Bad O2 sensor?

[LeVeL]

Symptoms:
-It seems like my exhaust has been slowly getting louder... maybe I'm just imagining things
-I was slowing down for a red light a few weeks ago, not braking very hard, and as I downshifted into 2nd, I heard a distinct loud pop that seems to come from the front of the car. Before, during, and after the fact, the car drove perfectly fine. My brakes are fine. No issues with the clutch or shifting. Maybe a small exhaust leak causing a backfire?
-I have a rough idle, especially when cold. Cleaning the throttle body helped a little bit but only temporarily.
-When the engine is cold it revs unevenly. That is, there is hesitation and in neutral at a steady throttle the revs oscillate within about 100rpm
-Tonight it cranked over for a good five seconds before starting. It was warm too; sat for maybe half an hour after I'd driven it.
-Over the past year or so my mileage has been dropping. From an average of 27 or so I'd gone down to 25. Last tank was 22mpg. Just for reference, last year when I autocrossed I got 21mpg, so 22 is pretty bad when I'm not racing.
-No CEL/codes.

Thoughts?

edit: changed plugs and wires less than a month ago and it didn't seem to have much of an effect. Friend suggested I go back and double-check the gap so I'll do that

[_HighVoltage_]

It does sound like it could be the front O2.

Another possibility, easy to check and clean is the MAF - or mass air sensor. It should be somewhere between the air filter and the throttle body along the piping.

[Spectre]

Symptoms:
-It seems like my exhaust has been slowly getting louder... maybe I'm just imagining things
-I was slowing down for a red light a few weeks ago, not braking very hard, and as I downshifted into 2nd, I heard a distinct loud pop that seems to come from the front of the car. Before, during, and after the fact, the car drove perfectly fine. My brakes are fine. No issues with the clutch or shifting. Maybe a small exhaust leak causing a backfire?
-I have a rough idle, especially when cold. Cleaning the throttle body helped a little bit but only temporarily.
-When the engine is cold it revs unevenly. That is, there is hesitation and in neutral at a steady throttle the revs oscillate within about 100rpm
-Tonight it cranked over for a good five seconds before starting. It was warm too; sat for maybe half an hour after I'd driven it.
-Over the past year or so my mileage has been dropping. From an average of 27 or so I'd gone down to 25. Last tank was 22mpg. Just for reference, last year when I autocrossed I got 21mpg, so 22 is pretty bad when I'm not racing.
-No CEL/codes.

Thoughts?

edit: changed plugs and wires less than a month ago and it didn't seem to have much of an effect. Friend suggested I go back and double-check the gap so I'll do that

What year is this thing again?

[LeVeL]

The year was nineteen ninety nine...

[Spectre]

It should be OBD-II, then, so unless someone installed a rear O2 sensor eliminator, it should be throwing codes indicating the O2 failed if that was the case.

My guess is a vacuum leak that's been getting worse and possibly bad coolant temperature sensor. That said, replacing the O2 sensors on general principle if you don't know when they were last changed is usually not a bad idea.

[GRtak]

If it has an exhaust leak anywhere near the O2 sensor it can cause problems.

[syncview]

^and this sounds alot like a small exhaust leak since he also mentioned that the exhaust is getting 'louder'...

[73GMCSprint]

Look very closely for an exhaust leak somewhere before the cat and O2 sensor(s).

I had a VERY similar problem with my Cobalt. Exhaust getting louder, idling a little rough, decreased mileage and power, and no CEL whatsoever. I suspected an exhaust leak but couldn't find it at first. Eventually I found a split in the flex coupler between the exhaust manifold and the "downpipe" that was hiding in one of the seams in the coupler. I just couldn't see it until it got big enough. Replaced that and the problem was fixed.

[93Flareside]

Also check connections like between the exhaust manifold and Y-Pipe/down pipe.

