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Old March 18th, 2006, 10:04 PM
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Oxygen Sensor Testing

I just tested mine using the method outlined in the EFI test procedure:
http://www.rangie.com/files/167_394.jpg test #27, 27a
http://www.rangie.com/files/167_395.jpg Test #27b

I only did test 27b since it was simple and I was clear on what was necessary. What's not so clear are the results.

Both results were changing the whole time. The left hand result was staying at .013V quite a bit and fluctuating up to .8-.9V. The right hand result was fluctuating constantly from .023V-1.250V The EFI procedure says that the voltage should be fluctuating between .5-1.0 something doesn't look right here already.

I'm not exactly clear on how to do tests 27 & 27a.

On 27 it shows the ohm meter connected between the #4 wire and the 87A slot of the fuel pump relay plug. There is also a line connecting wire #4 to the 85 slot of the relay plug. Does this mean it needs to be connected with a separate wire while performing the ohm meter test?

On 27A, it shows a loop between 87A & 30 in the relay plug. Like above I assume that means they need to be connected while measuring the volts at the O2 sensors, correct? And how exactly do I test the voltage at the O2 sensors? Do I have to go through the insulation on the wires?

I'm running the original O2 sensors with 89k on them.
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  #2  
Old March 18th, 2006, 11:45 PM
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The test 27b is testing the actual mixture output that the sensors are giving.

The voltages may have been a bit off, but the high and low really don't matter. The key is that it IS switching. The computer is looking for a specific voltage about half-way, generally about .7v or so. Anytime the voltage is below that the mixture is lean and the computer will bump the fuel up a bit. Anytime it is above that, the mixture is rich and the computer will bump it a touch leaner. It never actually
stabilizes, but constantly switches around.

The left side is either running lean, or the sensor THINKS it is running lean and the engine is trying to richen the mixture. If the heater element isn't working, it would cause that as the sensors just don't work until they hit 300 degrees or so.

Tests 27 and 27a are moronic ways of doing things. If you can get to the sensors themselves it will be easy to check.

Instead of test 27a.... With the ignition switch on and the relay in place, the red wire going to the O2 sensors should be a 12volt power source when the key is on. Either a test light or voltmeter would be fine for this. Since you are getting voltage returns on both sensors, it is probably going to test ok here too. You can do this by disconnecting the sensors themselves and testing the truck side of the connector. They have a removeable connector on the wiring about a foot away from the sensors.

Replacing Test 27 may be a bit trickier, as you want to measure the ohms through the heating coil between the red and white wires on the sensor. It should be between the 2.5 and 6 ohms as shown. If you get infinite resistance, the heater is busted and the sensor needs to be replaced. I'd check the resistance both cold, then after driving for about 5 minutes when its heated up. It should be within the 2.5-6ohm range (or close to it), but it should also go up or down after getting heated up..... depending on how the heater works. This might get tricky, only because the exhaust will be hot and you need to be careful not to burn yourself.

But I would also do another test. I'd drive the truck around for about 15 minutes or so, this should get the exhaust and o2 sensors heated up enough by the exhaust itself. Then do test 27B again on the left side and see if the voltage switches around more actively, similar to how the right side is acting. If this is the case, that will confirm a bad heating element in that sensor.


-Hans
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Old March 19th, 2006, 02:30 PM
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Ok well I got test 27 completed and got a result of 5 Ohms so that's OK. I wasn't able to get my ohmeter leads both into the sensor plug so I just used the #4 ecu plug wire and the 87a fuel pump relay slot.

Then I tried testing the voltages (27a) as you suggested and got a reading of zero volts on both sensor plugs. The truck side of my plugs have a blue wire that matches up with the red wire coming from the sensor. Any part of the engine should give me a ground right? I even tested all three sockets on the truck side plug just to make sure I wasn't doing something stupid.

Then after driving about 15-20 minutes I retested the voltages. The left side tested the same as before fluctuating from 0.14 - .998, but spending more time below .5. Today the right side was only giving me a static reading of zero volts. I'm pretty sure I tightly reconnected both plugs but I'll have to double check when the truck cools down.
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Old March 19th, 2006, 04:42 PM
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Those charts in the Rover procedures just keep confusing me. But, honestly, if you are getting voltage reading back from the sensors in test 27b it means they have power being supplied to them and you don't even need 27a.

Everything so far tells me that your right side sensor is ok (at least, once you get it reporting back voltage again) , and I think the left side one may be also. There is really only 1 test left to do to finally isolate if it is a sensor fault or something else wrong. This is to swap the O2 sensors between the left and right side, but it WILL be a pain in the behind to get those sensors out at that mileage as they are easy to strip. Only use a 6 point socket or 6 point box wrench, don't use an open ended wrench or 12 point socket .

If, after you swap them, the left and right sides is still acting the same as they are now, then your sensors are definitely fine and the engine is running lean on that side and we start looking for that problem. If the sympoms follow the sensors, then I'd replace the one that is currently in the left side.

At your mileage, I am starting to think that you probably have some clogged/dirty injectors instead of a sensor issue. Maybe low fuel pressure too, and your other issue might be related to this. Both sensors are registering the mixture, it's just that one is reading lean and if the heater was bad in the left side it would have been acting different after a 20 minute drive. E

-Hans
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Old March 19th, 2006, 09:57 PM
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Thanks again, Hans. I just re-tested the right sensor and it was reading just as it was before. Now begins operation liquid wrench. I sprayed down both sensors as they look like they aren't going anywhere without a fight. I'll do that for a few days while I find an appropriate wrench or socket and report back with what I find.
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Old April 1st, 2006, 03:30 PM
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Ok Well I think the O2 sensor fits a 17mm wrench, but it doesn't seem as tight as it should be. I couldn't get the 16mm on the sensor but I couldn't see really. Has anyone removed the sensors? Know what size they are? The auto parts stores only have a 7/8" - 22mm oxygen sensor socket and that's definitely too big. I bought some six sided flare wrenches, but I'm going to need some extra leverage to get these things to turn.

I've sprayed them down for at least 3 days with liquid wrench, doesn't seem to have helped much
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Old April 1st, 2006, 10:24 PM
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I used a regular socket on mine, but to be honest that was when I was swapping the Y-pipe and had the exhaust next to the truck. I can't remember which size I used, but have you also tried SAE sockets? Maybe something like 7/8 or so would fit better if it seems to be between metrics.

-Hans
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