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1982 500 SEL 126 V8 Euro Model; Engine type: 117961; tranmission: 722350
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563 Posts
Discussion Starter #1
420SE V8 1986, 125K miles; Car was operating in top shape with smooth idle no cold/hot start problems, excellent power, just as a MB in top shape does. Out of nowhere after a short 5 mile run to the store, it started idling rough and idle surging just out of nowhere. Has had recent timing chain/rails, plug wires/cap/rotor 1 yr. old, plugs are all OK and just cleaned. Did electrical check plenty of juice to all plugs; fuel pumps working; changed OVP relay; did fuel check loosening injector lines all showed plenty of gas coming in; checked distributor all OK; no vacuum leaks anywhere. Fuel filter has less than 5K miles. Tapped idle selenoid in the engine and idle relay under passenger floorboard with no change. I'm puzzled because this situation arose immediately after I started the car at the store parking lot. Car still has plenty of power although it seems to have a slight misfire. Placing hands in the tailpipe discloses smooth idle then surge with a ppppt, ppppt, (uneven exhaust flow). I am puzzled and would appreciate some help.
 

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Slightly creased
2015 ML 400 x 2
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16,601 Posts
Hmmmm..... you have certainly done your diagnostics....

Perhaps you need to open the idle relays rather than just tap them and check the boards for solder cracks?
 

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1991 300 SE
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18,534 Posts
fuel pumps working
Are the pumps original? If so replace them. Failing pumps will run fun for a period of time then exhibit the misfire/hesitation you’ve noted.

did fuel check loosening injector lines all showed plenty of gas coming in
Perhaps but only a fuel system pressure test will check the condition of each fueling component and possible system leak that may go undetected.
 

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Registered
1982 500 SEL 126 V8 Euro Model; Engine type: 117961; tranmission: 722350
Joined
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563 Posts
Discussion Starter #4
Fuel pumps were installed new @ 1 year ago when I rebuilt the entire fuel system e.g. new injectors, seals, air tubes, fuel pumps, EHA valve, FPR, etc.
 
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