We've had on the forums, in the XJ, s-type, and xk areas an unusually large amount of multiple misfires and lean/rich conditions related to one bank only (me included). There is not a single thread that goes into the amount of detail that I think would be most useful for all jaguar model owners to benefit from referencing.

For those of us who have a OBDII scanner that can measure, view, and record 02 fuel trims, both long/short and that wideband option, I was hoping I could find a volunteer with the knowledge and experience to both explain how to diagnose the fuel trims, what to look for in good and bad data, is % or V what we should be setting it to... perhaps providing a small video or some screen shots of the trim graph showing a problem and what an 'acceptable' trim reading should be, and the difference between each bank...I have no idea if my difference between O2 readings no each bank is normal, or truly indicative of a bad sensor and/or air leak.

I guarantee a FAQ addition and a very popular thread if anyone wants to take the time and effort to create one. If I can be of assistance in making it look pretty, I would gladly help.

 

Stand by.......
I'll see if I don't have what I need to whip somethin' up.

Cheers,

 

OK, Waterboy,

Maybe this will get things started. Here is a short document outlining fuel trims. Keep in mind this is not a scientific white paper, just an illustrated overview of what they are and how they work. Some people already have a grasp on trims, others just need something to get them started with the concept. This is meant for the latter.

Even has a quiz in there!

I'll leave it up to you as to where to post it.

Cheers all,

 

Steve:
Thanks. That is inded a great write up.

 

Horrors! I realized I originally uploaded the wrong revision of the primer. That has now been fixed. The little quiz is now correct and my comments make sense!

My apologies!

 

xjrguy...

The quiz in Rev 1 made sense to me, but not Rev 2. In both cases, the result was the short term trim went way positive, meaning the cpu was trying to richen up the mixture because it was too lean. Disabling the coil would make the exhaust in that bank rich, causing the fuel trim to go negative. Regardless, this is a very nice writeup.

 

It’s the Oxygen that the O2 sensors measure, so no ignition, lots of oxygen heading the O2 sensor, so the ECU will react to provide more fuel as designed.

There is a lot you can read from the STFT/LTFT, these are very helpfull values in case of trouble shooting is required, nice document!

 

GOTCHA!!

I had the right illustration in the first one, I just had not put the right text there. That was exactly my point in the "quiz". To show how the first thought one has, is usually the thought you had; misfire means no fuel burned and rich exhaust. Now, what sensors do you have in the exhaust stream? What are they measuring? They aren't, and never are measuring FUEL. They measure OXYGEN.

Normally the fuel and oxygen taken into a cylinder is ignited by the spark plug and mostly CONSUMED. If the coil and therefore the spark fails, the unconsumed fuel pumps right into the exhaust, that's the first thought. Now think a little further and oops, there is oxygen pumped it there, too! What are we sensing?? OXYGEN.

That's why misfires are so problematic. A misfire pumps fuel and oxygen into the exhaust, the oxygen sensor sees more oxygen and reports to the ECM, the ECM adds even more fuel because it thinks it needs more to consume the perceived excess oxygen.

I should add that if the injector fails and cuts OFF fuel to that cylinder, the fuel trim response is the SAME. Only in this case there is no fuel there to consume the oxygen, so what the ECM perceives is basically the same thing.

Ain't science grand!!

My whole point is, there is a lot going on there, and it takes a while to fully get a grasp. And unfortunately not even all technicians have the understanding that they really should have.

All thoughts are welcome!

Cheers,

 

Apologies steve, just now saw my thread had been replied too..I thought it had died after my first bump!

Love the writeup, that's just what I was looking for, and a sincere thanks from me and everyone else who's ever wondered the same thing. The quiz was a bonus indeed, and of course, I failed it.

I'm going to go get a snapshot of my car's trim graphs and repost. My AutoEnginuity also monitors crankcase pressure and air rate...thought that was cool. Any idea what the 'target' pressure should be?

 

I have been wondering what was meant by fuel trims.Thanks for the write up-got the quiz right too!

 

I'll share a Porsche Boxster story that relates to fuel trims. As mentioned in another thread, beer and Ebay don't always mix, and I ended up with a Boxster as a result. Great car, but it threw the engine light on fairly regularly. I bought a tester for my laptop, similar to that seen in the (very good) tutorial above.
The code said basically that the fuel trims were at the limits of their authority.
The ECU could add or subtract 25% of the scheduled fuel. They were at that limit, and still the O2 sensors weren't happy.
Long story short, an internet search later, and the fix was to pull the MAF and spray the sensor clean with contact cleaner. The observed readouts of air mass at idle dropped noticeably, the fuel trims went back to nearly zero, and the light never came on again.

