Hello, I have a 1998 Saturn SC2, Automatic (136XXX miles). Lately I have been getting bad gas mileage and have noticed a loss in performance. Since these problems have started, I have replaced the ECTS (Brass) and the upstream O2 sensor (Bosch). I do have one SES light, P0420. I am just curious about possible solutions (Besides buying a new Catalytic Converter).

Thank you, John

 

When's the last time you just tried a simple tune up on it? Ever? Maybe try running some Techron fuel injector cleaner through a couple of tanks of gas and clean the throttle body. Basic common sense things to do on a regular basis. Tap the cat con with a hammer and see if you hear stuff rattling around in there. If so, that could very well be your problem. That crap can also come loose and clog your muffler. Same test for it as the cat con.

 

my s cars run like crap on the bosch O2's, I prefer the denso part used as the saturn oem part -- only you save 50% by not buying it thru GM.

It's entirely possible that the rear O2 is sufficiently poisoned/fouled that it cannot properly sense the true O2 %age in the exhaust, thus reading low and throwing a catyl. below effic. code.

I would try Rube's test first. Also, if you have access to a scanner that can display data in realtime, you can watch what's going on w the two O2 sensor readings.

136K is not unreasonable for the lifetime of the front O2.
The lifetime of the rear O2 all depends on what blows past it.....

Did you throw the code first or have the performance issues first?

 

I haven't done a tune up yet, but I checked the condition of the plug wires and coils. The prong for the #3 plug wire on the 2nd coil had some minor corrosion, but the wires look new. I'll have to pull the plugs and take a look at them, as well as the hammer test for the Cat and muffler. I will surely clean my throttle body (when i buy a new gasket) and get some techron in my tank.

BTW, my car also idles at high rpms sometimes. Around 1600rpms when shifted to park after driving for a while. Also, when I decelerate my car sputters a bit.

Would a faulty downstream o2 sensor cause bad gas mileage?

 

You might have a vacuum leak somewhere. Take some WD-40 or carb cleaner and spray it around the intake and throttle body while it's running. If the rpm's increase, then you need to find where it's "sucking wind". I THINK a funky 02 sensor will screw with the mileage, but am not sure.

 

front 02 greatly influences air/fuel ratio; rear O2 is just a "policeman" to ensure the cat is doing its job. ODBII was mandatory for 96---one of the benefits was no more tailpipe testing if car's computer said emissions were within range.

Hopefully you replaced the ECTS with one sourced from GM , and also replaced the connector/harness which is prone to corrosion and will screw up the reading badly. The aftermarket ECTS's are in general not calibrated the same as the GM part and therefore the air/fuel mix is off because the coolant temp reading is off. It's like $45 for both. Wiring harness is dealer only. Any GM dealer can order them for you.

Does the idle return to normal on its own within like 10-15 seconds? Or is it stuck there until you pulse the gas pedal? And does that bring it down?
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You still did not answer my question of which came first? The code or the driveability issues.
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Per Rube, start w a throttle body cleaning and see if that addresses the 1600 rpm in park issue (assuming it comes down eventually).

You may indeed have a vacuum leak around the throttle body gasket where it attaches to the intake manifold, or at the intake manifold gasket. The DOHCs did not have a reputation for this but the SOHCs were ill designed and would suck part of the gasket into the #1 opening. Per Rube, spray some carb cleaner around the throttle body gasket and intake manifold -- if the rpm changes, you've found your leak.

If there is a vac leak, this would cause the air/fuel mix to be leaner than the PCM intended, and the exhaust gases would reflect a lean condition, which would request a richer mix, then have it leaned out again by the vac leak, and ask for a richer mix.....total speculation on my part.

What rpm does your car idle at after warmup?