engine dying problems
 

On a 1988 126 300SE 6 cylinder. I have a problem with deacceleration dying such as coming down a offramp or deacceleration in traffic situation. I have to put into neutral and the engine will key restart immeditaley. Behaves normally at idle and when in drive. in park with engine running sometimes idle will change between 700-1200 RPM

Possible bad OVP relay...........

Over Voltage Protection relay is one, cracked joints inside the relay is another, I'm going through the same grief with a 500sel at the moment. Idle Control Valve can be faulty, vacume leaks from the rats nest of hoses that live in the engine bay. Check also the rubber hose connectors on your I.C.Valve. These are fine on a cold engine, but once warm, they swell and you loose vacume. It probably runs fine, just when your slowing for a corner or stop sign/red light it stalls, but starts again immediately. Fuel pump relay is another gremlin. Check the cheap easy things first, Plugs and leads, distributor cap, coil before throwing big dollars at parts in an effort to find the problem. There are a heap of posts on idle problems, most point to vacume leaks or I.C. Valve, O.V.P Relay. The Idle control valve can be removed and the gunk cleaned out with Carb cleaner, a few success stories. Another is to disconnect the two wires to the I.C.V. start engine, if revs don't jump to 1500-2000rpm its probably deceased. Im no wizz on your problem and mine is still ongoing, I wish you luck, check the other posts, lots of knowledgeable people here.
Brian

Engine dies on decel
 

There are several things to check before replacing the valve
#1-O.V.P. must be good. Do you have power to the valve?
#2-Is mixture too lean?
#3-Make sure idle contact in throttle body is good.
#4-Make sure Decel switch is functioning.
#5-Make sure the EGR valve is not sticking open.
#6-Check the solder joint on the Hall effect speed sensor for Cracks
There are a few devices that use TD signal from the ignition, Make sure that one of these arn't shorting to ground.
Good luck and post your fix
Mike

Mike, Great checklist, as this appears to be a common problem, is there any chance of expanding on the methods to check these parts. Would assist me and others in tracking and eliminating possible faults. Thanks in advance.
Brian.
280SL
500SEL