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Does a 90° O2 sensor adapter act as a spacer also? From what I understand adding an O2 sensor makes the engine run leaner since it senses less o2 (But I don’t know if that’s how it works so please correct me!).
If I add this 90° adapter will it make my engine run lean enough to hurt it? Reason for using is because there’s not enough space with my new headers for the O2 sensor now.
In order for the O2 sensor to work correctly, the head of the O2 sensor should be located in the exhaust stream. This helps keep the sensor itself hot, which improves it's effectiveness. If you put something like a 90deg bend and relocate the sensor, it's not going to work correctly.
If you've installed headers and the position of the sensor in the headers doesn't allow for installation, you need to seal off the old O2 bung and place a new one. You can purchase O2 bungs all over the internet, so that isn't an issue. You drill a hole in the headers at the collector, then weld the new bung in place, ensuring a proper seal at the weld. You don't want air to leak past small openings in the weld, which will cause the sensor problems and give false readings. The bung should be placed parallel to the ground when being installed. If you place it above on the collector, you'll pretty much have to drop the header to change out the sensor. If you place it straight down, you'll have issues with clearance and with moisture forming (or collecting) on the sensor, which can cause rust as well as fouling the sensor itself.
All-in-all, if you have an electronically controlled fuel injected vehicle, you need an O2 sensor. The sensor is going to give you the best reaction to fueling needs, the best performance, and the best economy. There's just no way around it, the engine needs it if you want it to run correctly.
I would not recommend using a 90° adapter here.
As @Paulster2 mentioned in his answer, the O2 sensor needs a steady stream of exhaust gas in order to function as expected. With the 90° elbow, the exhaust gas that runs across the sensor will be slow and haphazard as the elbow will act as a pocket that traps some of the incoming exhaust stream. This is not the condition that the O2 sensor is designed to operate in - if it were possible to do so then manufacturers would have done this a long time ago for the sake of reduced exhaust restriction (cost-permitting).
Also, a properly installed O2 sensor would not cause the fuel management system to operate leaner.
Any fitment other than perpendicular to the exhaust tube, and with the sensor standing upwards, is going to have a worse effect than that. If you've got limited space, you can mount the sensor sideways, but the closer it is to horizontal, the worse it will clean out internal deposits. Gravity will hold them there, and if the sensor doesn't get hot enough, they won't burn off.
The tip of the sensor is made specifically to get a good diffusion(mixing of gases), when in the exhaust stream. If it's not in the stream, it will react much much slower, and possibly less accurate.
Mounting it wrong may give you a leaner(or richer) running system, but not by a safe amount, a fixed amount, or a known amount. E.g. if it runs slightly leaner at idle, it may run far too lean at cruise speeds.
Instructions usually explain how the sensor must be mounted, and how far from the engine. Respect these instructions, or you'll get an unpredictable unstable sensor output, that can develop worse over time. If you want to play with the mixture, rather tweak the sensor's output than it's location. And even that required skill, experience, and knowhow. I wouldn't recommend it. O2 sensors and their dependents are far more complicated than analog MAF or MAP or TPS sensors.
