03-18-2016, 01:46 AM
Join Date: Dec 2009
Total Cats: 0
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.
Last edited by Greasyman; 03-18-2016 at 09:47 AM.