Started this on another thread, but I need some help understanding! Fuel trims. 2002 xkr

At idle, they look like this. So I'd say, "OK, small vacuum leak."



Where I get confused is at higher rpm.. Here is steady state 1500 rpm. According to what I've read, the stft should drop as rpm increases as a small leak is less a % of total air flow, not rise. So what gives?



And at 2500 rpm.



I'm keen on learning how to interpret fuel trims, so please chime in!

 

Hi mhminnich,

Fuel trims are a big subject, but there's plenty of info easily accessible online. Just google "Analyzing Fuel Trims." Here are a few of the first links to come up:

https://www.motor.com/magazine-summa...agnostic-tool/

https://www.autoserviceprofessional....for-you?Page=2

https://gearsmagazine.com/magazine/2083/

https://www.autonerdz.com/yabbfiles/...AF_testing.pdf

Cheers,

Don

 

From my understanding its the LTFT you should be looking at ..
Yours are high +10.94 at idle and they drop at higher revs so indicating an air leak.
Both bank 1 and 2 are the same indicating something affecting both banks the same..
Check air intake tube for cracks and give the maf a good clean.
Smoke test will help to identify any leak/leaks..

Others may give a better understanding of such a big drop in your LTFT at higher rpm.
Good luck.

 

Yes, it's LTFTs.... but, on the basis the STFTs are stable and that will also mean close to zero. This being parked with a hot engine.

The STFTs if not near zero normally have not migrated into the LTFTs.

However, the above STFTs are nowhere near zero! What gives? More info needed.

 

I've read a lot of Articles and watched a lot of YouTube videos, but I still can't find anything online that reflects what I'm seeing.

When the engine is running above idle, the short-term fuel trims indicate that it's adding fuel to correct for a lean condition, while the long-term shows that it's trying to subtract fuel to correct for a rich condition. In the end, the codes being thrown are for a rich condition which matches up with what I'm seeing in long-term fuel trim.

Without throwing parts at it, are we just looking at a MAF sensor issue?

 

As its something affecting both banks equally its doubtful its o2's or injectors,
So the maf could be suspect....
(after having a quick google) it could be a defective fuel pressure relief valve?

Cleaning the maf is a good start (if not already been done)..

 

I would check the easy stuff first. Air leak will typically screw the trims, so I'd check the intake concertina underneath for cracks, or around the throttle body etc., finally the MAF sensor.

 

Still sounds like it hasn't stabilised. It takes time to learn (the L in LTFT).

 

Have you done a hard reset or had the battery removed lately ?
Take the Jag for a run out, Then back home re check your trims at idle and 2500 rpm

After the run out your trims should stabilise quickly when doing idle and 2500 rpm..

 

I went on a 150 mile jaunt last Saturday and continue to see similar results. Above idle the stft rise to +25 and stabilize there, while the ltft drops to roughly - 18 as if compensating.

The only time they are anywhere near 'normal' is at idle where stft is near 0 and ltft stabilizes around +10.

Thinking out loud, it can't be an intake leak or the idle stft would be excessively high and drop, not rise with rpm.

It couldn't be fuel leaking into the intake or the stft wouldn't be correcting high with rpm, though it would explain ltft.

I'm going with bad maf. That's at least a cheap part if my guess is wrong.

 

A quick way to check the function of the MAFS is to monitor its signal in Live Data. A rough rule of thumb is that a MAFS will read about 1 gram per second of air for each liter of engine displacment, so expect something around 4 to 6 g/s at idle, climbing smoothly with revs.

Cheers,

Don

 

Cool. Thanks!

 

• Start by doing a good cleanup of the air flow meter. Fill the cavity with cleaner, let it rest upside down, dump and clean again. Keep your expectations low, cleaning is more of a maintenance procedure and not a repair
• Do the battery reset. This should bring your trims to zero
• Do the throttle cable adjustment to remove the slack
• Do the throttle learn procedure: ignition on, engine off, slowly press and release the gas pedal several times all the way to the floor and back up
• Consider a smoke test to locate all the leaks, there are just too many areas to list

Altogether, trims are a matrix of numbers representing percentage of fuel to add to the base fuel table. In theory, they should all be zero, i.e. the base fuel table is perfect. The matrix is indexed by load and rpm. It means there is a separate trim for each rpm range AND each load range (not sure how wide these ranges are). These numbers are tracked separately. There is one such matrix per bank. As the O2 sensors are not able to detect O2 very quickly, the trims are separated in Long Term and Short Term. Long Term starts at zero, and changes over time by adding the average of the Short Term trims over the last 2 minutes (at that particular load and rpm). Short Term is typically a bit jumpy.

For example, if your car has a small air leak requiring 5% extra fuel at idle and low load, Long Term will start at zero, and the average Short Term should be 5%. After 2 min (at that load and rpm), the Long Term will be updated to 5%, and the Short Term will now hover over 0% on average. So, when you engine's conditions reach that Load and rpm in the future, the Trim table already knows to add 5% instantly (as per the Long Term), without requiring a read from the slow O2 sensor (Short Term). Obviously, staying at idle happens all the time, so there is a lot of data to update the Long Term at idle. For the higher rpms, one would assume there is nice data during acceleration (range of rpm, and pretty constant load). Now, when is the last time you "gunned" it at 3000 rpm in Neutral (low load)? Accumulating 2 minutes of this load/rpm might never happen, so your trims might be out of whack (high Short Term).

For better visuals, consider adding load and rpm to your screen in TorquePro, and move the trim gauges vertically per bank (bank 1 is passenger side, so right side of the screen). From memory, it is also possible to log these numbers as you drive and even graph the trim matrix from the app.

Hope it makes some sense.

 

[QUOTE=mhminnich;2152921

I'm going with bad maf. That's at least a cheap part if my guess is wrong.[/QUOTE]

One man's cheap is another man's O.M.G how much !!!
Make sure you get the XKR and not the XK8 maf,
If you do go for a new one go for original...

 

I didn't realize there was a difference, and I see it is not as cheap. .
Good tip!

 

from what I've found , the XKR part is only found in one other car, the XJR of the same years. Whereas the XK-8 MAF is common across several different manufacturers.

Z