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impact of headers on o2 sensor


 
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dave_g_WJ
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Joined: 09 Nov 2006

PostPosted: Thu Nov 09, 2006 9:29 pm    Post subject: impact of headers on o2 sensor Reply with quote

Hi, i have recently had the 'cracked' exhaust manifold replaced with some headers on my 4.0L WJ. From then on, a few times a day, (at good speed) the thing majorly cuts out, chokes and shudders. It often recovers before i loose too much speed with a bit of nursing of the acceletator. Idle is pretty rough overall, and power is lower than usual.

The DTC indicates the oxygen sensor is bad, which makes sense, as this has been moved off the old exhaust to the new.

My real issue may be that the old exhaust effectively had 6-1, with the sensor in the single pipe. The new headers are 3-2-1, (the 2-1 join is under the trans), so the sensor is plugged into one of the two pipes. I am guessing the the 'oxygen' ratio will not matter much, running off 3 banks instead of 6.
I am wondering about the sensor getting up to temperature - as 3 banks are probably not going to make as much heat as 6. It's 4 wire sensor with a heater, so unless the ECU only 'heats' during startup, that too should be ok.
I connected an analog voltmeter up to the sensor and have been driving with it on the dash. It appears to work ok (swings around from .1 to .9v)for about 2-5minutes, then it goes 0v pretty much from then on. I need to turn the engine off for about 1 to 2 minutes, then the sensor starts operating again (for a short time). This would explain the rough idle, and underpowering, but could it contribute to random HUGE power loss? It's faulty, so i need to fix it regardless.

Does the sensor heating only happen for a short time during startup?
My next step is to connect another voltmeter to monitor the supply voltage to the sensors heater circuit, in an attempt to establish if the ECU turns the heat circuit off once the warm up period has completed. If this is true, i am still unsure if this particular sensor will happily operate within the correct temperature range if i wire it up to be always heating. I have had no luck trying to find any spec's for oxygen sensors.

Moving the sensor down to the 2-1 is probably pointless, as it's going to be colder.

Replace the sensor? Hard-wire the sensor heater circuit? or go through the pain of getting an original manifold?
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dave_g_WJ
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Joined: 09 Nov 2006

PostPosted: Fri Nov 10, 2006 5:21 pm    Post subject: Reply with quote

I checked the supply voltage to the o2 sensor and found it's always on (with the engine), and remains on, even when the sensor signal faults and drops out to 0v. This means the headers are not (and will not be) a problem. The sensor was probably damaged during the change over.
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