AFR Meter Issue

[Strawdawg]

When the exhaust valve first opens, we have a positive pressure peak in the wave form but that is followed by a negative pressure as the gas picks up speed and begins to cause a suction behind the positive part of the pulse.  This will suck atmospheric air into the system via the leaks and lean the measured a/f out thus giving a false a/f.  This is exaggerated by cams with a lot of overlap and duration.

This is magnified at lower rpm and begins to diminish up around 3000 rpm in most cases.  In some prior thread I posted the suggested distance from an exhaust opening in terms of multiples of pipe diameter that is recommended by some wb manufacturers.   

When running closed loop in an fi system, it will totally screw up the O2 and cause the system to add a lot of fuel to correct a "lean" problem and the engine actually then runs rich because it was fooled by a crack in a header tube.

[cwestra]

Lotsa people here that will tell you that cannot happen because the exhaust gas pressurizes the pipe but such will really screw up the O2 readings on new cars. 

Glad you figured it out!

The guy at Innovate told me that under load (which I have not run yet) my lean condition, caused by the leaks, probably would have gone away due to the increase in exhaust pressure.  It was the low pressure condition at idle that elevated the readings to the out-of-sight lean levels I saw.  I suppose that the readings under load still may not have been completely accurate, just more reasonable, or within a more reasonable range.

[Strawdawg]

The guy at Innovate told me that under load (which I have not run yet) my lean condition, caused by the leaks, probably would have gone away due to the increase in exhaust pressure.  It was the low pressure condition at idle that elevated the readings to the out-of-sight lean levels I saw.  I suppose that the readings under load still may not have been completely accurate, just more reasonable, or within a more reasonable range.

That was why I asked about the readings over 3000 rpm...of course if you are not driving it to put a load on it, it is not the same as running it in place.  As the rpm picks up the positive pulses are much closer together and don't leave much time between them for the negative pressure areas to be able to pull much in.  If you had mounted a bung in a location such that it was seeing all 8 cylinders instead of 4, the problem would be even less apparent.