Engine Idle Fixed

The idle speed was getting steadily worse as time passed. A few weeks ago I put the Merc away for a while because the high idle was becoming intolerable. I would be idling at 1600 rpms while hot and in neutral. Also, while underway, the car would speed up to about 60kph with my foot off the throttle.

I was able to switch IAC valves with the buddy with another 560 SEC. This brought the idle speed down a couple hundred rpms. We knew his was probably bad as well, but it was certainly in better condition than mine. His car is the ECE version, with a different fuel injection system, so minute differences in how the engine controls idle probably resulted in a measurable difference in idle speeds despite having the same basic engine and same IAC valve. Mine idled a bit higher than his with the same valve.

Anyway, it was clear that despite getting a clean bill of health from my mechanic last winter, my IAC valve was in dire need of replacement.

It's a relatively expensive part. I didn't even dare ask how much it costs at Finnish retailers. I know from reading forums that it's in the hundreds. So, we organized a group order from Autohaus AZ with a couple 560 SEC owners. They stocked both the VDO (original) valve and related rubber hoses at a fair price ($185 for the valve and about $10 for the hoses). Autohaus AZ has a great stock of OEM M-B parts even for rare models, and very reasonable prices.

After about a 3 week pause, I'm driving the Merc again. I have to say I was beginning to miss it pretty bad. For the past weeks I've been driving my dad's 1969 Renault.

First impressions with the old friend 560: damn it's big, damn it's comfortable, damn it's quiet, damn it feels heavy, damn it has a great soundtrack.

Here's where the IAC valve goes. Removing it is really easy, literally a 5 minute job. You only have to take off the air filter casing. After that, and removing the electric connector, the IAC valve lifts off the metal and plastic pipes you're seeing here. Originally there's a rubber fastener around the valve, as seen in the next picture, but mine was broken and didn't really do anything, so I just left it off. Better take the right side rubber hose off first, while dodging the vacuum connectors on the left. They are fragile and cause problems if they break. After this the IAC valve wiggles off the downward facing pipe.

Here's my old IAC valve. You can feed it 12 volts, polarity does not matter. If I understand correctly the valve should close completely with 12 volts. In the picture it's powered up but has only closed the amount you can see there. I think this was responsible for the high idle speed.

Here's the new valve installed. As you can see it's really polished and looks like a quality part, as it should, since it's a VDO (original part). The new rubber hoses were pretty soft and had a really snug fit, totally unlike the old hoses. I was probably sucking in lots of extra air through the rubber hose fitting imperfections with the old parts. That probably contributed to the high idle issue.

All said and done! Once I fired it up it immediately felt more healthy.

It did the usual rpm adjustment routine like it should, ran 10 seconds or so at 1000+ rpm, then brought the idle speed down a notch, and again after a while. I'm now idling at about 500-600 rpm while warm and in gear, and about 800-900 rpm while in neutral. No irregularities or anomalies in the idle speed now.

This fixes a lot of issues. Now since the idle is so low (normal), engaging a gear doesn't feel as violent as it did before. Also, the car sounds much healthier since it's not constantly "fighting" the high rpms. I'm also now probably using a bit less fuel. Win-win.