'90 Trooper oxy sensor?

Group, My '90 Isuzu (4 cyl. manual trans) Trooper has an idle problem. When I shift into neutral, (at a red light) the idle goes up and down from 1000 to 1900 rpms.
Up and down, up and down.
I just had a new head gasket installed. The shop said I needed a new oxy sensor since the old one was contaminated. They said if I didn't change it the worst would be the check engine light would stay on. I told them to throw it back in there, and haven't seen the check engine light yet. But the rpms sure do oscillate.
What do you guys think. How hard would this be to swap out myself, with minimal time, tools and initiative? where is this puppy located?
Thanks!
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I know on my jeep that whenever the rpms do that and nothing else is noted, it usually mean that the idle air valve that controls the idle is all carboned up. I have seen it happen to many makes and most of the time it is the IAC. it is usually on the throttle body and usually next to the TPS (throttle position sensor)

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Yea, that sounds like the same thing I experienced on an old Ford Probe I used to have.. had the throttle body cleaned and the idling smoothed out tremendously.
Chuck
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I forgot to mention that the Trooper is fuel injected. What else can I look at, or is it the oxygen sensor? Thanks guys

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It's probably not the oxygen sensor. 1. An overly hot engine will cause the idle to fluctuate as there is a valve that senses that and increase the idle to cool the engine down...then the idle gets too high and drops back down..and back up... 2. An idle set too high will also result in a fluctuating idle at standstill. 3. I vacuum leak that allows too much air to enter the throttle body will cause a fluctuating idle as well.
Do you have problems keeping the engine going when it's cold...won't hold an idle until it warms up? Maybe someone cranked the idle up to compensate for it? Let me know, Jim in Parker CO 88, 2.6ltr, 226k miles
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IF you just had the head gasket changed, it is probably a vacumn leak. It sounds like the shop missed a vacumn line, or something leaking around the intake manifold gasket, especially if it was running ok before you took it to the shop.

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