Hello All. I'm having a hard time finding the cause of persistent P0171 and P0174 codes and high Fuel Trims in 99 XJ8. Both upstream A/F ratio sensors have been replaced recently. I have replaced the fuel injector seals, checked the air intake tube for cracks. I have also sprayed an entire can of carb cleaner all around the intake area and all vacuum hoses coming off the intake manifold, but still can't pinpoint any massive air leak. Fuel trims are running very high with STFT Bank 1 fluctuating between +12 to +25 at idle with warm engine and LTFT on both banks at 19.5%. See attached images of live data screens. I am totally stumped and frustrated. Any help would be greatly appreciated 🙏🏼

 

Maybe clean the MAF sensor; run a can of SeaFoam through a tank of gas (always solved a spitting injector on one of my Range Rovers); check the o-ring seal on the oil dipstick; clean the throttle plate and bore. Clean the part load breather . . . change the fuel filter.

 

Do XJ8's have a part load breather tube? If so, that tube will leak if you look at it wrong because it gets very brittle over time. Try spraying carb cleaner under the air intake, which is where (I assume) the tube runs. On an XJR it runs under the supercharger, but I've never worked on an XJ8, so I can only guess. I fought a vacuum leak for a long time before I finally found this.

 

I had the same codes from a leak around an injector. Spraying starting fluid did not find it. I used a Macanudo and a hand pump to find it. I later built a homemade baby oil smoke machine to find a leak in an EVAP canister. I did a thread on the smoke machine if you want to build your own.

 

If spraying, watch short term fuel trims as you do it. Do not rely on hearing revs or the like change.

 

Yes, I already did that and found a couple of injector seals leaking, so I replaced all of them. Now when I spray carb cleaner around them, there's no change in the STFT on both banks. Since the high LTFT is the same on both banks, I'm assuming the leak is from the manifold or one of the hoses connected to it or the Throttle Body. I tried a homemade smoke machine that didn't work too well, so I will try a smoke/fog machine next.
I was hoping someone had had a similar experience and found the actual culprit....
Thanks for all the replies!

 

Apologies if I've missed it, but how much have you used the car since replacing those injector seals? LTFT takes a while to change as its long term so wondering if the ECU has had time to notice?

Or you could disconnect the battery & touch the terminals together on the car's leads to fully discharge the system as I believe that wipes the adaptions memory in the ECU.

 

I doubt you’ll find your leak using carb spray. Better to use a smoke tester. Your leak could be anywhere from a loose clamp on an intake hose to a cut in a vacuum line or intake tube leak bad O-rings loose oil Cap bad gaskets lose throttle body bad oring on the oil dipstick etc.

 

There is a vacuum fitting on the right side of the throttle body that has a o-ring. It took a long time to find that problem on my XK8.

 

​​​​​​UPDATE on my frustrating issue:
I double checked and replaced all the O-rings on all quick-connect hoses that attach to the valve covers and throttle body. Also replaced O ring on dipstick as well. Then used up another can of carb cleaner while watching fuel trims and could not find a leak. The only hoses I found connected to the throttle body were the lines from the fuel pressure regulator, the brake booster and fuel EVAP purge valve. Those all were in good condition with no signs of leaking. Perhaps I am not seeing another hidden hose ? Then I did a smoke test via the hose from the brake booster using a home made contraption. I found a tiny wisp of smoke from the area where the brake booster line enters the throttle body. I sealed the outside of that area with ample silicone sealant, not wanting to yank the hose out and exacerbate the problem. Then I cleared all the codes, did a hard reset of the ECU and restarted the car. It ran fairly smoothly, but the STFTs were bouncing all over the place. After a couple of drive cycles the check engine codes P0171 and 0174 did not return and the car runs well. However both the STFTs and LTFTs have settled down, but almost maxed out, indicating an air leak still exists! (See attached screenshot) The amperage reading from Bank 1 AFR sensor is really high, hovering around 8ma, while Bank 2 sensor is more within normal range of 1.0 to 1.2. I also need to mention that the AFR sensor on bank one is aftermarket, while the one on bank 2 is an original Bosch sensor. The aftermarket one seems to be working fine but it's possible that false readings from that sensor could be contributing to the high fuel trim readings. I am completely stumped at this point, and any help would be greatly appreciated

 

denso air fuel sensors only