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I think this is worthy of a new thread. Yesterday I worked with T3hk1w1 on his recent drop in MPG on his 2000 XJ. He did not have an analog volt meter...etc.
First off two mechanics had worked on this jeep and given up already. There were no OBD-II codes at all. The jeep ran fine, no symptoms other that sudden drop in mpgs. O2 sensors were new. TPS, etc was new. Engine was even new.
I tested the CTS, it was good. The MAP tested good, and the MIT was off some (reading about 190 F at 150 F), but not enough to be the sole cause of the MPG drop.
So we tested the upstream O2 sensor. The two white wires, grounds, were good. The gray O2 sensor was reading lean, .05 volts, locked in place, not moving at hot idle. The black wire, 12-14 volts the O2 sensor (HEGO) internal heater was only getting 3-4 volts. The rear O2 sensor was only .5 volts, not 12-14 volts. The final test was at 2000 rpm, engine fully warmed up, where we got a nice steady 0.45 +/- .02 volts out of the upstream O2 sensor, which is what it should do with a working O2 sensor.
At 2000 RPM in park, hot, the O2 sensor gets hot enough to work properly, so the PCM (ECU) thinks the O2 sensor is OK and working, and thus does not throw any OBD-II codes or check engine lights. We checked the 12 volt path to the O2 sensor's heater, and it gets to, and past the ASD relay and 15 amp fuse, but drops off, or out on the last leg of the wiring to the O2 sensors, which the computer can not detect!
So in slow traffic, stops, low rpm operation the PCM gets a lying steady lean signal from the O2 sensor, and swings the A/F ratio to the max rich table values trying to restore the 14.7:1 stoich, thinking the O2 sensor is working. When RPMs pick back up everything works fine! So long story short, loss of the 12 volts to the heater on the O2 sensors can cause low MPG with out throwing any codes even a 2000 year XJ.
First off two mechanics had worked on this jeep and given up already. There were no OBD-II codes at all. The jeep ran fine, no symptoms other that sudden drop in mpgs. O2 sensors were new. TPS, etc was new. Engine was even new.
I tested the CTS, it was good. The MAP tested good, and the MIT was off some (reading about 190 F at 150 F), but not enough to be the sole cause of the MPG drop.
So we tested the upstream O2 sensor. The two white wires, grounds, were good. The gray O2 sensor was reading lean, .05 volts, locked in place, not moving at hot idle. The black wire, 12-14 volts the O2 sensor (HEGO) internal heater was only getting 3-4 volts. The rear O2 sensor was only .5 volts, not 12-14 volts. The final test was at 2000 rpm, engine fully warmed up, where we got a nice steady 0.45 +/- .02 volts out of the upstream O2 sensor, which is what it should do with a working O2 sensor.
At 2000 RPM in park, hot, the O2 sensor gets hot enough to work properly, so the PCM (ECU) thinks the O2 sensor is OK and working, and thus does not throw any OBD-II codes or check engine lights. We checked the 12 volt path to the O2 sensor's heater, and it gets to, and past the ASD relay and 15 amp fuse, but drops off, or out on the last leg of the wiring to the O2 sensors, which the computer can not detect!
So in slow traffic, stops, low rpm operation the PCM gets a lying steady lean signal from the O2 sensor, and swings the A/F ratio to the max rich table values trying to restore the 14.7:1 stoich, thinking the O2 sensor is working. When RPMs pick back up everything works fine! So long story short, loss of the 12 volts to the heater on the O2 sensors can cause low MPG with out throwing any codes even a 2000 year XJ.
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