Upstream O2 sensor?
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Upstream O2 sensor?
Hey guys, I have a '91 f150 I just bought from my dad. He had the exhaust reworked by a hack. I am currently doing it "right" starting from the manifolds and going back all new, including the cat, and new flowmaster muffler. I bought the new downstream sensor. I was cutting the old stuff off, and was unbolting the exhaust flanges at the manifolds when I noticed a sensor in the passenger side exhaust manifold right above the outlet flange. All of the wires were cut off of it about 1/4" from the top of the sensor. I cannot find a quick connector or loose wires that might have been hooked to said sensor. Is this an upstream o2 sensor? My wiring diagram in my haynes manual only shows one o2 sensor. If so, where do I look for wires to hook to it when I replace it? I am assuming some where around the bell housing, but can't find anything. My only thought is to pull the plenum, as it was just off when I changed the injectors. I don't know what I'm looking at or what to do. I am a carb. guy. Any help will be greatly appreciated!
Logistics Pro
Joined: Mar 2005
Posts: 4,698
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From: Mexico, Missouri
There was only 1 HEGO sensor, located in the (H) of the exhaust before it entered the CAT on the early Fords from 87-96, I believe, before Ford went with OBDII. That's when they added another 3 sensors. I know another FTE can correct me on this, as I'm a bit sketchy on 3 or 4. I knew of (1) in each exhaust manifold, and one after the cat. Could have also been one before the cat and another after...but you are right about the manual showing only one for the early fords.
As I was just under mine this evening, I know after looking at it numerous times it goes into the wire loom just above the pass. side, inside fender well, sorta where that coffee-can looking thing is. It follows up the wire loom shared by the starter "HOT" wire from the POSITIVE battery cable then goes in to a weatherpack connector near the Fender-mounted Relay.
Hope that helps you!
As I was just under mine this evening, I know after looking at it numerous times it goes into the wire loom just above the pass. side, inside fender well, sorta where that coffee-can looking thing is. It follows up the wire loom shared by the starter "HOT" wire from the POSITIVE battery cable then goes in to a weatherpack connector near the Fender-mounted Relay.
Hope that helps you!
Lead Driver
Joined: Nov 2002
Posts: 9,557
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From: Concord, NC
Upstream and downstream is relative to the catalytic converter. Only the OBD-II trucks had a downstream O2 sensor, as the others have said. The original location for the O2 sensor on your 91 is just before the two pipes enter the first cat. The later models used two sensors mounted in each exhaust manifold. The O2 sensor you found with the cut wires is probably just plugging the hole in a replacement manifold since it was not used for your '91.
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Thanks guys you have saved me a lot of unnecessay work. Dad bought the truck with 80k on it, it now has 155k on it. I suppose it is possible the first owner had to change out the manifold. I don't remember dad ever having to. Now hopefully I can find time to get the exhaust done this weekend! I appreciate it!
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