I know this is an old thread, but after new spark plugs and wires plus adjusting my TPS for a third time, I decided to replace the O2 sensor on my '90 GXL. It is still only $18 for the universal Bosch sensor #11027 at AutoZone.

I am really amazed at how much of an drastic improvement this simple change made. No more chugging at part throttle or tip in or feathering during corning. It has a much smoother and consistence idle speed, much smoother pulling under load from lower RPM's as well. Heck there is even less engine braking available on deceleration while in gear. The car has 80K miles, but the engine was replaced 11 years and has 26.7K miles on it. I have no idea how long the previous sensor (custom fit NGK/NGT unit) was in there - but it must have been way too long. I should have changed this when I did the tune-up this spring. For only $18, it maybe worth changing the O2 sensor at the same time as the spark plugs.

I sure hope it helps to improve the mixed city/highway gas mileage as well, since the last couple of tank fulls have only been returning 16-17 MPG :(

Just my luck to get the darn car finally running near perfect and I will have to put it into storage in about 6-weeks. Oh well.

As a follow-up and I know this is on my FC, but for the 726 miles before the O2 change I was averaging 16.8 MPG. In the 669 miles since the change my average has been 18.5 MPG - that works out to a 10% increase with the same driving style and over the same route.

Now about the same time as when I changed the O2 the gasoline was also changed over from summer to winter blend so that could also have a big impact on the MPG as well. But based on how much better the engine is now running, I think most of this was from the O2 sensor change.

May fix 3K RPM hesitation - it did for me
 

I was again having the 3K-RPM hesitation/bucking during mild acceleration. So I repeated the secondary throttle plate adjustment that I noted in my original post.

Secondary throttle plate adjustment
First, I moved the secondary throttle plates by rotating their exposed linkage. The plates were noticeably sticky coming off the closed position. I adjusted the secondary stop screw 1/6 of a turn tighter.

The sticking AND the 3K hesitation have again disappeared. :nod: