Unknown Problem

[taz4432]

Background: About a month and a half-two months ago my CEL would come on very infrequently and usually not for too long. Checked out the code and it was my Rear O2 Sensor. Mileage was same as always as was performance and I'm a broke student so I left it for the time being. Fast forward to two weeks ago - CEL comes on and stays on (same code as before - Rear O2 Sensor) but again, mileage and performance are the same or indestinguishably different. Around a week ago my mileage dropped like crazy (we're talking 3L/100km for 90% country roads and highway driving...so, it's significant)...something that usually happens when the Front O2 Sensor goes. Went to do my e-test, failed miserably. So, I ordered in a new Rear O2 Sensor from Mazda.

Last night I pulled the old one off and put the new one on. Drove the car home (from my buddy's place) and it drove fine, mileage hadn't gotten a ton better but it seemed to be improved ever so slightly. CEL was still on because I hadn't reset the ECU.

This morning I reset the ECU and let it sit for a while to re-learn idle and all that jazz. Everything seems fine, CEL is gone and away I go to work with a little detour to help it 're-learn' quicker. I start driving and I notice things are sluggish - betweein idle to about 5k RPM it feels like it's holding back and then after 5k it feels like boost/VTEC kicks in because the thing just comes alive and rips like it should/always has normally. Figured it was just the ECU learning and kept revving it out during my drive to get it back to normal. Seemed to work a bit better by the time I got to work but still sluggish.

Come the end of the day today, I get in and start my drive. Mileage is back to normal but it's still sluggish as hell (worse than when I reset the ECU). So, I go for a drive and as I'm accelerating my CEL comes back on and it's even more sluggish. It also doesn't 'return to normal/open up' until 5.5k RPM. I do a lot of driving in big trucks and this thing feels slower than just about anything I've ever sat my butt in but when it hits 5.5k RPM it's like *HOLD ON!!!!* and goes back to normal. Anyhow, I don't have a code reader and the one at Part Source is broken, so for now I have to go on knowledge/intuition. My plate sticker expires in a few days and I need to get this e-test done but more importantly, figure out what the heck is wrong with my car.

I've had a lot on my plate lately and am getting really frustrated with all the setbacks I've been dealing with related to cars and can't think of what might be causing this.

Any ideas?

P.S. Anyone in the Frankford/Stirling/Belleville/Quinte West/Trenton/PEC area with a code reader?

[-cj-]

Maybe go get your car e-tested and see what comes back? (Like, can you reset the ECU and have them test your car before the CEL comes back on?). The reason I suggested that is, 1) It might give you some hints at what part of the car is the weak link (IE: Maybe you have a clogged cat or something?), 2) If your repair ends up being expensive, you now have a limit on how much you can spend before you get an automatic pass w/ the emissions people...

I'm guessing plugs or clogged cat... or maybe you need another reset.. :) Shots in the dark... You gotta find out what code it's throwing.

[taz4432]

I believe it'll pass with flying colours. Cats are relatively new (<= 70k on either of them) and right now I'm getting the best mileage I've ever gotten in the car, so I know it's not dumping loads of fuel like it was this past week. The issue at hand now is something that definitely needs to be fixed but based on all variables that I can monitor, I don't think it will affect my e-test (which I am re-doing tomorrow).

My response to some posts on another forum:

"No way to check the CEL atm and I'm not spending $100 at a dealer/mechanic/Crappy Tire. May do 'use&return' with a code reader but not sure how strict they are one returns for that kind of stuff.

Funny thing is, I'm getting the best mileage I've EVER gotten in this car right now, so I know it's not dumping tons of fuel anymore. Doing my e-test re-test tomorrow so we'll see what happens.

Car has 175.5k but engine, tranny, exhaust (includes manifold and both cats), shocks, motor mounts, alternator, belts, i.e. EVERYTHING mechanical except TB, Front O2 Sensor and Clutch have <80k on it, if even that. (It's a coil-on-plug setup, plugs have <80k on them as well). This car has been the ultimate lemon from the day I bought it. Such a POS.

As for plugged cats, doubt it. Was driving perfectly fine (albeit dumping fuel) until I did the ECU reset. I can also rev it up perfectly fine (I do strictly rev-match/heel-toe downshifts and revved it in neutral while parked as well..no resistance). Now mileage is great but drives like crap. And, it behaves the EXACT same way every time. Hit 5.5k RPM and boom, like boost/VTEC kicking in except it's really just driving like it should again. "

[bman13]

Possibly spark plugs

[taz4432]

Possibly spark plugs

Will give 'em a look tomorrow evening. Would love to find out what this CEL is so I have a clue...hmm.

The only thing that makes me think *not* spark plugs is the fact that even after the new O2 sensor (as well as before), the car accelerated as hard as normal. Once I reset the ECU, it became a whole different beast. That's what makes me hesitant to think it's anything like plugs/plugged cat/etc...hmm. Will check as much as possible as time allows.

[Thrizzl3]

maybe the MAF or the distributor cap?

[Flagrum_3]

Alternate possible cause; it may be the purge solenoid valve as you have 'some' of the signs of a PSV failure or leak.

_3

[taz4432]

Alternate possible cause; it may be the purge solenoid valve as you have 'some' of the signs of a PSV failure or leak.

_3

Ohhh? I'll have to look into this.


My car is definitely running way too lean which is causing knock which is causing timing retardation which is in turn causing the 'lag' I feel. It explains my crazy good mileage (way less fuel used), my passing the e-test cause of a super high NOx reading and everything else. Now I need to figure out what's causing it. I'm buying a code-reader tomorrow which should hopefully provide some insight.

[taz4432]

Got the $100 Princess Auto scanner. Does freeze frames and all that. Definitely not your Mazda dealership scanner but it's pretty good for what it is.

P0031 --> HO2S Heater Control Circuit Low Bank 1 Sensor 1 (Front O2 which is actually a wideband and not O2)

P0037 --> HO2S Heater Control Circuit Low Bank 1 Sensor 2 (I think this was a stored code from before I changed my rear O2)

P0134 --> 02 Sensor Circuit No Activity Detected Bank 1 Sensor 1 (Front O2 again)

I'm going to do some electrical testing to see if it's the circuit but it all seems to point to just a bad O2 (wideband) sensor, but what's weird is that the car is running beyond lean.

Any kind of acceleration results in insane retardation/roughness/knock (I'm assuming). Typically failed sensors make it pig-rich. This leanness accounts for the super-high NOx on my e-test (passed everything else) and the fact that I'm getting the best mileage I've EVER gotten with this car. We're talking a drop of 2.0L/100km+ on country road driving (normally ~8-9L/100km average) , so hugely significant.

[-cj-]

Is that a factory wideband, like on the speed3's? I don't know very much about the non-speed's. If it isn't factory equipment, it's possible it fails in a different state... or perhaps it isn't in a failed state yet, thus the weird readings. A weird reading O2 sensor could definitely explain your problems.

[em3]

yeah it looks like a bad A/F sensor. The car doesnt know what the fuel mixture ratio coming out of your cylinders and its just dumping a whole bunch of fuel in to be on the safe side. better rich than lean right

[Cosmo77]

You have to test the front a/f sensor with meter that will read mA. The front sensor reads in mA and uA

[taz4432]

yeah it looks like a bad A/F sensor. The car doesnt know what the fuel mixture ratio coming out of your cylinders and its just dumping a whole bunch of fuel in to be on the safe side. better rich than lean right

I'm running dangerously lean. My car is knocking itself to pieces it's so lean. :S