O2 Voltage Dropping

[ginsu417]

Okay, I've run this across a few seasoned tuners, but I'm getting mixed reviews. Hoping some of the tech geniuses here can assist me before I trash this car and buy an SRT-4.

90 GSX, 550s, Walbro 255 high pressure, SAFC, 2G MAS - and before you assume i'm an idiot and don't know what I'm doing... all of this was installed over a year ago, and the car's been running fine up until about 2 weeks ago.

About 2 weeks ago, the O2 voltage started to fall off under boost. A lot. In 1st and 2nd gear, they stay where they're supposed to, but in 3rd, 4th and 5th, they slowly start to come down until they hit nothing. Under normal conditions (part throttle), the O2 voltage bounces up and down like it's supposed to (though it does drop over-all after the car warms up). The sensor was replaced about a month ago.

I richen up the AFC like 10% and it doesn't change the O2 voltage at all. I suspected a lack in fuel pressure, so I replaced the pump (already had a walbro 255 in it, but I popped for a new one anyway), filter, FPR and checked the fuel line for kinks all the way back to the tank. Set the rail pressure to 39 lbs. Changed the capacitors in the ECM (they didn't appear bad enough to cause problems, but I did it anyway). But i'm still showing a significant drop in O2 voltage under boost. So much so, that under full throttle in 4th gear, the O2 voltage drops to almost nothing. (no high EGTs, no sputtering, no hesitation, but I'm afraid to push it much further) EGTs do not hit even 800C under full boost in 4th gear. They used to. In fact, when accelerating uphill in 5th gear from cruising at 60 to about 80, EGTs actually went DOWN 250-350 degrees C. I can actually smell raw fuel sometimes under hard acceleration, but I've already checked 6 times over for any leaks and have found none.

I haven't pulled the plugs yet, but I plan to today to see if they indicate a lean condition. Anyone think is this just a bad sensor or faulty O2 circuit or is the car really leaning out under boost? I was under the impression that as the car leaned out, EGTs go up, but some people tell me they only peak out at stoich, and anything under that will cuase them to go down. Any help would be appreciated.

 

[92redman]

Is your ECU pulling timing when the voltage appears to drop? How are you monitoring O2 voltage?

 

[ginsu417]

I've been monitoring with the A/F gauge. Checked it with a digital voltmeter - gauge is accurate. Did a few runs with the voltmeter hoked up, to get the actual voltage readings. Since then, I've just been swapping stuff out and seeing if the symptoms go away.

I can't really tell if it's pulling timing, I haven't logged it yet (planning to do so tomorrow afternoon). But if it was pulling timing, I would expect the car to feel sluggish, and EGTs to go up. Which is why this is all so puzzeling.

Starting to suspect that O2 sensor, even though it does cycle properly under part throttle.

 

[92redman]

If your adaptives are going up and it isn't pulling timing, then you are probably fine. I guess you'll hafta wait for somebody else to chime in.