Experiencing starting problems and rough idling with your classic 1991 Mercedes 300e can be frustrating. This article delves into a real-world scenario faced by a 1991 Mercedes 300E owner and explores potential causes and troubleshooting steps for these common issues in the W124 model.
The Case of the Hard-Starting 1991 Mercedes 300E
Donna, the owner of a 1991 Mercedes 300E with the 3.0-liter engine and approximately 79,000 miles, reached out for advice regarding intermittent starting and idling problems. Her meticulously maintained vehicle, primarily used for short local trips a few times a week, began exhibiting issues that evolved over time. Initially, the car would start without a hitch in the morning but struggled to restart after short drives. This progressed to hard starting even on the first attempt of the day, accompanied by a rough idle and occasional stalling.
Initial Symptoms: Intermittent Hard Starting After Short Trips
The problem first manifested as difficulty restarting the 300E after brief trips. Donna described that after driving less than three miles and turning off the engine, subsequent restart attempts became challenging. The engine would turn over but would be reluctant to fire up, requiring multiple tries and flooring the accelerator pedal. Once started, the car would run normally for the remainder of the day. At this stage, there was no noticeable fuel smell, and the issue was intermittent. A local repair shop suggested vapor lock as a possible cause and advised keeping the fuel tank full. Spark plugs were replaced as a preventative measure, but the problem persisted.
Evolving Symptoms: Hard Starting on First Start and Rough Idle
The starting problem then shifted its pattern. Instead of occurring after short trips, the 1991 Mercedes 300E began experiencing hard starts right from the first attempt of the day. This was now coupled with a rough idle characterized by shaking and vibrations in the front of the car. The rough idle would typically subside after a few minutes, and the engine would return to its normal smooth operation. Stalling also became more frequent, sometimes occurring multiple times in succession after the initial start. Despite these challenges, Donna was always eventually able to get the car running.
The Appearance of ASR and ABS Lights
Adding to the complexity, the ASR (Acceleration Skid Control) light illuminated on the dashboard. Initially, this was accompanied by a perceived reduction in engine power, as indicated in the car’s owner’s manual. However, surprisingly, the next day, while the ASR light remained on, the engine power returned to normal. Shortly after, the battery completely died, necessitating a replacement. Following the battery replacement, the ABS (Anti-lock Braking System) light also illuminated, alongside the persistent ASR light. The repair shop attributed both warning lights to the battery failure and suggested that the fault codes could be reset, although they lacked the specific Mercedes-Benz diagnostic equipment to do so.
Fuel System Suspicions
A different repair shop, after hearing Donna’s description of the symptoms – hard starting, rough idling that clears up – suggested a potential issue within the fuel system. They speculated about a possible fuel pump or related component malfunction. They dismissed fuel injector problems as unlikely in Mercedes-Benz vehicles. A fuel filter replacement was performed, but it did not resolve the underlying issues. The shop reiterated the advice to drive the car until it definitively breaks down, unless Donna was willing to invest in potentially costly and speculative parts replacements.
Past Throttle Actuator Issue
In 1997, Donna had experienced a stalling issue with the car, which was resolved by the Mercedes-Benz dealer replacing the throttle actuator under warranty. She later discovered that this throttle actuator issue was subject to a recall, of which she was never informed. While acknowledging that the current symptoms were different, Donna raised the possibility of a connection to the previous throttle actuator problem, wondering if the replacement part might have also failed or if the original repair was incomplete.
Potential Diagnostic Areas for a 1991 Mercedes 300E with Starting and Idling Problems
Based on the described symptoms of this 1991 Mercedes 300E, several areas warrant investigation:
- Fuel Delivery System: The repair shop’s suspicion of a fuel system issue is reasonable given the hard starting and rough idle symptoms. Potential culprits could include:
- Fuel Pump: A failing fuel pump may not be consistently delivering adequate fuel pressure, especially during startup or idle. Fuel pressure testing is crucial.
- Fuel Pressure Regulator: A faulty regulator could cause inconsistent fuel pressure.
- Fuel Filter: While recently replaced, it’s worth ensuring the correct filter was used and properly installed.
- Fuel Injectors: Although deemed less likely, partially clogged injectors could contribute to rough idle and hard starting. Injector testing and cleaning might be considered if other fuel system components check out.
- Ignition System: While spark plugs were replaced, other ignition components could be at fault:
- Distributor Cap and Rotor: Cracks or corrosion can lead to misfires, especially during cold starts or humid conditions.
- Ignition Wires: Deteriorated wires can cause weak spark.
- Ignition Coil: A failing coil can result in intermittent spark issues.
- Engine Control System:
- Crankshaft Position Sensor (CKP): This sensor is crucial for ignition and fuel injection timing. A failing CKP sensor is a common cause of hard starting and stalling.
- Mass Air Flow (MAF) Sensor: A faulty MAF sensor can provide incorrect air intake readings to the engine control unit (ECU), leading to fuel mixture problems and rough idle.
- Engine Coolant Temperature Sensor (ECT): An inaccurate ECT sensor reading can affect cold start enrichment and idle control.
- Idle Air Control Valve (IACV): A malfunctioning IACV can cause rough idle, especially during cold starts.
- Vacuum Leaks: Vacuum leaks can disrupt the air-fuel mixture and lead to rough idle and starting issues. Inspect all vacuum hoses for cracks or disconnections.
- Throttle Actuator: Given the history of throttle actuator problems, and depending on the type of replacement installed previously, it’s worth re-examining this component, although symptoms might be slightly different from the previous issue.
Addressing ASR and ABS Lights
The appearance of the ASR and ABS lights, particularly after a battery failure, is not uncommon in older Mercedes-Benz models. While sometimes related to low voltage or battery issues, they can also indicate problems within the respective systems. A proper diagnostic scan using Mercedes-Benz compatible diagnostic equipment is necessary to retrieve fault codes and pinpoint the cause of these warning lights. It’s unlikely that these lights are directly related to the hard starting and rough idle, but they should be addressed separately for safety and proper vehicle operation.
Conclusion: Seeking Professional Mercedes-Benz Diagnosis
Diagnosing intermittent starting and idling problems in a 1991 Mercedes 300E can be complex and requires a systematic approach. While some basic troubleshooting steps can be taken, the most effective course of action is to seek professional diagnosis from a qualified mechanic specializing in Mercedes-Benz vehicles or a reputable Mercedes-Benz dealership. They possess the expertise, diagnostic equipment, and knowledge of common issues in the W124 platform to accurately identify the root cause of the problem and recommend appropriate repairs. While cost is a concern, investing in a proper diagnosis can ultimately save money and prevent further frustration by addressing the issue correctly the first time.
