SER­VIC­ING OXY­GEN SENSORS

Successful Farming - - SERVICE TEAM - By Ray Bo­hacz

Due to more emis­sion con­trol stan­dards, oxy­gen sensors can now be found in off-road uses such as the farm’s UTV. The terms oxy­gen sen­sor and lambda sen­sor are in­ter­change­able. A diesel en­gine does not em­ploy this de­vice.

Ev­ery fuel has a sto­i­chio­met­ric value. It iden­ti­fies the ra­tio of fuel-to-air for the most ef­fi­cient com­bus­tion.

For pure gas, the sto­i­chio­met­ric value is nearly 14.7:1 (14 parts of air to 1 part of fuel). As the ra­tio goes nu­mer­i­cally lower, the mix­ture is richer and the con­verse ap­plies. Hy­brid fu­els such as E10 have a lower sto­i­chio­met­ric value since the com­bus­tion char­ac­ter­is­tics are fewer and the en­ergy con­tent of al­co­hols is lower than petroleum-based fuel. The fol­low­ing are ap­prox­i­mate sto­i­chio­met­ric val­ues of blends.

Pure gas .... 14.68:1 E10 ..... 14.08:1 E15 ............... 13.8:1 E85 ....... 9.85:1 Pure ethanol (E100) ........................... 9:1

When an en­gine is la­beled as flex-fuel, it has an ad­di­tional sen­sor in the fuel sys­tem that mea­sures the ethanol con­tent of the gas. The en­gine con­troller mod­i­fies the amount of fuel de­liv­ered to each cylin­der via the in­jec­tor to cre­ate the re­quired sto­i­chio­met­ric value for that blend.

A gas en­gine is of­ten equipped with a cat­alytic con­verter. (A cat­a­lyst is some­thing that speeds up a chem­i­cal re­ac­tion with­out be­ing con­sumed it­self.) The pur­pose of the cat­alytic con­verter is to al­ter the ex­haust gas from the en­gine into a be­nign form. This is called the con­ver­sion process. For the cat­a­lyst to have a high rate of con­ver­sion, two things need to hap­pen: It must reach a min­i­mum of 600°F. and the en­gine-out fuel mix­ture must be at or near sto­i­chio­met­ric. When this oc­curs, the cat­a­lyst is con­sid­ered to be lit-off. The unit looks sim­i­lar to a muf­fler but con­tains var­i­ous pre­cious me­tals in a sub­strate that re­sem­bles a hon­ey­comb.

There are nu­mer­ous oxy­gen sen­sor de­signs based on the in­ter­nal ma­te­ri­als and other fac­tors, but their pur­pose is the same. Their job is to act as an au­di­tor and tell the en­gine con­troller if the mix­ture is at sto­i­chio­met­ric. If it is not, the in­jec­tion sys­tem ei­ther leans or richens the air-fuel ra­tio to sat­isfy the oxy­gen sen­sor.

The most com­monly used oxy­gen sen­sor pro­duces a minute volt­age of be­tween 0.100 volt and 0.900 volt. The volt­age is cre­ated by the chem­i­cal re­ac­tion of the ex­haust and the ma­te­rial in the sen­sor. When the mix­ture is rich, the sen­sor out­put is high (above 0.450 volt); when lean, it is low. Sto­i­chio­met­ric is around the mid­point of the volt­age range. The sen­sor has a port that sam­ples the oxy­gen con­tent in the at­mos­phere and mea­sures that pro­por­tion­ally to what is found in the ex­haust gas. That is how it de­ter­mines the mix­ture strength.

The sen­sor is con­sid­ered a con­sum­able and needs to be re­placed. As the sen­sor ages and is ex­posed to ex­haust, en­gine oil and coolant (from a head or in­take man­i­fold gas­ket fail­ure) skew the out­put. Then the sen­sor re­quires a richer mix­ture to pro­duce the same volt­age. Thus, it is telling the in­jec­tion sys­tem to add fuel when the en­gine does not need or want it. At first, the only tell­tale sign is an in­crease in fuel con­sump­tion. As the sen­sor fur­ther de­grades and adds fuel, it will re­sult in di­lut­ing the en­gine oil with gas-wash­ing oil from the cylin­der walls. This can cause ex­ces­sive wear to the en­gine bear­ings and pis­ton rings along with glaz­ing of the cylin­der wall.

A stan­dard pro­to­col is to al­low 25% cor­rec­tion from the sen­sor. That would mean the en­gine can be run­ning at an 11:1 ra­tio (ex­tremely rich) and there will be no di­ag­nos­tic codes in the sys­tem. Once the 25% thresh­old is passed, the mix­ture is con­sid­ered out of con­trol, and a trou­ble code would be stored. Many en­gines have been ru­ined by a de­graded sen­sor.

The sen­sor lasts long­est when the en­gine is tuned prop­erly, in­gests no coolant or oil, and isn’t ex­ten­sively idled or in­cur­ring nu­mer­ous cold and warm restarts. Since most sensors cost less than $100, it is a wise prac­tice to reg­u­larly re­place them.

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