Yesterday I started trying to diagnose the Check Engine Light on my “new” 1995 940 with B120FD non-turbo.

Generally I have avoided learning about the computer diagnostic systems either by being lucky (on my previous 740 models) or avoiding them altogether by driving old Benz diesels. Now it looks like I can’t avoid it any longer.

With the help of the FAQs I got the codes out, which are 221 & 231 for the mid-range lean or rich codes likely to be associated with air intake leaks, the oxygen sensor, or the air mass meter (at least I think lambda means O2 sensor). From a search of old threads, I have seen a number of other possibilities as well.

I did tighten up the air system by replacing some rotten hoses, and I have some confidence that I have cured that part of the system. However, there was no flame arrester in the cup that connects the manifold to the oil separator. Maybe the flame arrester also limits airflow from the crankcase? And, I have ordered a new oil pressure cap gasket because I saw some oil leakage. Maybe this could let in too much crankcase ventilation air?

I am hoping that my particular symptoms, which I did not find in my search may narrow the possible problems. They are: When the check engine light is on, there are no apparent starting, running driving problems except that the fuel mileage might be low at 20 mph (suburban driving, not city stop and go but not highway). However, when I clear the codes, I have a cold running problem. On cold start, after clearing the codes, I have the tip-in hesitation when I first crack the throttle. After warm up, the problem disappears, and the problem disappears if the check engine light comes back on.

The only other symptom I have noted is that, occasionally, the engine will “hunt” at a stop when my foot is on the brake. This might lead me to believe that the booster is ruptured, except it only happens occasionally. I have not yet put a vac tester on the booster.

About a half hour ago I pulled the fuses that seemed related to the ECU since this was advised as a more through clearing of the system than resetting at the diagnostic block.

Now I am going to drive it over to where I work on cars to pull a vacuum on the fuel pressure regulator, and see what other trouble I can get into. Any suggestions will be appreciated.

Thanks,
Charley