I have a cherry 560SL with 81k on it. Last week the battery was low so I put it on a charger overnight on trickle. Started up and ran great. Drove it a 10 miles to a friend's where it sat for a couple hours during Buc's game. Went to another friend's where it again sat for an hour or two. Started it up and she was idling at 1500+. Took it home and disconected battery for a few minutes thinking it would reset any codes even though no idiot lights are on(yes they all work). Started it up and same thing.

Next morning started it up and ran fine. Went to friend's house and let it sit an hour or so and when I started it she went to 1600rpm. At first I was thinking a vac line but they do not "fix" themselves so now I assume it is a sensor of some sort...Temp? Charging? cold start injector???? This car has only been driven 2k miles in 3 years but gas has stabil and inj cleaner. This is in roughly 60-70 degree weather as well(FL).

Any help on what to check would be greatly appreciated.

Nick

The Idle Speed Control Box is sometimes the culprit when these symptoms arise.

Where is that located?

Anyone know where this is or any other ideas as to what it could be? I read something about a voltage protecting fuse that keeps the idle in check??? anyone?

The control box is in behind the glovebox. Remove the glovebox and use a light to peer into the 'rats nest' of electrical components. It should be a black box (it is on my '86 560SL), behind the alarm control box, which has 'Becker Radio' on it). You have to reach in and wiggle it out of the socket - it's tight.
However, nothing to do with it except open it up and look for obviously burned component(s)(not likely), or you can re-solder (melt and re-heat) all the connections. Also, check the part number. Mine was the wrong number for my car. I did resolder every conection, but that didn't help much. A used one off EBay, correct part, seems to have done the trick.
Your post referrs to the OVP, overvoltage protection device. I swapped mine for a known good one, but it wasn't the problem. You'll find it behind the passenger side kick panel, just above the fusebox. The panel (not just the access door to the fusebox) needs to be removed to get at it, if I recall. It's an aluminum can with a red fuse on top.
I'd suggest you do a search under idle and OVP in the tech and SL forums, lots of good stuff to help you. Also look under idle control valve, which is the other likely source of your problem (that's the main item the control box controls).
Finally, you should thoroughly check all round the engine bay for a possible vacuum leak. That will cause high idle too. From your symptoms, it may just be a hose that has come loose...it could be 'fixing itself' by sealing/unsealing from engine vibration...

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1986 560SL
2002 Toyota Camry
1993 Lexus

Thanks!

About 650-700 rpm is just about right for the 560 engine idle, when you first start it up.

These engines are notorious for their high idle problem, many have it at one time or another, luckily it can be fixed.

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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)

The check is to find the idle air valve, which is a silver cigar-shaped gizmo on right at the center front of the intake manifold. It has a two pin connector on the front. Pull the connector about halfway off and with engine idling, measure the voltage at the pins - one test lead on each pin. If the idle relay is working, you should see 3-5 volts.

Various online suppliers sell units rebuilt by a company called ProgRama.

If you have voltage, the unhook the connector and touch battery voltage to the pins. If you don't hear a soft click, then the valve is bad.

If the valve clicks, then take the valve off, plug one of the air passages, and fill it up with carb cleaner, shake it up, and let it sit for an hour or so. Dump out the cleaner, and repeat until no more crud comes out.

Reinstall the valve and maybe it will work.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Seems to have fixed itself(I know it never does). But I took her for a 30+ mile trip yesterday and shut it down a couple times and she ran flawless. Might have just been from sitting that something stuck? I will keep an eye out for it and a copy of what people have posted here in the Glovebox.