41 and 91 codes persist even with new O2 sensors

[globaltefl]

1990 5.0 Mustang: When I first start the car it dies, unless I give it some gas. When warm it idles and runs rough. KOEO test reveals both 41 and 91 codes, both before and after the new sensors. During the high RPM part of the test there is also a hollow sound (hard to describe). In fact, the high RPM test failed once. It also has a strange smell, but not raw gas. I have replaced the IAC and tried a proven EGR valve and sensor. Could it be catalytic converters? Smog pump or what? I have not done your O2 harness test given I get both codes.

 

[HISSIN50]

Did you clear the codes after adding the new sensors? If not, they might be artifacts.

What test did it fail? The balance test or did it give an idle limit code?

Have you looked for vacuum leaks and ensured the MAF is not dirty and functioning properly?
A vac leak is where I would start - it would explain the lean codes and ereratic idle.

 

[globaltefl]

The codes were cleared. I cleaned the MAF, but don't know how to tell if it is functioning properly. I don't know about the balance vs idle limit codes. How do I get this information? I need to continue to pursue a vacuum leak. But I thought that a vacuum leak would only generate the 41 code. An I wrong?

 

[HISSIN50]

The balance test is a test you actively do after pulling codes. The limit code is something that would be seen when pulling codes.

41 and 91 suggest a total air-fuel issue, not an O2 sensor issue.

 

[Mustang5L5]

The only codes you get are the 41/91 codes?

 

[globaltefl]

I also get a 21 code that disappears when fully warmed up. I also get a code 15, but that is recent and post dates the other problems and the way it is running.

 

[Mustang5L5]

Usually when you get a 41/91 together, it's not o2 sensors and may be other things like a vacuum leak, exhaust leak, and/or wiring issue.

Basically, if extra oxygen gets in somewhere, and the o2 sensors see it, it can trip the code.

Might be time to start looking upstream for a cause. Do you have a larger exhaust leak at the connectors? Is thermactor tube open to air? Any vacuum leaks? Etc

 

[01GTSTANGER]

Hey bro really check ur vacuum closely... I had the same problem on my 91 and it was driving me crazy!! It turns out I had a vacuum leak underneath my intake, one of the hoses was completely torn. Once I fixed it and cleared the codes ive been good since not to mention car runs so much better. Good luck bro