If your short term trims are fine and stay that way your long term trims will fall back in line. They are not separate issues.

The way it works, in a lean condition for example, is that the STFT's will see a lean condition so the computer will attempt to 'fix' the problem by increasing the injector 'pulse rate' to richen the fuel mixture to compensate. That enriching (trimming) will show up in your LTFT's so that they will show relatively rich mixture.

When you (or mother nature) correct the lean mixture the STFT's will for a short bit show a rich condition due to the computer still ordering a 'rich' mixture based on the old LTFT's. That will go away as the computer 'learns' the new normal from the STFT's and both will drift down towards zero.

They're never stable as the car is constantly accelerating or decelerating, under load and just idling, and the weather also effects fuel trims as cold days produce denser air just as rainy or very humid days cause heavy air, and all those cause the trim, both short and long, to vary as they attempt to keep the air fuel mixture at the most efficient 14.7 pounds of air to 1 pound of fuel.

A 'lean' condition can be caused by several things which in no particular order generally fall into the following. A bad or dirty MAF, a vacuum leak AFTER the MAF, a failing injector that under-charges a cylinder, an O2 sensor that's going south (usually just one side as both sides at the same time is unlikely), and the ECM or other ignition/fuel controlling module going bad.

Which of those and how many depends to a large degree on the other codes that are popping up in an attempt to pinpoint the problem area. Codes that indicate only one side point to a different scenario than codes involving both sides and dictate what you should look at for a possible fix.

Not your problem but for example my wife's '04 XJ8 is currently showing an ABS/DSC fault message and light but NO CODE. That problem has 51 possible codes and 30 pages of pinpoint tests to be used to zero in on the culprit !!! And with no codes showing in the SDD scans where do I start???

Ain't life just grand?

 

Here's an illustration of where your MAF readings should be according to Jaguar. This is for a 2004 XJ8 non-S/C car at full operating temperature (190+degrees).

Your OBD may be able to give you a reading in air flow through your MAF but it may be in pounds per minute so you'll need to do a metric to U.S. conversion to make sense of it as the illustration is in grams per SECOND.

Find the air flow data on your in your scanner's functions and run the illustrated tests to compare your readings with the optimum that Jaguar says you should see.

Since one pound of air weighs 453.6 grams (take my word for it) then in one minute at idle in park your MAF should be reading close to 306 grams per minute or about 8/10's of a pound per minute. At 2500 rpm's in park it should be reading approximately 2 1/4 pounds per minute, and so on.

Last but certainly not least don't ignore what Don B posted way back at post #4. If your only remaining code is in fact P0101 then follow the leads listed in Don's post as it would seem as if your MAF is at the center of that storm somehow and someway.


It may appear confusing but it really isn't once you know how your scanner is reading the data.

 

That is what makes me think it's a leak at the intake manifold, with a new high end M A F sensor already installed, after everything else looked to be in great shape. The old one just may not be bad after all.That is why i am trying to fully understand W T F is going on exactly, understand WHY, then know what your doing will cure, & not miss the problem again

 

That may be incorrect since those values are for the 4.0L, not the 4.2L. I don't how much of a difference that may make.

 

No.

If the STFTs are small then the LTFTs have stabilised. If the STFTs are near zero and the LTFTs are not near zero (say within 3 of 0) then something is wrong.

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In case the STFTs are not near zero, the LTFTs are still changing. Normally this won't happen unless you've re-zeroed the trims (e.g. disconnected battery) or a fault has arisen.

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The PCM stores many learned trim values (for various loads/revs/etc).

 

OK here's the question, does it look like the intake manifold gasket, or a crack in the manifold, is the most likely cause to anybody else????Want to get on with putting things back on track with fluid changes,etc. Hard to forget about a slight issue if it keeps turning your check engine light on.Brake fluid, coolant change, backflushing the heater core with the old coolant, the old aux water pump, with a bucket,& couple hoses.That should catch up on most of the neglect.Water pump, alternator, both look to be fine at the moment, BUT an automobile with 90,000+ miles on it could be due any time. Getting all the necessary tools together, i did not have,(engine support bar from Harbor Freight was one) so i can be ready when the time comes.

 

If the long term trims stay high once the short term has stabilized near the zero point then there would be something amiss in the ECM as the LTFT's are supposed to be 'learned' response to the data from the STFT's are they not? The LTFT's work to correct any STFT's variance (either + or -) from the base line air/fuel ration of 14.7 to 1 so that if the STFT's go + the LTFT's will go - to try to get as near that base line as possible. The LTFT's are a 'trailing indicator" but shouldn't be far behind any change in the STFT's.

If they continue to be + or - for any period of time after the STFT's then I'd tend to think the problem is electrical (i.e. module or sensor related) rather than mechanical.

Don't the long term numbers come from the exhaust portion of the system as opposed to the intake side and would therefore require a problem in the downstream hardware?

In essence the long term trims are the effect of whatever causes the short term trims to be going lean or rich.


That is, except for a problem in the exhaust/downstream readings caused by a by 02 sensor...or the gravitational pull of the moon on the 3rd Tues. of every off numbered month<G>

 

I had incorrectly stored that illustration in my 2004 XJ8 folder and it should have been in my '99 XK8 folder.


