C126.044 500SEC High Idle after a run

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Active Member
Mar 15, 2013
Dublin, Ireland
E250 CDI Coupe(C207) Year 2009 & 500 SEC(C126.044)

I recently bought a 500SEC, there is work to be done to make it a nice car, but I'll tip away it over the next few years.

I've taken it on a few runs and have noticed that putting it into Park after that it idles high, around 1100-1200 RPM.

When I start it, it seems to be fine, idles at around 1050 RPM for 30 seconds, then drops down to 550 RPM, which I believe is normal behavior.

I've read on various forums that the OVP Relay can be the culprit, 10A fuse and its contact are fine, but cleaned and filed just in case.

I removed the OVP and checked the pins, all were clean with no corrosion or oxidisation.

With the OVP relay removed, ABS light stays on as expected, I started up the engine, runs like a dog, revs go up from 1000 RPM to 1300 RPM. Replaced the OVP relay and the startup sequence returns to normal.

I think that rules out the OVP relay as the problem. I wonder if should I consider replacing the Idle speed control module in the footwell?

Any advice would be very welcome.
Few ideas:

Check the throttle (and linkages) aren’t sticking?
Sticky idle control valve?
Air leaks on inlet side?
OVPs can be quite sensitive and cause a myriad of issues. Could be worth trying a replacement to rule it out (I appreciate the tests you’ve done also)

Does it have cruise control?

I bought a 126 420SE years back, cheaply, that had been sitting for a while (from a mate). It wouldn’t rev up.

Got it home and fixed within 10 mins. Throttle linkages had seized up 😀

First thing I would say, don’t start making any adjustments eg idle speed or mixture or anything until you find what the cause is 👍
Thanks Will,

It's 4-5 runs(20 minutes - 1 hour) that consistently produce the high idle, so I'll check the throttle and linkages, but probably not the issue.

I'll look into the idle control valve, they are not cheap new!

I'm waiting on the components to arrive to build an OBD1 reader, very simple LED, push button, and banana plugs. That might show up errors to point me in the right direction.

No harm in getting a new OVP, just to rule it out.

It does have cruise control, did a quick test the other day, seemed a bit flakey. Is there any relationship to a high idle after a run?
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