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Old 06-08-2021, 03:32 PM   #1
lummert
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Default Failed O2 Sensor?

Think my 88 765 TIC has a failed O2 sensor. My fuel mileage average went from 21 to 25 mpg. This suggested something being wrong such as a bad vacuum hose to the FPR which had happened years ago. I tried checking the baseline fuel mixture but could not get the LED to blink, it was steadily lit no matter what I tried.

I then did the O2 sensor tests. O2 sensor output was .8 volts on every test. I tried the tests at 3000 rpm with sensor disconnected and result was still .8 volt. I tried with vacuum hose disconnected and plugged at the FPR, still .8 volts. I tried with a vacuum leak, still .8 volt.

Luckily have have a couple spare O2 sensors handy. But, I'm wondering if I can just run it this way in open loop with the POT screw adjusted to 300 ohms. It starts and runs fine as it is LH2.2. I increased the POT screw setting to make up for the too rich running and will check the fuel mileage average at next fill up.
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Old 06-08-2021, 09:35 PM   #2
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You are either running rich, or it is a bunk sensor. You could try a snap-throttle test. It should go rich, then lean, then go back to oscillating. You could try to induce a large air leak and it should lean out the mixture and voltage would drop.

I have had LH2.2 cars with the mixture go way off and fail emissions testing, but the car runs fine. Leaking injectors could cause a rich condition.
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Old 06-09-2021, 07:34 PM   #3
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Checked again today. No output at all from the O2 sensor. I have located 2 OE sensors here at home to take to my friends junkyard to get it swapped out.
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Old 06-09-2021, 08:49 PM   #4
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Report back.

If you check cold, there will be close to zero volts. Put a test lamp between the two white wire terminals, key ON. This tests the heater circuit. The heater is there to bring the sensor up to temp faster for better emissions. Your o2 sensor will work without the heater.

The sensor should start out at close to zero volts on a cold start. Once the coolant is to a certain temp, the ECM starts "tuning" the mixture. 0.9v as it goes to the rich side, 0.1 V as it goes to the lean side.

The sensor is actually making its OWN voltage. Crystals and s**t!!!

You can have some fun with burning propane and bench test the sensor with a voltmeter. Put a one meter lead to the GREEN wire and the other meter lead to the sensor THREADS (ground) Here is a 1 minute video:


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Old 06-09-2021, 09:13 PM   #5
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Old 06-10-2021, 08:50 PM   #6
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I managed to drop the downpipe today and swapped out the O2 sensor with one of three I found at home. O2 sensor voltage is switching but I can't get the LED to flash when checking the baseline mixture setting, it is constantly lit. I'm thinking I have a failed brake booster check-valve causing a vacuum leak. I've been noticing that the brake pedal seems to travel too far. I have no vacuum leak into the car from the brake booster.
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Old 06-10-2021, 08:53 PM   #7
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Quote:
Originally Posted by ZVOLV View Post
Report back.

If you check cold, there will be close to zero volts. Put a test lamp between the two white wire terminals, key ON. This tests the heater circuit. The heater is there to bring the sensor up to temp faster for better emissions. Your o2 sensor will work without the heater.

The sensor should start out at close to zero volts on a cold start. Once the coolant is to a certain temp, the ECM starts "tuning" the mixture. 0.9v as it goes to the rich side, 0.1 V as it goes to the lean side.

The sensor is actually making its OWN voltage. Crystals and s**t!!!

You can have some fun with burning propane and bench test the sensor with a voltmeter. Put a one meter lead to the GREEN wire and the other meter lead to the sensor THREADS (ground) Here is a 1 minute video:

I did this test on the sensor that I installed. I didn't test the sensor that I removed because I cut the wires loose trying to get a socket on the sensor. I just dropped the pipe and easily loosened the sensor with a wrench.
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Old 06-11-2021, 02:12 PM   #8
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I believe LED shows the cross counts as the voltage swings to each side of 0.45V. If it's pegged on, I think that means your sensor is pegged/bias toward rich or lean.
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Old 06-11-2021, 03:23 PM   #9
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Quote:
Originally Posted by ZVOLV View Post
I believe LED shows the cross counts as the voltage swings to each side of 0.45V. If it's pegged on, I think that means your sensor is pegged/bias toward rich or lean.
Does that assume the rest of the system is working correctly? Since the mix being off could also peg the sensor, no?
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Old 06-12-2021, 07:28 PM   #10
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Starting to think the coolant temp sensor has failed. Runs fine until engine is hot, then idle is not so good. I have tested a spare sensor and will swap it out after it gets dark. Need for things to cool down and don't want my neighbors to see me doing something that takes 15-20 minutes.
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Old 06-14-2021, 05:18 PM   #11
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765T finally running good again. On Saturday afternoon I swapped out the FPR without any change in running. Saturday night I swapped out the ECT with little difference. So Sunday I swapped in a new set of Autolite XS62 Iridium plugs ( last set from the 3 sets I purchased at 50 cents a plug). This time I didn't apply copper anti-seize to the threads. Before replacing the O2 sensor I had installed a set of re-gapped XP63 plugs using anti-seize. After replacing the O2 sensor and still having rough idle I swapped in a set of take out XP64 plugs, which would be one range hotter, using anti-seize. When I went to take out the XP63 plugs I found two of them to be loose in less than a week.

The O2 sensor had to have been seek for years as after it's replacement I was able to set the baseline mixture to 300 ohms from 133 ohms.

Info: Bosch Gen III white body 0 280 155 811 injectors at 2.5 Bar. These injectors are used as replacement for Ford Racing redtop injectors which run at 2.7 Bar.
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