Diagnosing engine issues in a classic Mercedes-Benz, such as the 92 Mercedes Benz 500sl, can be a complex task. Owners sometimes encounter perplexing problems that require systematic troubleshooting to resolve. This article delves into a real-world scenario of engine hesitation and misfire in a 1992 Mercedes 500SL, outlining the steps taken to diagnose and address the issue.
The owner reported a recurring problem that manifested primarily during cold starts and after the engine reached operating temperature, particularly after “hot soak” periods. Initially, upon a cold start, the engine would crank for a couple of seconds, followed by a brief stumble and misfire before settling into a stable 900rpm idle.
However, the issues became more pronounced under specific conditions:
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Warm Ambient Temperature & Short Hot Soak: When the ambient temperature was around 80 degrees Fahrenheit and the car was driven for 20-30 minutes, followed by a 10-15 minute rest (hot soak), restarting the engine would result in a momentary stumble and misfire, succeeded by a more prolonged hesitation and misfire lasting 10-20 seconds before the engine ran smoothly.
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Warm Ambient Temperature & Longer Hot Soak: With the same ambient temperature but after a 45-minute drive and a subsequent 45-minute rest, the symptoms worsened. The engine experienced a 3-second stumble and misfire at startup, followed by a significant 1-2 minutes of hesitation and misfire. Interestingly, after this prolonged issue, the engine would run correctly for about 7-8 minutes before the hesitation returned, only to disappear again after another 8-10 minutes.
These intermittent and condition-dependent symptoms led to a frustrating troubleshooting journey. The owner, drawing upon community resources and online forums, compiled a list of potential culprits:
- Over Voltage Protection Relay (OVP): While the fuse on the OVP relay was intact, its unmounted state raised concerns about potential damage.
- Crank Position Sensor: A common suspect in ignition and fuel delivery issues.
- Fuel Accumulator: Crucial for maintaining fuel pressure, especially during hot starts.
- Engine Temperature Sensor: Faulty readings could disrupt fuel mixture and engine management.
Seeking professional assistance, the owner initially consulted an independent mechanic. Extensive work was performed, including replacing spark plugs, distributor caps and rotors, ignition wires, both ignition coils, and all eight fuel injectors. Despite these replacements, the core problem persisted. Notably, the mechanic, described as excellent and reputable, did not charge for the labor and returned the replaced original parts, recommending a Mercedes-Benz specialist.
Moving forward, the owner took the 92 Mercedes Benz 500SL to a different independent mechanic specializing in Mercedes-Benz vehicles. By this stage, the symptoms included a cold start rumble, occasional misfires after warm-up, hesitation during idle in drive, and hesitation upon acceleration. An additional observation was a drop in the oil pressure gauge below one bar and an unstable “hunting” idle when in park or neutral.
After a week of diagnostics, the second mechanic identified the fuel distributor as the likely source of the problems. Replacing the fuel distributor seemed to initially resolve the issues. The engine idled smoothly, without misfires or hesitation. However, after the car sat for a short period following the repair, the hesitation recurred. Further attempts to resolve the issue involved replacing the fuel accumulator with a used spare, which worsened the problem. Reinstalling the original fuel accumulator did not improve the situation.
Ultimately, the mechanic concluded that replacing the fuel distributor and fuel filter, combined with adjusting the air/fuel ratio, was the most effective solution. As of the latest update, after these interventions, the 92 Mercedes Benz 500SL showed significant improvement. The cold start rumble persisted for a brief 2-3 seconds, but idle stability was restored both during cold starts and after warm-up. Hesitation was no longer present, and performance felt strong. However, the oil pressure gauge still exhibited a drop below one bar when hot, suggesting a potential separate issue to monitor.
This case highlights the intricacies of diagnosing engine performance problems in older vehicles like the 92 Mercedes Benz 500SL. A systematic approach, combined with expert knowledge, is often necessary to pinpoint and effectively resolve these challenging issues.
