Dyno with O2 Sensor

[Berkeley and Bill Conner]

I recently reported losing 5-7 WHP on the dyno, with moderately lean mixture
readings.  Thanks again for all the information and helpful ideas.  This is an
update.

As a reminder, my mods include: PR CAI, S3s and S3 ECU, HS header, and HKS
exhaust.

Here is what I did before returning to the dyno:
1. Had a leak down test performed  Results:  2%, 2%, 2%, 4%.  The engine is pretty
tight..
2.  Replaced thermostat and temp gauge sending unit -- either the sender or the
t-stat had gone flukey.  Now all better.
2.  Adjusted TPS voltage from 0.46 to 0.50 (right in the middle of the spec now).
3.  Replaced the fuel filter.  Nissan parts only, please.
4   Installed underhood fuel pressure gauge.  Pressure at idle is 36psi and with
no vacuum hose it is 46-48 psi.  Within spec here.
5.  Wired up a mulitmeter to the MAF and ran wires into the car so I could monitor
MAF voltage as I drove (with a flashlight no less).  On the road, MAF voltage was
4.50 at the fuel cut in 2nd gear.  I saw it run as high as 4.53 at tip in around
6000 RPM.  These voltages would argue against the bad MAF line of reasoning.
6. Purchased a NISMO AFR in case I needed to tune with fuel pressure.

Then I returned to the dyno and found that something I had done restored the
power.  The ponies are back!  Also the A/F ratio is richer, though still a bit on
the lean side (13.5 - 13.8 at 3800 - 5000 RPM) and then 13.1 - 13.3 up to 7400.

So, I tried to install the AFR while on the dyno and found that the outlet for the
fuel return is aimed in the wrong direction and hit the linkage on the throttle
body.  The regulator would not fit properly.  So, I was disappointed that I was
unable to try and tweak a few more HP using the FPR on the dyno.

I'm thinking it was the fuel filter.  It had 40,000 miles on it at least.  I'd
love to hear your thoughts.

Bill Conner