Hello to all & Happy Thanksgiving.

New user here, as well as fairly new 1991 F250 Custom owner. Truck has ~134 K miles, 5.8L engine, & 4 speed + OD manual transmission.

I am required by state law to smog this vehicle annually (and of course it has to pass) in order to get it registered. I purchase this truck about 6 months ago & it passed emissions at that time. I had to re-test it in November 2014 due to a technicality and now it fails miserably.

First re-test it was reading in the 2800 PPM range for both idle & loaded hydrocarbons. Test limit is 300 PPM. I did a few things -- set timing per the under hood instructions, replaced many rotted vacuum lines, cleaned/gaped plugs (all looked pretty good), replaced distributor cap & rotor, & replaced HEGO (aka heated O2 sensor). All simple, easy to do things. At second re-test still failed, although hydrocarbon readings were reduced by roughly a factor of two.

Somewhat perplexed, I purchased a ODB-I (aka eec-iv) DTC reader/tester & the KOEO tests all passed. However, the KOER test failed with code 41 which I believe maps as follows:

Code 41- HEGO sensor circuit indicates system lean
Code 41- No HEGO switching detected

Is this pointing to an O2 sensor issues? Can there be something else that can result in this particular DTC? I checked the heating element resistance of the old sensor, & it seems to be very close to the 6 ohm reading specified by the on-line repair manual. So the old sensor was/is probably OK.

This is a weird problem because:
[1] The truck passed about 6 months ago & I really didn't drive it too much & did not do anything to it other than change the oil, air cleaner, & other mundane routine maintenance.
[2] The hydrocarbon readings are so far out of bed relative to the last test which was compliant.
[3] It seems like the HEGO is bad, but it isn't.

Does anyone know what fuse is in line with the O2 sensor heater element? I have some documentation that says fuse 5, but the panel label does not map that fuse to the O2 sensor.

Any suggestions or insight is appreciated.

 

Intake/vacuum leak.

Have you physically inspected the vacuum caninsters? If they (2 total canisters if your truck is equipped with cruise control) are rusted badly, then I would check to see if they are functioning. They must be able to hold vacuum. These are most commonly overlooked when diagnosing vacuum leaks. Be sure that all "brittle" nylon lines are intact and not "just looking" ok. I did notice that you said you replaced some vacuum lines. Don't forget the notorious brittle "white" vacuum line that controls the defrost/vent switch for the cab, that sits atop the cowl, just back of the passenger side of the hood.