Day and a half after the use of Seafoam which I put in the gas tank and in intake manifold on my 1992 300E I started having idle problems, it begun to have rough idle intermittently. It was progressive, the fourth day of use it was really bad, it would stall and loose at least 30% of power. While driving I exercised the gas pedal and found positions where it would run very good, but as soon as I got to a stop the engine would tend to stall and idle would be very erratic, engine would stall and loose at least 30% of power. I could also smell stinky sulfuric gases from the exhaust and when hot some plastic overheated smell from the catalyst converter. In the morning during cold start idle was ok, but as soon it warmed up to about 70~80C (when it went to idle) it would begin very erratic again. First thing I thought I clogged or contaminated the fuel injection system with debris removed by the Seafoam, so I drove it in the highway until I burnt most the gas in the tank, refueled then took it for a ride no help, it behave the same. So I decide to remove and clean all injectors in ultrasonic machine, no help
I replace the fuel filter no help
I read the lambda ratio on pin 3 of X11, and I could see that the voltage would read 7~7.5Volts while warming up and would drop almost to ' 0' volts when warm (70~80C)
I checked all vacuum connection, temporarily strangled some, no difference.
I read the error codes out of pin 3 of the OBD1 connector (Fuel Injection controller) and found no errors.
I read the error codes out of pin 14(MAS controller) of the OBD1 connector and found no errors.
I checked the ICV and cleaned as well, no help
I replaced all spark plugs and verified operation of the cables, spark was getting there.
I disconnected and checked the EHA with an ohmmeter and found ok.
I checked the coolant temp sensor resistance (double sensor) and it read normal for the temperature.
I disconnected the coolant temp sensor and put two 2500 ohm resistors across to the pins of the coolant sensor connector to fool the computer (as if engine was cold), no help. engine would still misfire and be very erratic.
I checked the voltage out the oxygen sensor which read ok while warming up (0.4~0.6 volts), then would drop to ' 0' at normal operating temp.I disconnected the O2 sensor while running, no help.
Car ran so erratic that it made me believe that the ignition timing was being shifted by the computer.
I hooked a strobe light and verified that the ignition timing was correct (10~12deg BDT), and it would not change from cold to hot operation of the engine.
I decided to put an inline spark monitor on cylinder #1 to observe the spark and eureka !!!! it fixed it. It was the connector pipe on cylinder #1, I don't know how it affected seemed like all cylinders, remember one of the things I did during my troubleshooting one day when the idle was messed up was to pull each spark connector at a time to see the effect and I could not see a significant drop on any of them. Anyway to be 100% sure of my findings, I removed the inline spark plug tester hooked the pipe back and there it failed immediately, then I put the inline spark tester, took it for a drive and worked perfectly, so I order a new pipe connector a U$14 part, and problem solved.