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Old 07-25-2014, 01:53 AM   #1
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Question Strange wideband readings. Swinging rich to lean to stoich

Here's some back story. I bought my NB1 about 1.5 years ago and it performed well. A few months later I took it on a road trip and I noticed my head was bobbing slightly cruising at highway speeds. I replaced the TPS, the forward O2, and nothing changed so I assumed I was imagining it and disregarded it.

Fast forward to earlier this year. I picked up one of those really cheap BPSX widebands off of someone locally for cheap (he didn't want to cut the display into his gauges) and I installed it with ease. Immediately I noticed that the AFR was reading rich to lean to stoich to lean to rich to lean etc. I called the company and they said it didn't sound like anything they'd heard of with their widebands and they'd gladly swap out to make sure. I installed the new one and the same thing was still happening so I borrowed my friend's AEM gauge to make sure it wasn't just my cheapo system and got the same results.

I am now ready to install the turbo system I have been piecing together for the last few months and I want to fix this possible lean-out-and-blow-my-****-up issue before I boost it. When I put the car into open loop it reads a solid (though slightly rich) AFR with no swinging. I have searched all kinds of forums and asked a fair number of colleagues and I'm still at a loss.

Anyone know what this could be? I'm in a 99 base with no major mods other than a 4-2-1 header and a custom in-bumper cold air intake. Attached is a gif of the idling AFR in traffic in closed loop. Notice that the RPMs do not change with the AFR change.
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Strange wideband readings. Swinging rich to lean to stoich-al75h.gif  

Last edited by teefish1; 07-25-2014 at 12:48 PM. Reason: I forgot how to paragraph because I'm a noob and it was late.
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Old 07-25-2014, 12:27 PM   #2
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1) Put it in paragraph form.

2), Yeah, that's how it should work at idle or cruise. If you tip into the throttle it should then stabilize and go a bit rich, then when you let of the throttle it should start cycling roughly around stoich again.

3) If I am reading this right, that company is a pack of fools.

4) Wow. Just Wow.
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Old 07-25-2014, 12:45 PM   #3
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3-way Cat Converter requires AFR to tic- tac lean to rich at about IIRC 1 Hz for maximum
efficiency. So stock ECU drives that using a narrow band O2 sensor. Typical is 14.2 to 15.2.

I don't think that is what caused your head to bob. Radio, maybe
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Old 07-25-2014, 12:57 PM   #4
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It's now in slightly readable format.

I still don't think it's normal. I've driven a few other Miatas and none of them had the same feeling. Isn't 16-17 way too lean?

They're not the best but they didn't give me any issue and their product is worth the money.
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Old 07-25-2014, 01:12 PM   #5
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See my previous post as what is normal in FRONT of the cat conv.

You didn't mention 16:1 in your original inquiry.
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Old 07-25-2014, 01:23 PM   #6
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Watch the .gif...
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Old 07-25-2014, 01:43 PM   #7
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If you're about to turbo it, what kind of plans do you have for engine management? If you've got an aftermarket ECU go ahead and install it, tune the car, and more than likely that'll take care of the issue.
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Old 07-25-2014, 02:52 PM   #8
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I'm using the old Powercard Pro and BTC method until I have the means to go standalone. Any other ideas?
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Old 07-25-2014, 03:46 PM   #9
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I don't have anything else constructive to add beyond if stable in open loop, and that's where you're going to blow stuff up. As far as closed loop bouncing around, it can make the car stumble or lose a little gas mileage but I don't think it's really going to hurt anything.

Like DNMakinson was saying, the stock O2 sensor is anything but precise. I wouldn't be terribly surprised if this wasn't fairly normal on the stock computer with boltons.
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Old 07-25-2014, 03:49 PM   #10
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I guess I need to look into going standalone a lot sooner than expected. Mostly I just wanted to know if this was an actual issue because it gets awfully lean and it may not be ideal under boost.
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Old 07-25-2014, 04:29 PM   #11
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Do you have your bandaids installed in the car already?

Factory ECU will oscillate to try and please the hippies in California.

If you're truly worried about blowing your car up, just take the time and do it right. This will all but guarantee the life of your motor. This includes using a real ECU to control the fuel/timing. Install larger than stock injectors and tune reasonably.

AFR up around 16 are only scary in boost. Other than that it's saving gas, lol.

AFR gauges on a stock ECU are for entertaining the passenger, (light show).
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Old 07-25-2014, 04:42 PM   #12
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Yeah the bandaids are already in and everything else is fabbed up and ready to go in as well. They don't do anything yet as there's no positive pressure so it's not caused by the cheap boost controllers. It just dawned on me that my car was born a Cali emissions car but by the time I got it it had a 49 state manifold. Could the Cali ECU be doing this?
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Old 07-25-2014, 06:02 PM   #13
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Yes it's the ECU. That is how they work.
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Old 07-25-2014, 10:34 PM   #14
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Any consolation my afrs are crazy until I get into boost, then they stabilize. But I also have a massive vacuum leak I'm fixing this Sunday.....
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