just an FYI - I had my engine apparently lean out last year, requiring a rebuild. It appears that my original, 21 yr.-old fuel pressure regulator is faulty, fuel pressure is 8lbs or so down from what it should be, and the probable cause of leaning out. I would highly recommend doing a fuel pressure test at this point in any E30's life (particularly if you track it), and/or are above 150k miles, even more so if 200k or more. I think the only indicator you might get of low fuel pressure would be a check engine light. Perhaps Paul can offer up some comment on which conditions trigger check engine lights. Not sure if it's triggered by both low and high out-of-range readings. I didn't see any running problems w/ the pressure being a bit low, but it is certainly enough to be dangerously lean @ full-throttle, high-rpm running. If you do a pressure test yourself, unless you do something to de-pressurize the fuel lines, be prepared for fuel to spray directly at you when you detach the fuel line (keep a fire extinguisher at hand, wrap the junction w/ a rag or something, and watch your eyes). Make sure you use high pressure fuel-injection hose for T-ing in gauge.
There are only 4 things that can light the check engine light on an E30: AFM signal O2 sensor signal ECT (coolant temperature sensor out of range) and dammit I forget the other one. The CEL on an E30 is basically useless. You will not know unless your computer lights it for O2 sensor, and even then, it may not be enough to light that, because old style O2 sensors are narrow band. You should notice the power los if you are really really good, but 90% of people would not know. Buy a new one, they are cheap insurance HOWEVER, lube the new O ring with silicone lube, and push it in all the way by hand before tightening the bolts!!!! IF YOU KNICK THE SEAL IT MAY LEAK DIRECTLY INTO THE ALTERNATOR AND SET YOUR CAR ON FIRE!!! A friend of my old shop did not listen to this, and put it in dry then tightened it down forcing it in with the bolts. A week later he was driving his car and a friend of his called from an upstairs window to tell him his car was on fire. He left a flaming trail behind his car, and luckily his friend was able to out the fire out before he did any real damage.
