95 S500 ECU problem? poor fuel mangement

[chiwanga]

I have spent the last couple of days working on a 95 S500 that I acquired. It starts and dies, unless you keep your foot on the gas, then it will run poorly. I was fortunate enough to have a good running 1994 e420 in the shop that I could switch parts from for the diagnostic process. Here is the list of diagnostics performed:

scanned car with star scan tool- found bad mass air flow sensor hot wire, cam sensors adj,

Switched out the mass and cam sensors-no change

pulled the spark plugs and found 2 plugs on each bank were clean but wet with fuel, while the other 2 per bank were black

checked the wires, cap and rotor for proper installation(all the tune up parts were new when I got it)

checked the coils

found a Service bulletin that reads " if the on/off ratio is anything but 50% when ignition is on, and if ratio does not oscillate with engine running at temp, then ecu is suspect(ratio was at 20% with ignition on and did not oscillate with engine running)

I switched the ecu in the S500 with the one in the E420 and I noticed an improvement in that it idled better, but it still miss-fired badly.


My question is, do I need to replace the engine control module? is the ecm in the e420 incompatible with the S500? I am getting spark, but improper fuel management.

[mbdoc]

Did you still get the air mass code?
Have you inspected the wiring harness?

[chiwanga]

I am still getting the mass code. What do I need to do to inspect the harness? Peel wire insulation back?

[MEPEH]

Originally Posted by chiwanga

I am still getting the mass code. What do I need to do to inspect the harness? Peel wire insulation back?

Yes, especially on the MAF. Pull the insulation and check for stripped wires. You can do that on a few other connectors to determine the condition of the harness.