Originally Posted by
Padgett
Have been having a reoccurring P0170/P0173 (P2017/P2086) in my '01 SLK320 (same chassis/drivetrain as Crossfire). Have been told that when in a long hot idle (takes about 150 minutes) the Secondary Air Injection is supposed to come on and correct a long slow move from a near zero STFT in both banks to +25 (Torque Pro) which trips the codes. Has been mentioned that the Secondary Air Injection i(SAI) is supposed to come on to correct this. Mine is only coming on when cold and not when hot (added a LED to tell when on).
What you were told about the secondary air pump coming on after long slow idle doesn't make sense to me. Adding air to the exhaust increases burning and heat which the O2 sensor interprets as a lean condition (excess oxygen), giving low voltage to the PCM/ECU and thus the PCM will increase short term fuel trim. Thus adding more oxygen to an already lean condition as noted by the short fuel trim is the wrong thing to do. IF I INTERPRET THIS CORRECTLY!
If the 02 sensors are operating correctly and the PCM is responding correctly giving you the increase in short term fuel trim the voltages the 02 sensors are producing should be 0 to no more than .5 volts. Checking the voltages of the O2 sensors against the short term fuel trim could help you determine if the PCM is going bad as you suspect.