hello I'm new to this site, have a 2003 XJ8 VDP check engine light on, lean codes 171 and 174 had it smoke tested at intake system at jaguar dealer! found vacuum leak at the full load hose and around throttle body seal.installed a new MAF sensor,new load hose and adjusted throttle body seal. restricted performance comes on but goes back off. checked fuel pressure,44 psi with regulator disconnected. 37 psi with regulator connected. so can any of you folks direct a guy on what to do next? would really appreciate it!! thank you from Nova scotia canada! Great site!

 

Maybe bad seal at dipstick; leaking around air hose between air filter and throttle body.

If it just started with cold weather, maybe temperature sensor is bad and not enriching during cold start.

Is the throttle plate clean?

Does your OBDII reader give you live ST and LT fuel trims? Plenty of threads for researching issue. Start by searching P0171 P0174

 

thank you for the advice! never thought of the dip stick.the hose from the air filter to throttle body has been replaced. just picked this car up in the summer so that rules out cold weather. i do have a scanner that does live fuel trims, this is all new to me, trying to educate my self as i go. been a bit of a chasing your tail thing! but i love driving my jag!
ill do more digging on here about where the fuel trims are supposed to be!

 

Its rather simple, you will have to find the STFT (Short Term Fuel Trim), and the LTFT (Long Term Fuel Trim).
Each has a value for the left, and a value for the right bank, so total 4 values.
They will start showing and changing once the engine and its systems are up to temperature.
You will see that the the STFT will move up and down directly related to the engine load and the position of the accelerator, the LTFT are much slower.

Theoretically, all values should be close to zero, and the left and right bank values should be the same.
In reality, the STFT should move within a range of some -10 till +10, the LTFT within a range of some -3 till +3, perhaps a bit more (note: a plus or minus is important for judgement).

Try to do a reading while at idle for a minute or 2, and try to grab some values while driving at different rpm's.
That will give some ideas to start with.

 

+1 except...

Hot engine, then parked. Find idle LTFTs. Then rev to 2500rpm or so and watch the LTFTs. If at idle they were (say) 10+ and drop a lot then you have an air leak. (Because the slight leak doesn't matter much at higher revs when the air flow is much bigger.)

 

Thanks again folks for your advice!! hooked OBD scanner up and this what i got STFT's were o to -1.57 to -2.35% would fluctuate higher or lower pending on speed.... LTFT's were 8% all the way up to 19.5% so i take it thats to high? forgive my ignorance about this tec stuff as its brand new to me... looked at the o ring on dip stick it looks pretty intact!! but here's an other question? i have a small oil pan gasket leak, would this also cause an air leak? thanks again for the support really appreciate it!!!!!!!!

 

Maybe have a better try at following post #5.

 

i tried post 5 and the LTFT did drop significantly!!!! so i guess its back to square one!! thanks folks!!! just got to find that air leak

 

Next... smoke test or TINY squirts of such as propane while watching STFTs.

 

So fuel trims reading -0.8 on both is good then?

 

If you mean the Long Term Fuel Trims at idle, and after a bit of riding, yes, they are perfect.
It means your ECU is reducing the fuel by 0.8% from the standard fuel map, which is basically no correction at all.
You can leave the reader connected, and next time do a further check on let say constant highway speed (so higher, constant rpm), and double check.
It will probably be slightly different, but as long as it is a couple of points around the zero, still perfect.

 

Better than mine!

 

Back to primary question: when you cleared the codes, did they stay cleared?

Go back to tightening all fittings and double check that air hose to assure no air leaks.

Does the car still run well?

 

Let's go back to the primary problem: did you clear the codes P171 and 174? Did they come back? Are you still getting a restricted performance message?

How does run?

Might need to review the whole line from the air cleaner to the throttle body, for any tears, (don't care, do it again!); clean the throttle plate; double check that the MAF sensor is plugged in correctly with no bent prongs; run a can of GUMOUT or SeaFoam through a tank (used to get the 171 on a RRS/SC when one of the injectors on bank 1 got dirty).

From your STFT and LTFTs, you should be ok (?), those figures are pretty damn near perfect.

OBTW: this thread is a pretty tutorial on fuel trims.

 

hi i clear the codes and they come back, restricted performance pops on and off real quick. the car runs fine doesn't seem to bother it the odd time the restricted performance will force the trottle to cut back. but when the warning pops off car is back to normal running but of course with engine light on

 

There will be freeze frame data and it might give clues as to what's happening.