Please help with o2 sensor readings

Hello there and thanks for reading. I've been having some major performance issues with my 2006 Mazda 3 2.3L for a while now and I'm still trying to figure out whats causing the problem. The car has a lean condition and a decent lack of power. The Short term fuel trim looks ok, bounces back and fourth without really going higher than 7+ or - but the long term is constantly at or above 10% usually maxing out around 17%.

Symptoms:
Heavy clutch and labored shifting (the clutch/shifting isn't always heavy but the worse the symptoms get the worse the feeling gets. Sometimes its really not that bad but it still isn't correct as to the way it drove when everything was ok.)
Power steering is stiffer. (Again not always the case)
Lack of overall power/ struggles under WOT.
Can hear the fuel pump running. (It should be noted that I installed a new fuel pump to try to fix this performance problem but it didn't help and the new fuel pump makes noise right from the first time it turned on. I performed voltage/voltage drop/ground tests on it and everything looks fine afaik. The voltage drop was 0.37v and its my understanding that anything less tan 0.5v is ok.....unless I'm mistaken. I also found that the hose that leads from the fuel pump to the fuel rail was a little kinked in one area so i replaced it with a good one but this didn't help anything.)

I've been all over the car with water/propane/brake cleaner trying to find vacuum leaks and I'm pretty confident there are none. There was a little soot as well near the exhaust manifold too so I changed the manifold gasket but I'm unsure if there are any other exhaust leaks, it doesn't sound like it though when I'm driving but I'm going to try to do a smoke test at some point to be sure.

So onto the O2 sensors. I just got a decent scan tool recently so I can finally look at some numbers although I don't quite know how the downstream sensor should look. The upstream sensor is an A/F sensor and its readings look good. It osculates its current usually between -.08 and +.4 at idle and it doesn't really get to crazy but the downstream O2 sensor just seems funny to me. It will read very rich almost all the time in around the 0.845v range give or take a little but then if you just let it sit it will drop down to 0.1v or below and sit there. Am I correct to assume that it should not go below 0.1v? It doesn't hit zero but it will fluctuate between 0.07v and 0.1v but its usually below 0.1v. On the flip side if the numbers are sitting up in the rich range and I snap the throttle a bit, it will drop right back down to around or below 0.1v then it will sit there for about 10-30 seconds then climb all the way back up to the 0.8 or 0.9v range.

I find these readings strange and I'd like to say the sensor is pooched but I would like to hear your guys input on this. Do you think its the sensor or does this actually sound like unmetered air/exhaust leak/catalytic converter territory?

Thanks again

Ok so basically the engine just won't deliver power............. I wouldn't be concerned about the readings you are getting from the rear O2 sensor.I can see why you suspected t the fuel pump and hose and it is true a blocked cat will cause severe loss of power but I would want to look at comparing the fuel trim numbers at idle with those at higher rpm. Also if your scantool will read it then airflow from the MAF.
If it were in my workshop I would investigate a dirty MAF first , low power is classic symptom of a dirty MAF. On some engines a quick and dirty test on the MAF is simply to disconnect it and force the engine to use a fall back fueling strategy, this won't work on some engine types but on many engines if engine runs better with the MAF disconnected then the MAF is dirty or failed.

Thanks for getting back to me. The MAF is new as well but maybe ill check the voltage to see if its working right. Also ill check the fuel trim data like u said and ill post more info tomorrow.

Without a pressure gauge quick and dirty (and anti-social) test for a plugged cat is simply to remove the front O2 sensor, if the exhaust coming out just about blows your head off then the cat is history.

Andy.MacFadyen wrote: Without a pressure gauge quick and dirty (and anti-social) test for a plugged cat is simply to remove the front O2 sensor, if the exhaust coming out just about blows your head off then the cat is history.

:lol: :lol: So accurate, and true!

I agree with Andy's suggestions about a fuel delivery problem or possible plugged cat. Some scan data during a WOT run could definitely help point us in the right direction. We'd be interested in watching that upstream A/F sensor, the downstream O2, MAF, MAP, calculated load, absolute load, engine speed, commanded equivalence ratio and equivalence ratio. As an example, here's an shot of some data I took off an '08 Mazda 6 with a 2.3L:



With those PIDs, we'll be able to tell if the engine is getting enough fuel under load, as well as check for a volumetric efficiency issue, an intake restriction, AND confirm the rationality of the downstream O2.