Hoping someone can provide some education regarding Rich Fuel 175 code and Fuel Trims. Yesterday my 1998 XJR (160,00 km) displayed the CEL and intermittently went in and out pf Restricted Performance. I put my OBDII scanner on and retrieved a P0175 code - Bank 2 - too rich. The car seemed to be running fine, so I cleared the code. I had to run out of town on a trip of about 40 miles, so had my son watch the fuel trims as we went. Short term were well into negative territory -12 to 15 for a short while, then they settled back into 0. (fluctuated slightly positive and negative readings near zero).

Long term remained rich and seemed pretty much stuck on -15.6. Before heading out on the road, I checked codes again after clearing and got a pending code of P0138 (O2S sensor circuit high voltage), but only a pending code. I have occasionally wondered if the O2 sensors should be replaced as a matter of routine maintenance at mileage like mine, but I do not know for sure.

I checked again today and got the all clear. My trip out of town and back was smooth and it was sufficient to clear all the tests, so I had a P1111 code this morning. Everything seems fine, except the long term trims are reading -15.6 on both banks and do not fluctuate at all.

Can someone help shed some light on this? I do not know much about fuel trims, but decided I should look at them after getting the P0175 code. I assume Short Term should be near zero, and assume the long term should also be near zero. As mentioned above, I really do not know and the JTIS manual does not provide any input that I could find.

If anyone can enlighten me on how to interpret the trim readings in general, I would appreciate it.

 

The P175 is bank B combustion rich, P138 is 02sensing circuit short circuit to high voltage, grounding open circuit.

Not an expert in this area but how old is the battery and have you checked it to see if the voltage is normal. We recently had several problems related to the battery. You should take it to a place like Auto Zone and have it checked first before you begin digging deeper.

 

Have you done things like cleaned the MAF sensor with contact cleaner, cleaned out the part load breather and the throttle body? What is you fuel pressure like? Injection clean and fuel filter change done any time? 100k miles is when things need cleaning if not done before now.

These engines if not abused and well maintained can see into the 300k mile mark with no fuss.

I'd say clean what you can, service what you can, oil/filter/air filter etc, then think about O2 sensors when they throw codes, they are specific. Rich running is in my mind an engine management system over compensating for furred arteries somewhere. Maybe even egr pick-up at the back of the induction elbow restricted.

What's you fuel mileage like, has it stayed the same, is it going down? How old are the plugs?

 

Thanks Sean & Gus,

I replaced the battery about 3 or 4 years ago, so I suppose it could be worth looking into. I have cleaned the MAF, TB and breathers in the last 10,000 km.

Fuel mileage averages around 18 - 19MPG, which I assume is OK as 90% of the driving is city.

I will keep an eye on the mileage to see if it drops, but the codes only showed up yesterday, and so far are gone today,

I am curious as to why long term trims seem to be static at -15.6 on both banks, and also not sure what they should be, as it is only my uneducated assumption that they should all be near zero.