Essentially, the ECU is programmed with a baseline fuel schedule, that the O2 sensors can trim. Problem is, the MAF (contaminated somehow) was misreading the actual airflow, leading the ECU to schedule the wrong amount of fuel. The O2 sensors then tried to fix it by trimming the fuel back, but ran out of authority. The best part was, it cost nothing to fix, once I had the OBDII unit.
I can't say if it's right or wrong, but many people in the Porsche community feel K&N filters and the oil they use can mess up the MAF.
If you had a V8 with two separate MAFs, this same situation could apply. If your engine has one MAF like mine, then it can't cause the one lean bank. But hopefully my experience may help explain fuel trim, although Steve did a great job, and my story may be superfluous.

 

For those who'd like to see a picture of the typical sensors & actuators in a modern engine that the PCM (ECM) is using and expects to be working: OnBoardDiagnostics.com - Engine Basics

See how early on in the intake the MAF (mass air flow sensor) is? That's why air (vacuum) leaks cause trouble - the MAF can't measure the extra air. (The MAF has to be in a clean air stream, so can't be after the PCV or EGR etc valves.)

A small air leak will tend to be compensated for by the PCM (because the O2 sensors will show the oxygen and the fuel trims will go a bit richer). This is shown in Steve's primer. If you rev the engine (say to 2500 rpm) the fuel trims (when there's an air leak) should head back towards zero, because the much larger amount of air flowing in past the MAF shrinks the effect of the leak.

Of course..... if you don't use an OBD tool (which can do so-called "live data", as even a cheap ELM327 can), you'll likely never know you have fuel trims giving you this sort of useful data.

John

 

MOST EXCELLENT!

Notice how a leak threw off the normal trim. I saw one Tech article that said if you see anything outside of the +10 to 0 to -10 range, your MAF is bad. That is why you don't want to throw parts at a problem.

 

Well, many, many moons ago, I told one of the Mods that I would have a follow-up to the Fuel Trim Primer I posted here. So, months and months later, here it is. Actually, Sparkenzap brought it to mind in a thread last week and I decided I needed to follow through and finish it off.

I hope some find it useful.


Cheers,

 

XJRGUY

Thanks for the very informative posts. I am going to have to read them a few times to make sure I have a decent grasp of the various connotations.

I have often wondered when I am going down hill with my foot off the throttle at 60 / 70 mph why the instant fuel read out gives a high mpg and then suddenly drops from 85 /75 mpg down to 17 / 21 mpg for quite long periods of time.

Is this normal in your experience or do you think I have a leak in the system which becomes more apparent under such conditions?

 

You are welcome, glad you find them informative.

Personally, I wouldn't put too much stock in the decell figures for the trip computer. The over-run fuel cutoff is the reason for the really high instant MPG figures, but you can never really tell when the fuel is cut back in. That depends of a host of factors. I do know the fuel comes back in at point sooner than you might think. It does that so the driver doesn't 'feel' the engine catch itself, so to speak; because it really does initially cut the fuel completely off. My understanding is the MPG is calculated from average speed and a 'measurement' of fuel used by tallying injector on time.

Cheers,

 

xjrguy, thanks for the very interesting descriptions of the fuel trims and their application in diagnosing faults. I'm an engineer and "get" the concept of closed loop systems, but admit to being "out of the loop" when it comes to details of my auto engines. But your descriptions make me think that I have a small grasp of the intricacies of modern engine management, and that's more a tribute to your powers of description than to my powers of understanding!

Thank you,
Steve

 

Well thank you, Steve, for the kind words. Always nice to know somebody got something out of them. Makes it worth the effort.

Cheers,

 

Very interesting write up. I'm running into a problem with saving the last pdf doc, PART TWO - USING FUEL TRIMS. It keeps asking me for an owner password. Do you have it available?
On the point of K&N filters, I have a 2008 LR3 and have heard much the same issue with the MAF sensor being screwed up by the oil in the K&N filter. So, for these fancier, higher-performance VICs I think we should stay away. Those filters work great on carburetors and old muscle cars but not on modern fuel injected cars with all the modern sensors. My humble opinion....

 

Gee, I don't know why you should be having that problem, the document is protected against modification, but not against saving. If your computer is actually opening it in an Internet Explorer window, you will need to use the 'save' button in the Acrobat portion of the screen and not the Internet Explorer file menu. If you continue to have trouble, send me a PM with your email address and I'll get it to you another way.

I personally agree with you about the K&N......lots of disagreement here about that though. Let's just have a civil disagreement.

Cheers,