My bad.


I do however suspect that the numbers are fairly close if perhaps a tad low given the 4.2's slightly larger cylinder fill needs.

 

Wingrider we seem to be talking past each other so let's start over.

First things first.

The short and long term trim numbers are a representation of the data collected by the four 02 sensors located (2 per side) in the exhaust system. They are not getting their data from fuel pumps, MAF sensors, fuel injectors, etc. They get their data from the amount of oxygen that still exists in the exhaust gases. The more oxygen they sense the higher the voltage and from that they can tell approximately what the air fuel ratio is. That data is sent to the ECM and from there to the applicable modules and their mechanical components.


If they read a "lean condition" (too much oxy in the exhaust) that pops a P0171 code that triggers the injector (s) to feed a bit more fuel into bank 1. Your STFT records the lean condition as a POSITIVE percentage and your LTFT will begin to respond as the injectors feed more fuel into the system and they will go NEGATIVE as the ECM attempts to maintain a programed 'balanced' air/fuel ratio between about 12to1 and 18to1. Ideally they'd like to be stabilized at 14.7 to 1 but that's impossible to maintain perfectly under all circumstances.

If too LITTLE oxy is in the exhaust the 02 sends a signal that the mixture is TOO rich and you get a P0172 and your STFT and LTFT will show corresponding but reversed numbers to try to balance it.

That's ALL the STFT & LTFT signify. They don't tell you in absolute numbers they only use percentages and they don't tell you the cause only the effect of what ever problem your car has.

The DTC codes are what you need to concentrate on as they're designed to give you a general idea of what the problem may be caused by and in some parts of the entire car they can be pretty good about aiming you at a specific part in a specific location. The engine, however, has so many moving parts and connections that the best they can do is make 'suggestions' as to where to look and as you're finding out there are a hell of a lot of places you can look.


If you haven't already go to the stickies section and download, at the very least, the list of DTC's for your year and model. Better yet download the 6 part svc. manual and go to the powertrain section and read the appropriate sections.

For a simple P0171 code that indicates a lean condition on bank one (passenger side) here is the list of possible culprits.


· Air intake leak between MAF sensor and cylinder head
· Fuel filter/system restriction
· Fuel injector restriction
· Fuel rail pressure (FRP) sensor fault (low fuel pressure)
· Low fuel pump output
· HO2S/catalyst monitor sensor harness wiring conditionfault
· EFT sensor fault (low fuel temperature)
· Mass air flow (MAF) sensor fault (low intake air flow)
Exhaust leak (before catalyst)
· ECM receiving incorrect signal from one or more of thefollowing sensors; ECT, MAF, IAT, fuel railtemperature


Eliminate those that by their very nature should affect BOTH sides such as fuel pump, fuel filter. After all there's only one fuel filter and only one fuel pump and only one common pressurized fuel rail that feeds both sides.

Eliminate those you've already checked or replaced.

Look at how many DO NOT deal with intake leaks but deal with failure in one or more sensors. O2 as there are 2 on each side, 02 wiring faults, ECT, MAF, IAT, fuel rail temperature sensors, and last but not least a plain old fashioned exhaust leak before the catalyst.

A bad 02 sensor can show a lean or rich condition and will continue to do so after you replaced every single vacuum line, connector, seal, and gasket in the entire engine !!! By the way that svc. manual I mentioned for the 4.2 XJ8's has very detailed and specific section on how to test the O2 sensors with a multimeter so you don't have to spend money replacing a good one.

You can choose to spend time throwing parts at it and spending hours under the hood or you can spend some time reading the portions of the manual and DTC's so you'll spend less time and MONEY replacing perfectly good parts.

In any case based on your latest that your STFT's are back to normal or at least in range I wouldn't suspect an intake leak. At least not until I'd ruled out a bad sensor or 02.

 

There's good information in it but it really doesn't address your problem. You're also posting to another thread opened about a P0171 code and I've gotten the two crossed.

If the only code you now have showing is in fact a P0101 the forget your fuel trims and most of the rest of it for now.

Way back at the beginning someone posted the section from the DTC's relevant to the P0101 and it would seem that based on what you've done and that you evidently have no other codes remaining and have replaced all the 02 sensors and checked for signs of an air leak that your problem is localized to a bad MAF. If you had an intake manifold leak (or any air leak for that matter) that would almost certainly trigger a lean code on one or both sides but you say that's gone. I'll assume that you've checked or replaced you air filter so we can rule that out. so what's left on this list of possible causes?


Blocked air cleaner

Air intake leak


Engine breather leak


Mass air flow (MAF) sensor to ECM sensing circuit:high resistance, intermittent short circuit to ground


MAF sensor supply circuit: high resistance


Throttle adaption fault (check throttle position voltage at Ignition ON)

 

Was throwing the 101- M A F sensor code, as well as 171, &174, which indicates, lean on both banks, or sides, as well as the dreaded restricted performance light. Pushing 90,000, with a filthy air, as well fuel filter, O2 sensors are considered a wear part to many, so replaced i them all, couple cans, Lucas fuel injector, treatment, & everything but 101 has left & it runs great. If my readings indicate no air intake leak, then will have to pay for a diagnostic test to see which electrical part is bad, then replace it, & get on to other things.