Hey guys and gals. My 98 4.0 appears to be running rich. I had a no start issue and fixed it with a new coil and cleanup. Since then it's been crazy rich. I have no code light on. Can my O2 sensors go without a code? Or am I barking up the wrong tree?
any help or opinions appreciated.

 

The 1st thing to look at is the O2 sensor the one before the cat the after the is there to see how good the cat works .

 

What are your short and long term fuel trims when fully hot? Fuel pressure ok? Any misfire?

 

The oxygen sensors will not throw a code unless their voltage is being held and maintained too high at equal or greater than 0.9 VOLTS for ten consecutive seconds at one time (for rich running). If the oxygen sensors voltages are turned off, or at 0.00 VOLTS this would be showing the opposite having a lean running condition.

If you are having symptoms of running too rich, their voltages should be high and close to 0.9 VOLTS. The P0138 code will show if the oxygen sensors voltages stay elevated equal or greater to 0.9 VOLTS for ten consecutive seconds.

So, they are able to be getting stuck at an elevated voltage, and to not throw a code. That is not likely, but sometimes it does.. It is certainly possible. It is also possible for them to run at equal or greater than 0.9 VOLTS for ten or more consecutive seconds, and also not to throw a code, but that is also not likely because they should based on there designs...

Having a scanner would be very helpful to see what is the fuel trim values, and for looking in the live data streaming function of the scanner for what the oxygen sensors voltages are doing in real-time. You would then be able to compare and decipher the data.

 

Actually, it's pretty common with these beasts. O2 sensor is a prime suspect when running too rich. An exhaust gas leak ahead of the O2 sensor will also cause a rich condition with no code. The O2 sensor sees a lean condition, so the PCM dumps fuel to compensate. Since everything electrical is working correctly, there's no code.