Before you start adjusting screws and dismantling parts on your Mercedes 450SL to fix a high idle, take a moment to systematically diagnose the issue. It’s crucial to begin with the basics, and often, the solution is simpler than you might expect. Let’s walk through some essential checks to get your 450SL idling smoothly again.
First, inspect your air supply hoses meticulously. Focus on the larger hoses leading to the auxiliary air valve (AAV) as these are primary air intakes. However, don’t stop there. Underline every vacuum hose under the hood and examine them carefully. In a classic Mercedes 450SL, these hoses are often aged and prone to cracking or becoming loose. Each hose should be pliable and fit snugly on its fittings. Interestingly, while some early D-Jetronic systems used clamps on the main air hoses for a secure fit, Mercedes later opted to remove them, potentially as a cost-saving measure. It’s a good practice to replace any hoses that appear suspicious and install clamps to ensure a consistently tight seal, eliminating obvious and easily rectified sources of vacuum leaks that can plague a 450SL.
To further investigate potential vacuum leaks in your 450sl Mercedes engine, use a vacuum gauge. Locate a manifold vacuum source; the area near the firewall at the back of the engine is usually accessible. Here, you’ll find lines extending from the manifold base to various accessories. Find a suitable point within this cluster to tap into for your vacuum gauge reading. A normal vacuum reading for an older engine like the one in a 450SL might range from 14-18 inches of mercury (in. Hg). Lower readings indicate potential problems. A reading in the low teens (12-13 in. Hg) or even single digits suggests a significant vacuum leak. As suggested earlier, manifold seal leaks are a likely culprit in such cases, and these can impact various engine functions in your 450SL. If your gauge shows a reading in the mid-teens or higher, the manifold gasket is likely in good condition.
Next, try adjusting the large brass idle screw. If turning it all the way in (bottoming it out) doesn’t reduce the engine speed of your 450SL, it’s a strong indicator that excess air is entering the system from an unintended location – a vacuum leak. A common cause of high idle in Mercedes 450SL models is a malfunctioning Auxiliary Air Valve (AAV). The AAV is designed to allow extra air into the engine during cold starts to raise the idle speed, and then gradually close as the engine warms up. If your 450SL’s AAV is stuck open, it will continuously feed extra air, resulting in a persistently high idle. Resources like the Jaguar AAV tutorial (while not Mercedes specific, the principle of AAV operation is similar) can provide valuable insight into how these valves work and how to diagnose them. Replacement AAVs can be costly, around $140, but thankfully, they are relatively straightforward to replace. In some instances, cleaning the AAV might be sufficient to restore its functionality and resolve your 450SL high idle issue.
A crucial point to remember when troubleshooting idle problems on your Mercedes 450SL is to avoid adjusting the throttle body throttle plate screw unless you are absolutely certain it’s the issue and understand the correct procedure. This screw is not intended for idle speed adjustment and messing with it can lead to further complications. If you suspect you’ve inadvertently adjusted the throttle body screw, it’s best to seek guidance from a Mercedes-Benz repair specialist or consult detailed repair manuals specific to the 450SL and its D-Jetronic fuel injection system to ensure it’s correctly reset. Focus on systematically checking vacuum hoses and the AAV first, as these are the most common culprits for high idle in the Mercedes 450SL.
