For the past few years I have been fighting with codes P0172 and P0175 which says the engine is running too rich. I have replaced or rebuilt everything I can think that could cause this problem. I comes and goes sometimes going over 1000 miles without an occurrence other times it pops up every 20 – 50 miles. Each time the restrictive performance light turns on for 5 – 20 seconds to 2 or 3 minutes (very seldom). The check engine light will come on until I reset it or leave it and it will go away after a few days. Using “Torque App” I can see that my short and long term fuel trims are always negative. LTFT are in the area of -12 to -18%. I have also had the car to 4 different shops to get this check out including the local Jaguar dealership. None of the shops could find anything major wrong with the car, but suggested different paths from using an injector cleaner to changing the O2 sensors. I have changes the gas filter, spark plugs, MAFS, Fuel pressure regulator, all 4 O2 sensors and part. load breather tube. I had the fuel injectors clean and tested. Had a smoke test done.
Last week the problem started again. I had read somewhere that if you pull the oil stick out a bit, you should see the fuel trims change. Using torque and my phone I tried it with no real change. I then tried to disconnect the Full load breather tube at the air intake hose. Within a few seconds the short term fuel trims seemed to show normal readings. If the LTFT were -15 the STFT were +16 or right where they should be. Today I did a reset on the ECU touching the + and – battery leads together and drove the car with the breather tube disconnect for about 30 minutes watching the fuel trims. The LTFT started out a zero and stayed at a +/- 3% the STFT stayed around +/- 6% with the exception of taking my foot off the gas after a hard acceleration, then is was showing -25 for a few seconds. I then went bact to my garage and, while watching the fuel trims, hook up the breather tube. Both STFT went to -25 at once.
Anyone have any thoughts on this. I feel a slight “sucking” at the air intake hose where the breather tube was disconnected. Should there be a vacuum on the engine side of the breather tube enough to change the amount of air getting into the intake?

 

There should be a slight Vacuum. Clean air from the Intake Tube passes into the Crankcase and is removed (along with normal Blowby) by the Part Load Breather into Intake Manifold Vacuum.

However, under Heavy Engine Load, Blowby can exceed the Part Load Breather's capacity and reverse flow into the Intake Tube.

 

I have had this issue on 2 occasions, both times it happened just after a hard acceleration, Jag owners don't do that, do they? and immediately hitting the brakes hard, to avoid hitting another stopping object. Has not occurred on any other occasion.


I figured it dumped so much gas in and I stopped before it could burn.

 

Dear Glenn:

I am starting to fight this exact same issue with my '00 XJR. First of all, where can I get the various value chart of our cars such as STFTs?