Noticed that my long term fuel trims have dropped negative over the years and am wondering if something is up with the positive crankcase ventilation system.

I am familiar with logging data so logged a couple hours with the breather hose connected (the first pair of trims) and then disconnected (second pair) and ran the results into a spreadsheet. Each time I allowed the engine to get to temperature and had separate drives to relearn between runs.

Looking at this I am wondering why the trims are higher, e.g. more positive with the breather hose disconnected. Obviously in that scenario I'm allowing unmetered air to the intake manifold but shouldn't there be just as much air coming with the breather hose connected?

This table is from a 2008 Subaru 3.0 showing just bank 2 as both sides have similar results. The PCV valve is replaced recently, the hoses are in good condition with genuine parts used.

0-10 -8.8 -3.0 -3.8 3.6
10-20 -8.7 -3.0 -3.8 3.4
20-30 -8.4 -3.0 -3.8 3.0
30-40 -8.0 -3.0 -3.6 2.6
40-50 -7.8 -3.0 -3.5 1.4
50-60 -5.9 -3.8 -3.0 -1.2
60-70 -5.8 -3.8 -3.0 -1.6

enter image description here

1 Answer 1


With the pipe disconnected and open to the air (not plugged), as you say, unmetered air will be getting in, bypassing the air flow meter. Therefore the mixture calculated by the engine ECU using readings from the mass air flow meter will be lean.

When the ECU reads the results of the lean mixture burn at the O2 sensors in the exhaust, it will trim the mixture to compensate, so that the mixture is no longer lean.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .