly. The Auxiliary Air Valve could also be a factor if it's not allowing enough extra air for a cold idle.

Q2: My car hesitates when I accelerate at low RPMs. What should I check?

A2: This could be due to a sticking or improperly adjusted airflow sensor, a binding control plunger in the fuel distributor, or potentially a leaking Cold Start Injector causing an overly rich mixture at low speeds. Also, check for vacuum leaks.

Q3: How do I know if my Cold Start Injector is leaking?

A3: The most common test is to remove the injector, reconnect the fuel line, and turn the ignition to 'on'. If the injector tip drips or leaks fuel, it's faulty and needs replacement.

Q4: My idle speed is too low, even when the engine is cold. What's wrong?

A4: This usually indicates that the Auxiliary Air Valve (AAV) is not opening or is restricted. The AAV is designed to increase idle speed when cold by letting in extra air. If it's not doing its job, the idle will be too low and potentially rough.

Q5: I've replaced the fuel filter, but the problems persist. What else could it be?

A5: A fuel filter is just one part of the fuel system. You should also check fuel pump operation, fuel pressure, the condition of the fuel lines, and the functionality of the injectors and fuel distributor. Don't forget to consider the airflow sensor and the idle control components.

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