FBolanos

got my 87 560SL serviced with a valve job last September where new valve seals, guides were put in plus the valves shims and everything properly adjusted. Prior to that had new injectors , cap and rotor plus cables. Compression did read well after all work done and rode smoothly.
past week developed a slight cold engine hesitation and random misfire that lasted fractions of seconds but disappeared until Christmas Day when this low rpm vibration appeared as if misfire has settled in out of the sudden. today changed all spark plugs and installed a new ignition coil, and aside of smoother operation at higher rpm 2000, the low rpm 1200-1500 vibration and apparently miss still persists. the ICV has been replaced with a VDO that has perfect rpm 500-750 readings and has been cleaned with ultrasonic so no issues there. Sometimes the exhaust fumes smells like pee or something weird in the mixture.
can some of you experienced in these gremlins point me to what to look after ? I am lost now after all these has been done I can’t believe the engine lose the just acquired smoothness it had after the last service just three months ago
thank you and happy holidays

chassis

How is the balance of the fuel system: pump, filter, lines, rail?

What is the sensor package on this engine? Throttle position, crank position, O2, intake air temp, coolant temp, etc?

Time to replace any of the above fuel or electrical components?

FBolanos

the 2x fuel pumps are brand new replaced last year as well , have not touched any of those you mentioned . I am not familiar on how to measure the values you suggested in your reply . I may have to learn how or take it to the shop if complex?

chassis

What condition is the engine wiring harness? Any brittle or broken insulation could create electrical issues that lead to misfire. I would give the harness and sensors a good going through.

ygmn

Exhaust smell..
IS it sweet? Like anti-freeze a bit?

I would do a compression test to see if head gasket has slight leak.

FBolanos

The smell it is more like sulfur or similar to urea kind

FBolanos

I took a stab at testing the vacuum on the EGR valve and does sustain pressure no leaks, I do not know if piston may need to be cleaned to prevent being stuck, but by now shows functional. I inspected the few months old cap and rotor and saw some carbon buildup on the terminals and on the rotor edge that I cleaned carefully with high grain sandpaper but plan on replacing this part as soon as possible. After that seems smoothing a bit the miss during driving on cold engine but becomes more obvious after engine is warmer in takeoff from traffic lights and between 500-1300 rpm….let’s see what happens after new cap and rotor are in.

FBolanos

today I did replace these:

-spark plugs - new Bosch copper ones. Old had no buildup but wanted to reassure they are new
-new ignition coil - still had factory installed
-new ignition cap (Bremi) and rotor (Bosch) they were with carbon deposits and worn contacts.
-checked vacuum status of the EGR valve and holds vacuum, so should work unless piston gets stuck altering mixture, requiring extraction and cleaning.
-checked all vacuum lines and all seem plugged correctly, none loose, including the injectors breathing hoses and all seems sitting well in place.

When engine is cold it appears smooth but as the engines gets to full warm operational temp around 30-45 minutes in traffic the miss condition becomes more obvious with the low vibration at low rpm and the exhaust noise

Any lead on what else sensors that could be related to combustion could I look after or replace before having no choice but Taking it to the shop?

Thank you so much

chassis

This is a version of a Bosch **-Jetronic injection system, correct? There aren't many sensors to check.

I would focus on tracing each wire harness section looking for cracked insulation and bare wires. These can short to ground and cause problems.

Ignition wires (spark plug wires)
Crank position sensor
Oxygen sensor (if present)
Mass air flow sensor
Intake air temperature sensor
Coolant temperature sensor

Any and all of these are candidates for replacement, including the engine wiring harness.