i have codes 41 and 91 KOER test on my 88 5.0 mustang. Both O2 sensors reading lean. I have no exhaust leaks and can't find any intake leaks. Replaced the O2 sensors with Ford sensors and Bosch sensors. Still have code 41 and 91. No opens or shorts in O2 sensor wiring harrness. I get a voltage reading of about .157 volts on the O2 sensors when hot. .380 when cold. no switching tho. always a constant voltage. I have a 8LF ecu with a custom diablo chip. my final conlusion is the possiblity of the ecu not operating properly. need some help before i sell the car.

 

your timing could be advanced a little too much.

 

Could it be somthing wit the fuel system? Do you have a adj. fuel pressure regulator and gauge to see what your fuel pressure is at?

 

timing is set at 14 degrees advanced. fuel pressure was changed to a BBK adjustable regulator and is set at 39 psi. but i did notice the regulator doesn't hold residual pressure. KOEO pressure drops. KOER pressure holds at a steady 39 psi. the residual pressure drop i can figure would only effect start up. did cylinder balance test to make sure all injectors were firing equally. and no problems there. checked all my grounds and can't find a problem. The plugs all look good. doesn't appear to be running lean. If anything running alittle rich. i did read somewhere in the forum someone having simmliar codes and the ECT was the problem. going to replace the ECT since it cheap but geting down to blaming a bad ECU.