EA1191

An Oxy sensor monitor has been running for some time and suddenly the display has changed.
When climbing a grade the light runs up and down as it should, but when cruising it now stays close to max volts.
Max volts would be a rich mixture?
The fuel pressure is OK. No hoses have fallen off.
The diagnostic test shows 11 (OK), and the engine runs smooth.
There are some valve stem oil leaks, but the plug colors vary from whitish to grey with a thin black carbon deposit near the threads.
Is there something which could be adjusted.
What could be the most likely cause?

 

EA1191

This has been on another forum where someone said that changing the fuel pressure regulator would change the A/F ratio?.
The fuel pressure is OK.
I will check the MAP when I think of someway to create a vacuum. Maybe a vacuum cleaner. The normal pulse rate is OK. Pulled a pressure gauge apart and moved the needle to make it read vacuum.
After lots of checks, now think it is most likely the fuel. Or a crack has appeared somewhere.
Or just wait and see if all returns to normal. Thought that the fuel may have been 'diluted' (ethanol?) and the computer has not adjusted.