I've been running down the list of diagnostics to find the source of a problem with rich running - haven't found the problem yet...could use some help brainstorming this one.

The car is a 1990 240 DL Wagon, manual transmission, A/C, 230K miles (I've had it for <10k). The fuel injection is the LH-3.1 system. Was running good, averaging 26-28mpg city/highway. Recently noticed the idle to be rougher, fuel economy down (17mpg), black haze in the exhaust and a foul smell; still starts & drives fine, perhaps a bit less torque noticeable while pulling hills in 5th gear. All 4 plugs are black & sooty. Recently got a "check engine" light; diagnostic codes of 232 & 113.

I found a couple of bad plug wires (end connectors broken loose)-replaced, along with the spark plugs. Pulled & cleaned the flame trap - wasn't too bad. Checked & couldn't find any vacuum hose leaks. No fix. Replaced the O2 sensor - no fix. Cleaned the grounds on the fuel rail bolts and the ground strap from the valve cover to body. No fix. Went through the mode 2 with the OBD; throttle switch full-throttle and idle circuit check ok. Went through mode 3; injectors, air control valve, and cold start valve cycle as described for proper operation. Started the engine and removed the harness to each injector (one at a time) & noticed a definite change in engine smoothness/rpm with each injector off line. Sprayed carb cleaner around the base of each injector - no change in idle RPM. Did resistance check of the air mass meter (heated film type). The nominal value is 108 ohms at ambient & I read 112. Pulled the return fuel hose from the fuel pressure regulator - lots of flow. Momentarily pulled the vacuum hose on the FPR - no fuel leaking past the diaphragm. Hooked up a multimeter to the O2 sensor-reads .92 & was slowly climbing at idle with no difference noted with engine at fast idle (~2000 - 2500 rpm) for a few moments.

I'm starting to run out of options here. All the cylinders are running rich. The possibility may be too high fuel pressure, but I have my doubts since there was lots of flow in the return and the diaphragm doesn't appear to be ruptured. It could be leaky injectors, but I doubt that would affect all 4 cylinders the same (sooty plugs) and their seal to the head seems fine (no air leaks). Possibly the cold start injector, but it cycled normally in the mode 3 diagnostics. Perhaps the AMM, although the only test described checked out normal. I haven't done the elecrical checks at the ECU yet, but I'm not too hopeful there either. I'm afraid its pointing to an internal problem with the ECU.

One last thing I wanted to check out; the intake hose running between the AMM & the throtle body has a suspicious wrap of duct tape on it. Could an air leak here affect the AMM signal the the ECU to cause a rich mixture?

Any comments/suggestions appreciated - Thanks!