I’m trying to isolate a problem that recently surfaced with my car. While I’m waiting for my copy of the Benly manual to show up in my mail box, I thought I’d tap into the collective wisdom residing on this list.

Background: 1990 240 DL wagon, 5 sp manual transmission, A/C (not functioning), 230k miles, purchased in Feb 2005, no previous repair records, running good until recently, starts easily (except when tank runs low – then takes more cranking, but will start), consistently yielding 26-28 mpg. This is a secondary car driven only by me for commuting to work and the occasional errand. During the warmer weather I mostly commute on a motorcycle, driving the car only occasionally.

Symptom: Since last fill-up (3 weeks ago) I’ve noticed the engine running rougher at idle and a foul exhaust smell at stops (windows open). After about 100 miles on this tankfull I noticed increased fuel consumption (gage normally reads about half full after 200 miles, now it’s below half after 150 miles). It also seemed a bit down on power, but only noticeable in fifth gear climbing grades at around 45mph. The other day I got out and checked under the hood with the engine at idle to look for loose plug wires or a vacuum leak. I noticed a slight black haze in the exhaust, a bit heavier with a rev-up. I momentarily pulled the AMM plug to see if anything changed (none to speak of) and re-connected within 15 seconds. I then closed the hood and drove it to the grocery store about 5 miles away. On my drive I noticed the check engine light had illuminated, and figured that happened when I pulled the AMM electrical plug. While at the store I removed the neg. battery terminal for a moment in an attempt to clear the fault After restart I noticed the check engine light had gone out but before I got home it had come back on.

Diagnosis: I spent some time on the Brickboard, doing searches & reading the 700/900 FAQ. I found the diagnostic module under the hood on the back side of the L/H shock tower, and with the wire plugged into socket #2 I got the following codes;

First Code: 2-3-2 Fuel Trim (Lambda Control) Too lean/rich at idle. Causes: too rich; high fuel pressure, leaking injector

Second Code: 1-1-3 Fuel Trim (Lambda Control) Too lean/rich. Causes: mixture incorrect, O2 sensor wiring fault

Questions: Is there a hierarchy to the order in which the OBD spits out the codes? It seems to me that the O2 sensor is more likely the cause of my problems since they are a common replacement part. But could a bad fuel pressure regulator lead to the failure of the O2 sensor, explaining why the check engine light didn’t come on as soon as the rough running was noticed a hundred miles earlier?

Any information appreciated – thanks for your time!

Joe in St Louis