Alright, to keep you updated I converted my 88 bronco to Mass Air about a month ago. It was running great. Smog time came around and I failed my high rpm test. It always passed before so I figured I would put it all back to speed desity and try it again. Well, I passed the high rpm test but failed the idle! My ppm was from around 200 to 410 and the limit here is 120. The truck runs great though. So I pulled my codes and I am getting a 41(lean HEGO reading) which must be causing my computer to burn rich. So I replaced my O2 sensor because the ceramic in it was in pieces but I am still getting the same code and failed smog again. I am lost. I am not joking when I say 99.9% of everything on my truck is new. I have new injectors, fuel pressure reg., fuel filter, so on and so on. Where should I start or has anyone else had this same problem? I have it for sale but I would still like to get it fixed for the next guy. Thanks!

 

Did you clear the codes after the new O2 was installed? Sometimes the code will still be present after the repair. Is the check engine light on? The fuel pressure regulator could make it run rich.

 

Yeah, I cleared them twice. The fuel pressure reg. is new.

 

Are you using a single O2 sensor or duals? Did you use a MAF conversion kit or DIY it?

 

I am running a single o2 but I am also running speed density. I swaped it back and then I was burning rich on the idle with code 41.

 

1988 F250 351
I had a 41 code for months after I replaced my old 351 - drove me nuts.

In my case, the idle was just too lean with the original fuel injectors. Even after I put in new injectors I kept getting an on again off again 41 and another code for air upstream during self test. What I finally found was that I had inadvertantly swapped the vacuum hoses comming off the TAD and TAB solenoids. Once I got that straightened out I passed the New Jersey State Inspection the first time through ( I don't think my truck passed inspection in a first time attempt in 10 years.)

You said your injectors are new so my thought is either the O2 sensor isn't being read (faulty wire or something) or that some air (and it could be very little) is being introduced somewhere along the line giving the lean condition.

Are you getting 17" vacuum or better at idle?

 

Yeah the whole thing has me lost but I actually just sold it this morning. So hopefully the next guy can figure it out.

 

Great! No more code 41 to worry about!