A month or so ago, I posted some messages here about the "check engine" light on my '89 240 wagon. The car has about 213,000 miles on it, and I bought it a little over 3 years ago at about 168,000. Other than routine oil and filter changes, plugs and wires, brake pads (and front rotors), the only really major fix was a water pump replacement about 2 years ago, and the timing belt (actually, twice... once when I got the car, and again a few weeks back). Anyway, the "check engine" light started coming on, although the car seemed to be running fine. I eventually noticed that it only occured when I was under hard acceleration. The only code I could get was 2-3-1, which translates as "lean or rich mixture", not very helpful! The light would go out by itself in a day or so, and I eventually found that I could extinguish it right away by pulling the main engine fuse for a few seconds the next time I shut the engine off. It would then stay off until the next time I had to accelerate hard, which could be hours or days! I checked everything I could think of, including the Throttle Position Sensor, a switch that closes one set of contacts when at idle, and another just before Wide Open Throttle (WOT). I found that both sets of contacts were erratic, so replaced the switch with one from the junk yard. Probably a good thing to do, but didn't help the problem I was having.
Then the other day, I went to do a routine oil and filter change, and found that I couldn't get the wrench around the filter! A little checking around revealed that the engine mount had separated, and the engine had slipped down and over to the left side of engine compartment! The rubber of the mount was actually resting against the filter, and I'm just lucky it didn't shear it off or puncture it when the car hit a bump! Obviously, the left mount must have been bad too, for the engine to shift so much.
Yesterday, a friend and I replaced both of the engine mounts (about $46 in parts, not too bad!). It took about 4 hours, mostly because the engine was so far out of position, but on the way home I noticed that the engine seemed to be running better, more power, and the light didn't come on when I floored the gas!
I'm guessing that with the engine against the left side of the compartment and free to move around a little, there was probably some vibration that the Knock Sensor would pick up and interpret as a misfire situation, retarding the spark timing and reducing the power. Under hard acceleration, extra fuel would be dumped thru the system, and I suppose the computer put the two signals together and decided there was a fuel mixture problem worthy of my attention, and turned on the Check Engine light for a while.
In retrospect, I had been noticing a bit of metallic tapping when I hit bumps, but thought it was probably just early signs of a failing strut or something like that....
In any event, it might be worth adding "broken engine mounts" to the list of possible causes of the Check Engine light being on!....
m9876c at yahoo dot com