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I need your advice. My second-hand, 145K-mile '95 G20 has poor gas mileage (~18 MPG), weak acceleration, and occasional hesitation. Also, it barely passed its last emissions test. I've had it almost 2 years, trying to nurse it back to health.

There is no Check Engine Light. Three garages couldn't find the problem. One said, "It's an old engine. Might have vacuum leaks, and it's not worth trying to fix." Discouraged, but I don't want to give up so easily.

Very slight improvement in mileage and response after getting a garage to do a throttle-body cleaning. Besides normal oil changes and adding fuel-injector cleaner about every 3rd tank, I have tried cheap & easy things:
- replaced the spark plugs with Bosch Platinum II's,
- replaced the fuel filter
- replaced the distributor cap,
- replaced the PCV, positive crankcase ventilation valve.
Wires & boots looked pretty new - no cracking - and a new set costs about $80, so I haven't touched those.

Next, should I mebbe try replacing the rear O2 sensor? Rock Auto has a universal on clearance for $20 vs. $100 for a new 4-wire; any recommendations? What's the best procedure? The link to Dabrits' explanation doesn't seem to work.

Or the TPS, throttle position sensor?
What about the EGR, engine gas recirculation valve?

Short of a full teardown/rebuild, what engine parts are easy or most likely to improve mileage or responsiveness -- valves, valve guides, piston rings? Some of these are on clearance at RockAuto, so I was wondering if maybe I should grab 'em. But I'd probly ask a garage to replace; I have zero experience with that. And to get all the possible worn parts would cost $$$.

I'm shootin' in the dark, guys. Thanks for any and all suggestions (besides getting a higher-paying job).

Al (Nashville)

P.S. I'm familiar with the modest power of a normal, stock G20; his was a replacement for the previous '95 G20 that I had bought new, which was totaled in an accident involving my teenage son. (Fortunately the airbag saved him from serious injury. It was partly my fault, since I had left half-worn snow & ice tires on it when not really needed. He hydroplaned and lost control on I-40 while changing lanes and braking one rainy February day.)

P.P.S. About 1500 miles ago I added Seafoam at the suggestion of an AutoZone salesman -- but didn't realize I was supposed to change the oil right after that! Haven't notice any oil leaks so far, but I'll get it changed pronto!
 
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