Spring is here and I pulled my 1999 XK8 out of hibernation. Really, not very surprised, it behaves rather strangely. The starts well, runs smoothly till the coolant reaches an operating temperature. Within a minute or two the car switches to an emergency mode and "Restricted Performance". When I keep driving it takes between 3 to 7 miles till the the engine control nit returns back to a normal driving mode and drives normally till stop and turn the engine off (for example at the gas station). A scanner shows P0171 code. I cannot find any leak around the engine's induction. Where should I look? Thanks.

 

Rather than looking around (there are quite a few spots where air can leak), try and do a smoke test. Inject smoke through the fuel pressure regulator vacuum line and see where it comes out. Resist the urge to dispense flammable substances around the intake in the hope of observing a change in rpm as any backfire would most certainly not be kind to the plastic intake.

Best of luck, keep us posted.

 

If you watch the STFTs you can squirt small amounts of a burnable substance to hunt for leaks and small amounts will not cause backfires.

You can use non-burnable if you have such (think what the trims will do then).

 

Check the accordion section of the plastic air intake tube, especially the bottom section. It is best to remove the entire tube and stretch it slightly with both hands while you observe the accordion section for any cracks and crevices that appear as you stretch it....

 

Plain water in a spray bottle works in the same way as carb cleaner in finding air leaks, and H2O won’t blow up or catch anything on fire. The small amount that finds its way into a combustion chamber will change the idle speed enough to alert you to an leak without undue concern about unintended consequences.


Z

 

gentlemen. All what you are recommending had been done. No vacuum leaks anywhere, or more accurately, no constant vacuum leaks. Read again - P0171 comes on when the engine reaches an operational temperature (with "check engine" and "restricted performance" warnings. After 3 to 6 miles driving the "restricted performance" turns off and the car performs normally. "Check engine" stays on but does not affect the performance. Question? Is there any sensor or valve that opens temporarily when the engine reaches an operational temperature,m or any other explanation.

 

Have you replaced the two O-ring seals surrounding the two Variable Valve Timing (VVT) solenoids? After a few years they get hard, brittle, and tend to allow both air and oil to seep out from around them. Replacing them fixed my hard-to-figure-out P0171 / P0174 codes in February 2017. I have replaced them again at least once since then. Part no. AJ82856 available from rockauto.com, amazon.com, etc....

 

Then check your (long term) fuel trims on both banks (normal OBDII tool) to see how far away from a P0174 code you are (they trigger at +25%). Start at idle, then try and check for other load/rpm data points. If both are high, then your next step is a new air flow meter.

 

I replace those seals every two years. No oil seepage around them.

 

It wouldn't hurt to clean the MAF, and check the o-ring seal around it. They're not real expensive either. I believe that's what fmertz was referring to in post #8 by air flow meter.

 

LTFT bank 1 - 16,5 - 19.8 %
LTFT bank 2 - 6.8 - 9.5%

the data vary from 2500 RPM to idle...

 

If those are idle and drop when revved you have an air leak.

 

Hello. I still did not discover any leak in the induction. All connection to the intake manifold and corresponding seals are tight as well as seals on the engine ventilation tubes. MAF cleaning did not help either. Thus, just on impulse, I decided to buy a new MAF for 17 dollars. What a change in data stream. At idle : LTFT bank 1 = 13%, LTFT bank 2 '= -0.8% STFT bank 1 = -3.9%, STFT Bank 2 = -7%....At 1500rpm LTFT 1 = 7.9%, LTFT = 2 3.12%, STFT 1 = -3.5%. SRFT 2 = -16%. There is still some imbalance but "Restricted performance" and the "check engine " warnings are gone. Also no trouble codes.

 

Sometimes a new MAF sensor is required to resolve these pesky issues. I ordered a new Denso one from amazon.com several years ago and keep it on the shelf because I know that sooner or later, I will need it....

 

As you changed your AFM, you might want to do a "hard reset" to reset the long term trims to zero. Other option is to let the short term trims slowly work their way and lower the long term trims. The fact that you have "high" long term trims and negative short term trims is good. Remember, for each rpm and load position (range really), there is a separate trim per bank. To my knowledge, the LT trims (for a load/rpm) are updated with the average ST trims every 2 minutes spent at that load/rpm. For load/rpm where you spend little time, this can take a while. That is why some post-reset procedures recommend working these trims at a stop, foot on the brake, and run the engine at the usual rpms for 2 min. Foot on the brake simulates load, like you would have driving off from a stop.

Best of luck, keep us posted.