1997 too lean at full throttle
 

Car is a '97 with 64k miles, stock engine other than complete Jackson Racing exhaust, including header.

I have two gauges telling me it is running lean at full throttle, about 16:1 according to my wideband. I also have a Ultragauge which shows the stock O2 sensor around .100.

There are no other symptoms, other than some mild surging now and then when the car is in closed loop.

From what I've read, the most likely sensor to cause that would be the MAF, but it seems fine. The Ultragauge indicates the MAF is working fine, there's no CEL, and if I disconnect the MAF the car stalls, which I hear it wouldn't do if the MAF was bad.

The throttle position sensor could also be the culprit, but it also seems to be ok, according to the Ultragauge.

As far as I know, no other sensor would cause this problem without having other symptoms.

There are no vacuum leaks that I can find, and my vacuum gauge reads 23 in hg at idle.

The car was supercharged up until a couple of weeks ago. I had a Powercard for engine management. With the Powercard the a/f ratio was around 10.5:1 at full throttle, almost too rich. This indicates there is no issue with the fuel system, since it was clearly capable of delivering all that was asked of it when the Powercard was installed.

I've had this issue only since I took off the supercharger, and I had it right away, though it seems that over time the car has run leaner and leaner. At first it was around 14:1, but now, a few days latter, it's usually around 16:1.

I'm pretty much out of ideas. The only thing I can think of is that the header and exhaust are so efficient that the engine is just flowing too much air for the ECU to handle, but I find that hard to believe. I think the stock a/f ratio should be around 12.5:1, and it seems unlikely that even the world's best exhaust would flow so well that it changed the ratio by 4 whole numbers.

Any ideas would be greatly appreciated, thanks.

Possibly a partly clogged fuel filter. Is timing set to 10 degrees?

The stock ECU doesn't use the o2 to compensate for mixture at WOT. It simply runs off the preprogrammed fuel map from the factory (open loop). You have made the engine flow more air but have given it no way to add more fuel. That is why it is lean.

I see you removed the supercharger but did you remove the powercard also or is it still installed?

His exhaust should not be flowing enough to make it go that lean. I have seen plenty of other miatas having header and exhaust on stock ecu's and not being anywhere near that lean at WOT.

I wonder if the PO did any other mods?

Thanks for all the replies. I did remove the Powercard, so that's not the issue. The car has no other mods other than the exhaust, it even has the stock intake. My plan now is to find a used MAF sensor for not too much money and see if that helps. While the one that's on there seems to work fine, I'm wondering if it could have a minor issue that is causing this. Most things I've read point to the MAF as the culprit when a car has the problems mine has. I know I'm just throwing parts at the car, but I don't know what else to do, and I'm not a good enough diagnostician to figure out the problem.

Last year I couldn't pass smog because three of my monitors wouldn't run and the car was never "ready", which meant the techs couldn't even begin to test my car. I tried everything to fix it, drove almost a thousand miles trying to get the monitors to run, replaced the cat, replaced the rear O2 sensor, tested the nearly new front O2 sensor, etc. Finally a tech somewhere suggested that because I had a Bosch O2 sensor I should replace it, even though it was almost new and tested fine. He said they didn't always work right in Miatas and I should use an oem Denso sensor. I didn't really believe him and felt he was grasping at straws, but two days after he told me this it went bad so I did as he suggested and voila, monitors ran, I passed smog.

Point of that long winded story is that sensors are touchy little things, and they don't always tell you when they aren't feeling well. Sometimes throwing parts isn't a bad idea.

Maybe there's a leak between the maf and the throttle body? If air is getting in to the engine without passing through the air flow sensor, it would cause the car to run lean, no? It sounds like you've eliminated leaks between the head and the throttle, so that seems like the next place to check.

I'd be hesitant to assume vacuum leak, they usually affect bottom end more than top end/WOT. Someone mentioned partly clogged fuel filter which sounds like the most likely issue, especially that it just got worse(maybe sucked some crap from the fuel tank and made it worse). If you are replacing parts that is cheap and easy to replace before assuming its the MAF that tests ok