[W126 Coupe] Strange Idle problem

[Dick Spellman]

Hi David,

It is likely that you have an over voltage protection relay (OVP) that 
has failed, check the fuse on top.
Check for the presence of an idle control module on the 83 (about center 
on your engine firewall) it will cause a floating idle not an engine cut 
out as you describe.when failed.
If the fuse on the OVP is good, the OVP itself may need replacing.  The 
manual walks you through the steps to test for failure.
The idle control module is available rebuilt with a new feature, an idle 
set screw at its base.  The cost $155-175US.
You should as a matter of course check your cap for arcing (carbon 
traces across the cap interior) or pitting on the lead contacts and the 
center carbon post.  On center post just be certain it moves freely as 
it is spring loaded.  Clean the cylinder lead contacts inside the cap 
with a small flat file.  Wipe the interior until clean with an alcohol 
wipe or cloth.  Rotor should be smooth.  If not, file it to obtain a 
smooth square edge.  Clean with alcohol wipe or cloth.  If you find 
traces on the cap or the carbon center pin is jammed or not smooth, 
replace.  The rotor should be replaced with cap if you find a new cap is 
needed.  Check your plugs for wear, gap and appearance.  Replace with 
oem (MB dealer) plugs not the mcparts store stuff as they don't have a clue.
Check your leads for arcing.  Water spray bottle, nighttime, engine on, 
spray the leads.  Look for sparks, arcing. (light show).  Replace the 
set with Karlynn (Jonathan Hodgman, on this list and a pro wrench) has 
these plus expertise and all other parts as needed.

The above is the electrics part of your MB.  The fuel side requires a 
set of fuel gauges to work through the diagnosis.   A nice set of gauges 
designed for your motor are available from Star Products for under 
$100.  Bosch fuel management book ($20-25) is on Amazon.  You  will need 
this if you go for the gauges.  Then there is the vacuum leak 
possibility that requires a vac gauge (Sears or mcparts $10-20US) and 
some electrical tape so you can isolate the systems to arrive at the leak.

That's it.  electrics detailed here, fuel (you need guages and we can 
walk you through the steps) and lastly vacuum leaks (vac guage to 
manifold) and drop off non-esential systems until the leak is located.  
There is a web page that walks you thorugh this for the 500, 560.

If you have the interest to learn and do it yourself, go for it.  This 
list has done it all and will provide direction and details as needed.  
Otherwise find a good INDY with a silver haired guy that knows the 126 
chassis and hand the problem over to him.  If you are near Atlanta, GA,  
USA seek J Hodgman at Blueridge Mercedes.  Young but, smart.

Good luck.

Dick

On 11/11/2010 6:42 PM, David Doubtfire wrote:

> This is the first time I have used this mailing list so I hope someone 

> can help me.

>

> I recently had a high idle, hard start problem with my car (Euro 

> 500SEC 1983) which I had recently purchased. I was offered a second 

> hand ICV from a 560SEC (US), which I was told would probably work. I 

> put it in, and hey presto, the car started first time from cold, and 

> after a quick test drive the idle settled down to a steady 500rpm, and 

> would start easily when hot. A few day later I took a short trip and 

> the car suddenly stalled when I stopped at a set of lights, and later 

> stalled at speed when I took my foot off the throttle.

>

> Back at the garage I checked everything and nothing seemed amiss. when 

> hot, the engine would start easily and idle at 500rpm. The revs would 

> then start to rise and fall, with the revs going higher and lower 

> until it would eventually stall.

>

> I took out the Fuel Pump Relay (FPR) and bridged the pins to keep the 

> pump running and try to eliminate it as a cause, but the symptoms remain.

>

> Firstly, does anyone know if the 560 ICV will work OK in a 500 (euro), 

> and has anyone had a problem like this. Once running the engine runs 

> sweetly.

>

> Many thanks

>

> David

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