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Old 04-25-2004, 12:18   #1 (permalink)
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Why does my 2.9 run better W/O o2 senser

This has been an on going problem, No codes!!
Everything runs fine until everything gets good and heated up.
Then It burns Gas like crazy, I changed all modules and including
the ox senser. I still have the problem. But when I disconnect
the o2 sen it runs better.

Also does anyone have voltage measurements for the o2 sen ?
The sen itself has 2 white and 1 black wire.

Going out is a green, Gray/yel , Black.

This is 1987 Bronco II with 2.9

I have a manuel but the wire colors don't match.
Ant help out there? Tnx Ron
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Old 04-25-2004, 12:36   #2 (permalink)
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Re: Why does my 2.9 run better W/O o2 senser

Testing O2 sensors that are installed.

The engine must first be fully warm. If you have a defective thermostat, this test may not be possible due to a minimum temperature required for closed loop operation. Attach the positive lead of a high impedance DC voltmeter to the Oxygen sensor output wire. This wire should remain attached to the computer. You will have to back probe the connection or use a jumper wire to get access. The negative lead should be attached to a good clean ground on the engine block or accessory bracket. Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do. Check the specs for your meter to find out. Set your meter to look for 1 volt DC. Many late model cars use a heated O2 sensor. These have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, ground on the other, and the sensor signal on the third. If you have two or three wires, use a 15 or higher volt scale on the meter until you know which is the sensor output wire.

When you turn the key on, do not start the engine. You should see a change in voltage on the meter in most late model cars. If not, check your connections. Next, check your leads to make sure you won't wrap up any wires in the belts, etc. then start the engine. You should run the engine above 2000 rpm for two minutes to warm the O2 sensor and try to get into closed loop. Closed loop operation is indicated by the sensor showing several cross counts per second. It may help to rev the engine between idle and about 3000 rpm several times. The computer recognizes the sensor as hot and active once there are several cross counts.

You are looking for voltage to go above and below 0.45 volts. If you see less than 0.2 and more than 0.7 volts and the value changes rapidly, you are through, your sensor is good. If not, is it steady high (> 0.45) near 0.45 or steady low (< 0.45). If the voltage is near the middle, you may not be hot yet. Run the engine above 2000 rpm again. If the reading is steady low, add richness by partially closing the choke or adding some propane through the air intake. Be very careful if you work with any extra gasoline, you can easily be burned or have an explosion. If the voltage now rises above 0.7 to 0.9, and you can change it at will by changing the extra fuel, the O2 sensor is usually good.

If the voltage is steady high, create a vacuum leak. Try pulling the PCV valve out of it's hose and letting air enter. You can also use the power brake vacuum supply hose. If this drives the voltage to 0.2 to 0.3 or less and you can control it at will by opening and closing the vacuum leak, the sensor is usually good.

If you are not able to make a change either way, stop the engine, unhook the sensor wire from the computer harness, and reattach your voltmeter to the sensor output wire. Repeat the rich and lean steps. If you can't get the sensor voltage to change, and you have a good sensor and ground connection, try heating it once more. Repeat the rich and lean steps. If still no voltage or fixed voltage, you have a bad sensor.

If you are not getting a voltage and and like you vehicle has been running rich lately, the sensor may be carbon fouled. It is sometimes possible to clean a sensor in the car. Do this by unplugging the sensor harness, warming up the engine, and creating a lean condition at about 2000 rpm for 1 or 2 minutes. Create a big enough vacuum leak so that the engine begins to slow down. The extra heat will clean it off if possible. If not, it was dead anyway, no loss. In either case, fix the cause of the rich mixture and retest. If you don't, the new sensor will fail.
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Old 04-25-2004, 13:12   #3 (permalink)
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Re: Why does my 2.9 run better W/O o2 senser

Tnx for the reply.
I do have a question.
The o2 sen has 3 wires,,, 2 white 1 black.
So one should have 12v and one 0 volts and one sig to computer Right?
Could it be that I have a ground problem if I have 12 v on both
white wires and .04 on the black?
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Old 04-25-2004, 14:03   #4 (permalink)
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Re: Why does my 2.9 run better W/O o2 senser

The black wire is the sensor wire and should read around .45. The two whites are power and ground. Only one white should be 12v.
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Old 04-25-2004, 14:10   #5 (permalink)
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Re: Why does my 2.9 run better W/O o2 senser

Tnx,
Ok I did fix the ground wire and thats the part thar works on heating the sensor, ? right.
I took it up to 2000 rpm for about 3 min and the sig wire did swing from
.o4 up to 1 volt, but as soon as I reduce rpm it stays at .04 . is that right?
Tnx Ron
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Old 04-25-2004, 14:53   #6 (permalink)
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Re: Why does my 2.9 run better W/O o2 senser

Yes the Ground wire works for the heating portion.

You should see constant fluctuation after the sensor is up to temp. It should not be steady at .4 it should go up and down while idling. You definately want to make sure that the sensor is up to temp otherwise you will get the wrong readings. I would just rev the engine between idle and about 3000 rpm several times for a few minutes and then retest after you have finished revving.

If after you do this there is not any fluctuation in the voltage then that is a good indication that the o2 sensor is bad.

In your first post you say that they are new. Have you checked any other things that could be causing you to run the way you are?

For continuously lean O2 sensor readings:

1. Check sensor output wire for possible grounding. A ground will cause a false lean signal.
2. Check the MAP sensor for proper vacuum to voltage output. A high vacuum signal will cause a lean ecu reaction. (Don't forget to check manifold vacuum first!)
3. Clogged injectors can cause a false lean condition. A cleaning may solve the problem.
4. Water contamination will cause a lean condition.
5. Low fuel pressure will cause lean conditions at any rpm or load range. Be sure to check pressure at all driving modes.
6. Exhaust leaks, especially near the sensor can pull in air and cause a false lean reading.
7. Check for proper air injection system operation. The air pump should not direct air to the exhaust ports during closed loop operation.

For continuously rich O2 sensor readings:

1. Check the fuel pressure. High readings will cause rich conditions.
2. Leaking injector(s) will cause rich exhaust. A leak down test and/or a power balance test can usually reveal the leaker.
3. A contaminated or malfunctioning canister purge system can very easily put uncontrollable amounts of fuel into the intake manifold. Simply disconnecting the vapor hose can reveal this as your problem system.
4. Check vacuum to voltage readings at the MAP sensor. A low MAP output will cause a rich ecu reaction. (Don't forget vacuum readings again!)
5. A false tps signal can cause the system to go rich if the Ecu sees a high tps output. Check tps readings at idle and for a smooth rise to wide open throttle.
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Old 04-25-2004, 15:20   #7 (permalink)
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Re: Why does my 2.9 run better W/O o2 senser

Yet another question :)
Am I right in saying after its up to temp its called closed loop?
And if it is then I only have rich run problems when that happends.
You wouldn't know there is a problem until your going down the highway at 65 for a few min.
But you have given me a few things to check.
Also could the computer seeing the lean sig. from o2 sen and try to put more fuel to it? I guess that only makes sense.
But Tnx for all your help
Ron
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Old 04-25-2004, 15:25   #8 (permalink)
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Re: Why does my 2.9 run better W/O o2 senser

yes when it is up to temp it is in closed loop

It sounds like you are having rich run problems and should investigate those as listed above. It is possible that the ECU is seeing a lean signal from the o2 and is trying to toss more fuel at it.
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