[Canadian_Stig]

An old trick is to put talcum powder on your exhaust (when it's cold!) and then have someone else start it up. If you see Talc blown onto the floor, there's a leak.

Sounds like my Nissan - I had a Catalytic converter that decided not to work anymore and the baffles inside moved around a bit, making for inefficient exhaust flow. I had the lower fuel economy, occasional rough idle and I also failed our equivalent to UK's MOT because of high emissions. As the O2 sensor was last replaced at 100K mileage (and now I have 300K!) I replaced it, and with a New Cat in by my mechanic - the problem was solved.

[tquattro]

My guess is a vacuum leak that's been getting worse and possibly bad coolant temperature sensor. That said, replacing the O2 sensors on general principle if you don't know when they were last changed is usually not a bad idea.

A bit late, but +1 for a vacuum leak. I also replaced the O2 sensors in vain because of rough idling before finding the real culprit...a 10$hose. At least the O2 sensors were still the ones from when the car left the factory, so maybe changing them was not in vain.

[GRtak]

Any update level?

[LeVeL]

oops, just noticed the new replies. I haven't done anything to the car and yet most of the problems have disappeared A proper warm-up fixes the cranking, rough idle, uneven revving; the exhaust note might very well be normal and I'm just imagining it to be louder; no more popping noises; the mileage still sucks though - I've been struggling to get over 24mpg. I supposed that's what happen when you're revving 4500rpm on the highway and redlining it all the time around town

[Canadian_Stig]

I'm willing to bet you have a baffle either in your cat or your muffler that's loose. It's just back to where it should be right now...Go over a rough road and see if the noise / bad fuel economy comes back.

That's exactly what mine did. "Cured itself" for about 2 months.

[Tomaserater]

Hey, to fix this issue, what you do is you remove the camshaft, and then you drill a hole in every cylinder approximately 1 mm across. Saw your camshaft in half and attach both pieces to the blow off valve. Then pour liquid nitrogen (N20) in your gas tank and start her up, cheers.

[Labcoatguy]

Hey, to fix this issue, what you do is you remove the camshaft, and then you drill a hole in every cylinder approximately 1 mm across. Saw your camshaft in half and attach both pieces to the blow off valve. Then pour liquid nitrogen (N20) in your gas tank and start her up, cheers.

Keep your day job.

[93Flareside]

No no, he must weld the diff!

Like this!

[ScarFace88]

Hey, to fix this issue, what you do is you remove the camshaft, and then you drill a hole in every cylinder approximately 1 mm across. Saw your camshaft in half and attach both pieces to the blow off valve. Then pour liquid nitrogen (N20) in your gas tank and start her up, cheers.

[Tomaserater]

What you could also do is unscrew the steering wheels, and use it as a spare tire, then weld the spare tire to the blow off valve, and pour a red bull in your gas tank. I was seeing whp increases of about 50 every day or so, cheers.

[WalkerPro]

Without any trouble codes it is very hard to help you diagnose your issues. If your vehicle has over 80,000 miles on it, you may have Oxygen sensors that are causing a shift in Air Fuel ratio control. The front sensors on your vehicle are Walker 250-24302 Ultra fast light off sensors. The post cat sensors are Walker 250-24001 sensors. An exhaust leak can cause a backfire and also cause a rich shift in Air Fuel Ratio control due to excess oxygen entering the exhaust system. You may also have a failing fuel pressure regulator that could contribute to the hard restarting. If you have the proper equipment, you could check the fuel pressure (should be around 38-40psi). If you have access to a scan tool, check the long term & short term fuel trim figures. If they are in the Plus ranges above 6-10% THEN SOMETHING IS CAUSING A RICH SHIFT IN A/F CONTROL. If they are in the Minus ranges above 6-10% THEN SOMETHING IS CAUSING A LEAN SHIFT IN A/F CONTROL. Hope these suggestions are helpful. If you have any further questions or concerns please contact us and we will be happy to help here at Walker Products. Walker Pro