I have been chasing down why my ltft are so high at idle for a while now. I have done everything I can think of to try and figure this out. I recently did a smoke test and nothing was leaking. When I start the car it shakes a bit untill it gets to normal idle then is ok with slight roughness. My ltft at that point are about +1.6. if I let the car sit and run at idle the ltft increase little by little but does not go over +12 and that's after sitting for 20 min or so. When I take it for a drive the ltfts go to - 1.6 and stay that way. Sometimes when going a steady speed they go to +2 to +6. What gets me is at 55 to 60mph I get the roughness I feel at idle. Higher speeds and lower speeds are normal running conditions. Any suggestions? I have replaced pretty much everything. Btw I do not have a cel so this is difficult.

 

Did you win yet? Are both banks the same? Really need LTFT and STFT for both banks. Vac leaks can be very hard to find and do not always show up when the engine is not running. Since it changes as the car warms up it could be a faulty temp sensor.

 

Both trims are different. At idle I get ltft +10 and stops at +12.5 at the same time my stfts are +0 to +4 but fluctuates between them. When I drive the ltft are at -2.3 to -5 again they fluctuate and the stft go to +5 to +12.5 while driving. I have checked the temp sensor and its brand new. Everything seems to be working as normal. No cel codes either so I don't know where to go with it besides throw money at it.

 

Which engine do you have? Still sounds like vac leak. Try spraying all vac hoses and joints with quick start or propane while the engine is running. Don't forget brake booster and oil dip stick.

 

I have the 4.2 V8 and yes I have done that already. Both dip stick and brake booster hose has been replaced. I have replaced every vac hose in the engine compartment. I have changed plugs and ignition coils. Checked for shorted wires. Like I said pretty much everything. I'm hoping it's something really easy other then bad timing or a sticky valve. The car has 260k

 

You are showing the LTFT for only one bank. Are they both doing exactly the same? If different or only similar it could point to the direction to check.

 

Been chasing a similar problem for over 6 months now. No vac leak after extensive smoke testing. The only thing people tell me is it has to be a vac leak.

May God have mercy on your soul.

 

How is your fuel rail pressure? Low pressure would require additional fuel pulse length.

 

Sorry to hijack this thread, but my question is relevant to the topic. My 2004 x350 has always had concerning STFTs and LTFTs, but it wasn't throwing a DTC so I was planning to get to it when I had some time. Unfortunately, I filled up on fuel the other day and it threw a P0174 on the way home (so not related to the fuelup), lean condition on bank 2. So I took a screenshot of the freeze frame:



Here is the freeze frame of the DTC.

The trims are obviously pretty far out. I cleared the code and recorded some values at different RPM. I may do a little more, but to me it looks like the main problem is a vacuum leak somewhere:



Here is a plot of my fuel trims in response to engine RPM. The LTFTs look to me like a vacuum leak.
The difference between the banks may be a result of a difference in O2 sensor sensitivity.

I have checked for vacuum leaks several times in all the usual places (using propane while watching the STFTs), but have yet to isolate one. Can anyone here help diagnose these results and suggest some other causes? I will go out again and check some other possible leak sources in the meantime. I will also clean the MAF sensor, which has less than 6k on it. The car has 150k miles and, as far as I know, has the original oxygen sensors as there is no record of them ever being replaced. Thanks!

-j

 

Here is an update. I found that the resonator had become partially separated from intake tube.


Here is the vacuum leak I found where the resonator attaches to the intake tube. I resealed it with JB weld. It looks to me to have been 'repaired' before.

Below is a comparison of fuel trims before and after the repair.


Here are plots of simple linear regressions of percent fuel compensation as RPM increases. This basically shows that fixing the vacuum leak dropped long term trims by 8% in bank 2 and 6% in bank 1. This fixed the DTC, but there appears to be another leak.

So I am on the right track and the trims are better, but there is still another leak lurking.

 

Another follow up. I drove the car around, roughly 6 miles and took some more measurements to see if the trims would improve with driving time and here are the results:


It appears that I lost another three or four percent of positive correction. I'll check it again in about a week to see if there is any more improvement. I am also going to swap the upstream O2 sensors across banks to see if the higher trims follow the bank 2 sensor.

 

Don't forget that the mid cat ones also affect fuel trims.

 

Great point, Edward! I will see how it goes with the upstream sensors first, and if higher trims don't follow the sensor, then I will try swapping the downstream sensors too.

 

My fuel pressure is between 54 and 55

 

I have an update, after a week of sitting I started it up this morning and found some cels that I have seen before but have gone away 6 months ago except for the very first one. That code is completely new

I did go ahead and replace the downstream O2 sensor and cleared the codes the only one that came back was p0442 my next step is to smoke the EVAP system. I have replaced the purge valve solenoid about 6 months ago. Codes have not come back after installation until now.