1995 MB E320 W124 90,000 miles
I had a bad pulley ( read completely eaten bearing) on my air injection pump and replaced it with a rebuilt pump. I drove it Monday, Tuesday and Wednesday (approx. 150 miles). On Wednesday, the check engine light came on (yellow). I immediately started researching the OBD I system in the MB forums. I saw several posts that thought that the “button and LED” to pull your codes was not present on this model (post 1994). In fact, it has an X11/4 connector which has a button and LED that function to give you the codes!! (positions :2 = button-& 4 = LED). I pulled the code (only one code came up) and it indicated code 4:
“4 Air injection system faulty hot film mass air flow sensor with hot wire”
I cleared the code and the CEL (check engine light) went off upon restarting the car. I read somewhere that some of the codes only come on after several of the same faults accumulate. Does anyone know if that applies to this error code??

Is this indicative of a bad air injection pump or some other sensor in the emission system?? It appears that the air pump is working “mechanically” (the electric clutch engages when the engine is cold and the pulley spins) or is it likely a bad pump or something else in the emission stream?

Now that I’ve cleared the code, I intend to drive it and see if the same fault occurs

Any thoughts will be appreciated.

Code reading/clearing on analog system:
* (you turn the key on and hold the #2 button down for 3 to 5 seconds and then release it and see how many times it flashes. I did it three times to see if there were any different codes. You then clear the code by holding it down for about 8 to 10 seconds. You then turn the key off and start the engine and the CE light should be off)

 

The next step is to read the faults that led to the tripped CEL light. This will help narrow in to the problem. Here's a link that will link you to the code reading tool you need to make.

Code reading information

You may have to search the archives to get more detail. After searching, let me know if you need clarification. Definitely clear the codes to see what pops up again. It's not unusual to get spurious codes.

 

I've driven about 150 miles since resetting the CEL and have't had any problems yet. I may be imagining it but the engine seems to be running very slightly rougher. Or it could just be that i'm noticing it more now that the pulley isn't gringing and screeching. I also changed the oil to Mobil1 0-40w. I did have a slight lifter tick at startup before and now it is plainly evident for a few minutes after starting but it smoothes out once warm. It isn't glaring yet. I read that it may be a problem with an "oil crossover tube" or the lifter/cam follower or shims. I may change the oil back to 10 - 40 syn or maybe 15 -50 to see if the change in the noise is significant... sigh, it is always something.

 

The CEL came on again and so I started another thread at this location.
https://mbworld.org/forums/showthrea...00#post1396800