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Old 10-29-2009, 08:14 AM
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O2 sensor question

If I correctly understand how these things work then switching during operation is more important than peak voltage? My question is about a 1999 F250 Super Duty Triton V10 I just bought that has an annoying miss in it that I can not find, the O2 sensors were sooty and the truck ran just awful but improved greatly after testing the sensors with a propane torch removed the carbon from them. I noticed that a new sensor reads a peak of .9 volts while both sensors from the truck read a max of .5 volts, the reaction times are about the same however. Testing on the vehicle I still get the max reading of .5 volts or slightly less and rapid, apparently normal, switching. Are these readings OK? I was under the impression that as long as a reasonable voltage was being produced then the switching rate or reaction time is what is important and higher voltage readings do not necessarily mean better readings?


Do I have this right or could those .5 volt peak readings be a cause of the problem which is a slight miss under load at less than 1500 RPM, it runs great at open throttle but did not until I cleaned those sensors.

Things I have done so far,

Fuel pressure is OK
Catalytic converter is open
New plugs and coil packs checked
EGR valve checked for proper operation
Air filter clean
No vacuum leaks


The only code I can get is 0P300 which is "random multiple cylinder misfire detected"

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Old 10-29-2009, 10:27 AM
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a swing from .1 to .9 is normal. A max of .5 would cause issues in most cases. If you have access to a good scan tool I would look at fuel trims also.
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Old 10-29-2009, 11:17 AM
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I don't have any way to scan any info but trouble codes, so you think maybe a .5 peak reading could cause a problem then? In the past every time I have identified problems with an O2 sensor it was switching erratically or not at all (like these two were before I burned off the carbon) but maybe I have learned something new here. Thanks.
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Old 10-29-2009, 04:55 PM
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Well $150 worth of O2 sensors later and still no change!

When I first removed the plugs they had 118,000 miles on them and were quite worn with no. 5 being black and sooty, no. 5 also had a bad boot on the coil pack which was replaced. Now after about 150 miles on the new plugs they look exactly like they did when they were first installed a week ago (including no. 5), I mean they look as if they just came out of the box so it must be running lean? It idles smooth and a vacuum gauge is steady at idle, above 1500 RPM it will pull really strong and smooth but below that it is jumpy and obviously misfiring.

Any suggestions, anyone?
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Old 10-29-2009, 11:40 PM
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The first thing I would do is call ford with the vin number,you might get lucky and it be a free recall(pcm update,etc)

If there is no recall.You really do need to get a scan tool and record the freeze data.
If will show the rpm,speed,sensor readings when it throws the P0300.
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Old 11-26-2009, 08:47 AM
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wiring harness?

did you check to see that none of the wires to the coil packs or sensors are damaged? inside the tubing the engine heat can cause wires to heat up and fuse together causing impossible to find and niggly problems. first suspects are near to the exaust parts.
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Old 11-27-2009, 07:36 AM
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I guess I should have posted when I found the problem, What it turned out to be was one of the small stainless lines going to the DPFE was plugged with carbon causing the EGR to malfunction even when it checked good itself.
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