Many owners of the classic 88 420 Sel Mercedes may encounter frustrating issues such as rough idling, surging, and hesitation. These problems can significantly detract from the smooth and luxurious driving experience expected from this prestigious vehicle. While diagnosing these issues can seem complex, often the culprits lie within the fuel distribution system and the idle control valve (ICV). This guide, based on real-world experience, will walk you through a potential DIY fix focusing on the often-overlooked adjustability of the ICV.
While a faulty fuel distributor is frequently suspected – and indeed, can be a source of these problems – it’s a costly component to replace. Before considering such an expensive fix, it’s wise to thoroughly investigate other potential causes. One area that deserves closer attention is the idle control valve. Many mechanics and owners have replaced the ICV, hoping to resolve idle issues, only to sometimes find the problem persists or even mysteriously returns with a gradually increasing idle speed.
Initially, when faced with a malfunctioning ICV, the natural inclination is to replace it. However, after bench-testing both new and old valves, no significant physical differences in plunger movement are often observed. This leads to a crucial realization: the issue might not be a drastic failure of the valve itself, but rather a subtle calibration problem. It appears that a “bad” ICV might simply lack the necessary “authority” to restrict airflow sufficiently to achieve the correct idle speed under all conditions. The valve might function, but not with the precision needed for optimal idle control, especially when the control module’s current output is within its normal range.
The breakthrough comes with the discovery that these ICVs are, in fact, adjustable. Hidden beneath a small cover on the plunger is a tiny threaded brass insert. This insert is connected to a spring that exerts pressure on the valve actuator electromagnet. By adjusting this spring pressure, you can effectively fine-tune the ICV’s responsiveness.
If your 88 420 SEL Mercedes idles too high – a common symptom – the solution might be as simple as reducing the spring pressure on the ICV. Here’s how you can attempt this adjustment:
- Locate the ICV: Identify the Idle Control Valve in your 88 420 SEL Mercedes engine bay. (Refer to your vehicle’s service manual if needed).
- Expose the Adjustment: Carefully pop off the small cover located in the center of the ICV plunger. This will reveal the small threaded brass insert.
- Find a Suitable Tool: You’ll need a screw that can engage with the brass insert. A 10-32 screw might work, or find a screw that properly fits the threads to gently “get hold” of the brass piece.
- Make Minor Adjustments: If your idle is too high, gently pull the center piece (the brass insert) out slightly. Small adjustments are key here as the optimal setting is quite sensitive.
- Test and Readjust: Start your engine and observe the idle. If needed, repeat the adjustment process. If the idle becomes too low, gently push the brass piece back in.
This ICV adjustment can potentially resolve idle and surging problems in your 88 420 SEL Mercedes without the need for expensive parts replacements. Remember to proceed with caution and make very small adjustments. By understanding the subtle mechanics of the ICV, you can potentially restore the smooth idle and responsive performance of your classic Mercedes. Give this DIY fix a try and see if it works for your 88 420 SEL!
