I have a 1991 240 Base sedan

CEL comes directly after the car is started. If I reset the light while the engine is running, it will come on after a few seconds of driving.

The codes are 2-3-1 and 2-2-1 (on port #2) -- seems to indicate:

Fuel trim (lambda control) too lean or too rich at part load and Fuel trim (lambda control) too lean at part load

I have replaced the O2 sensor, fuel pressure regulator, and fuel filter -- which has seemed to make an improvement in performance (imagined or not?) but has not affected the behavior of the CEL -- still comes on.

The car seems to be running pretty good -- but I would like to take care of the problem, whatever it may be.

Anyone have any insight?

 

This is going to sound obvious, but are you certain you have cleared the old codes? To the point where you get a 1-1-1 code and the light DOES go out, at least temporarily? It's a common error that one seldom does twice, but everyone does it the first time. If you are certain the codes are cleared, then I would say take a look at your AMM. Your 91 is most likely LH2.4 injection and uses a 016 AMM. Rather common used for cheap. If you happen to have LH3.1, I think it's a 001 AMM (guessing). Regardless, the LH3.1 AMM is considerably harder to locate used. Before I replaced it, I would first clean it out gently using electronic parts spray or, if on a budget, brake cleaner spray. Don't use carb spray as it is too harsh. Also, examine the pins in the 6 pin connector. They are quite dainty and can get bent and/or pushed back to the point they make no/poor contact. With a digital multi-meter, you can back probe the connector and see if voltage is in line w/ specs.
Keep in mind, a leaky exhaust will play Hell w/ accurate O2 readings. Air gets sucked into the exhaust and the O2 reads it as a lean mixture. Vacuum leaks, if small, will generally be learned around with LH2.4 and the system will compensate. Still, air entering the combustion chamber w/o flowing past the AMM is unmetered air and can cause some running issues.

 

Thanks for the feedback. I did verify that I was resetting the CEL (and getting a 1-1-1)... Light still comes on just a second or two after startup.

It is a LH2.4 -- the AMM looks good -- sprayed some cleaner on it anyway. I did notice the connecter only has 5 wires, but after a little reading it seems like that is normal.

This is a California car, with an EGR valve. I was getting 2-4-1 (Low flow) warnings awhile back, but a new EGR Vacuum Solenoid seems to have fixed that. I can verify that the EGR valve is working by applying vacuum on the lower vacuum hose while the car is running and the engine immediately starts to stall -- which I've read is a proper functionality test for an EGR valve. Would you think these two codes could be related to the EGR at all?

 

A bad FPR could also throw such codes, does the return line from the FPR have gas flow at idle. It should, and if not, it could be running real rich.