So, let's suppose that you're driving along and you get a lean condition code on your LTFT.

The X100 4.2 manual states that a lean code is actually triggered at 20% variance from optimal mixture and achievement of Stoichiometry (1.47:1).

Does this mean that your engine is (a) actually running a lean mixture at the cylinder at the moment the code is triggered, or does it mean that (b) the ECU has detected the amount of the lean condition and has adjusted the mixture by whatever amount of fuel needed to actually compensate the 20% back to 0%, and the mixture at the cylinder is correct.

In effect, the ECU has detected and fixed the problem, but the code is to warn that in the absence of the ECU's intervention, the mixture would be lean.

Thoughts?

Thanks

 

Good question.

 

Is the MIL on? What DTC is stored in the system?

The PCM is going to adjust the fuel/air mixture to get to the stoichometric ratio to maximise efficiency.

 

It started out adding the right amount of fuel for the air (using such as MAF), adjusting for temperature, barometric pressure etc.

Then it's found the O2 sensors weren't what it hoped so made more adjustments, and repeats looking at the O2s. Etc. At some point it's hit the limits.

In the case of a "lean" code it's had to keep adding fuel trying to get to the right AFR.

Note that unburned fuel&air counts as LEAN because it doesn't have fuel sensors in the exhaust, only O2s. So, misfires (e.g. bad coils) look like lean.

Similarly an air leak into the exhaust (bad weld etc) also looks like lean.

edit: was to be: The kinds of things a V engine's PCM cares about www.onboarddiagnostics.com/page02.htm but the page has gone

 

Thanks, guys. So the answer is basically (b), unless the vacuum leak is so far out of spec that the ECU (PCM) can't adjust enough to get back to stoichiometry. An example of this might be when the oil fill is left open, and the "leak" is so large the engine bogs down and dies. In that case the ECU will compensate until it can't do enough anymore.

Reason for the question was a guy on one of the Jag Facebook forums said he was getting a lean code consistently, after his local garage did some work, and was concerned that his engine had been damaged due to long term lean mixture. I didn't think it was likely.

 

x100 4.2 and the other early 2000s 32bit systems are different.

either way 171 and 174 are symptoms of long term correction for a lean mixture. ecm adds fuel to compensate for “lean” afr until it is under control and then subtracts.

some of them are 17-18 others go 25% to 30 long term correction before they set a fault. unless the vehicle is absolutely maxed out to the point it cannot compensate you are not in danger. i’ve had GMs go into +70% short terms before they go into open loop fault

 

Can fuel trims be measured in the SDD software - I have a basic training manual for the SDD, but a quick read didn't offer any info?

What other diagnostic tools are good for reading the trims?

 

My go-to, with SDD in the background, is Autel AP200 with the JLR software load. Pairs to smartphone. About $70 or so. Very comprehensive, very portable. Measures a hundred+ data points, including LTFT.

 

Thanks.

Just looking at my DashCommand app - after scrolling through menus it does have a fuel trims, AFR, O2 sensor, Lamba -will try these out tomorrow and look into your suggestions.

 

Thanks, Xalty. The only detailed manual I have on sensor data interaction and triggering is the X100 4.0-4.2. I'd like to find the X150 4.2. I appreciate your knowledge and experience.

 

After work done on my XKR last week, stripped down to replace valley hoses the idle is rough - reading from my DashCommand app - first pic at idle in park, second pic at around 2500 rpm, fuel trims did level at around +5 on both sides, there is a fair amount of bounce when increasing the gas, one side up the other down. Not sure if this is DashCommand issue - though I think not.Not sure what the LT and ST mean on the fuel trim bar.

So it looks like there is a vac leak somewhere.

 

I remember a few years back, I got one of those fuel trim too lean engine trouble codes. I don't remember the exact code, but I do remember the fix. It was a little bitty vacuum line that had deteriorated and once I found it, the fix was pretty simple. Look on top of the engine, next to the supercharger housing. I was able to disconnect the black rubber piece by the first arrow, slide the tubes out from under the big stuff, fix it with a new tube, and slide everything back under. I did not have to remove any nuts and bolts! Here's a picture of what I'm trying to describe:


Once fixed, the code did not come back. Hope this helps!

 

I had the same issue, but it had a very noticeable hiss which was easy to find.

 

Those little lines are the cause of a lot of leaks. Fragile as hell after 10-15 years, split when move a bit. Easiest solution is to break off the end plugs leaving 1" of tubing, then insert it into flexible vacuum line up to the connector. The new line supports the old line at the connector, and the rest of it is new. Works like a champ.