So since I bought my S6 i have slight rpm fluctuation issue. I have replaced all 10 injectors, new IM seals. Car had very bad negative fuel trims reaching -11% i belive. Now the partial load LTFT look about right with bank 4 being the worst (-5,1%). LTFT for idle are around +4%. I noticed my maf readings do fluctuate between 1,5 and 2,1 g/s while engine rpm go up and down. Also with that i noticed lamba control values go up and down between 4 and 24 (can’t remember if they were positive or negative). No codes registered in ECUs. I suspect it might be Y plenum as it has a crack on one throttle body bolt, and the second one has stripped thread. Also I had an issue with a moan when turning engine off, and disassembling and assembling the sound tube in right wheel arch seemed to resolve the moan issue. Also the PCV line between V valley and bank 1 intake tube was replaced as it had a broken clip.
Any suggestions on how to tackle this? My first thought is to monitor STFT while spraying something around the intake.
The 5.2 is notorious for it’s oddities especially when it comes to vac leaks. It’s easy to have areas like the Y and throttle body that have leaks right at the mounting point. -11 is a lot
going from -5 to +5 is still a swing of 10% things like a proper smoke test can help but doing what you said might be important as well. You want to test the instant fuel correction. as that should help make more sense.
Hi,
As a stated current LTFTs after injector replacement are between -1,0 and -2,5% for banks 1-3 and -5,1% for bank 4. Idle LTFTs on the other hand seem to be around +4 all around
Not bad results. I don’t think enough people realize just how bad the injectors get on these cars after a while.
These are ok, however the bank 4 -5,1% makes me wonder why it’s twice the values compared to the other banks
Intake leak located at bank 2 throttle body
That would do it. While you’re probably not going to get all of the fuel trims to always be on point due to injector age and other components abnormalities looking to get the fuel trims all within a close range is a good rule of thumb
Any idea on what would cause my higher long term fuel trim in bank IV? It was noticably higher before and after replacement of all 10 injectors. PCV was changed out too. It seems weird, that i have richer reading from bank IV which is the closest to the throttle body with vacuum leak
Have you thought about the condition of your 02 sensors?
After all, they are what close the loop that drives the fuel trims for each bank.
This would be a good answer, however it only affects Partial load LTFT. Idle LTFTs seemd to be around the same all over, so I guess it would disregard O2s as an issue
This would make logical sense.
Also you know you have a vac leak… Before you fix the leak you can watch the real time fuel corrections for each bank. It would be worth spraying starting fluid in the are where the leak is and then watch the fuel trims real time correction. Then you can take a wet rag and try to wrap or seal off the leak are and then watch the trims. This would help let you know in a loose range the effect of the leak on the trims
This is more or less the method I used to find the leak. However I have strong feeling that said leak had more of an effect on Idle LTFTs than on Partial load LTFTs. Partial Load LTFTs were all around -0,8 to -1,5%, only the bank IV had -5,1%. Now that does not really make a lot of sense, since if was affected by the leak I would have lean fuel trims, and -5,1% is clearly a rich condition.
So it makes sense that leaks like this make a big impact at idle as the motor is trying to restrict air flow to keep the rpms down. So with a restricted air flow the vac on the system is very high say -20 or so this is a very healthy pull and you’ll find most leaks shown themselves best at idle.
This is in part why people get so many misfires on a cold motor and at idle. The motor is working super hard to restrict air flow. Even a small leak will bring in a lot of unmetered air that un metered air will cause a lean condition so then the car will start to add fuel to off set. This addition of fuel starts the fuel adaption. Since part throttle isn’t going to be nearly effected by the leak. And it’s not as effected by the leak as there is load on the motor and usually at part throttle your seeing less vac than idle the unmetered air gets absorbed better vie the increased demand for air. Due to this the idle fuel trim will be much different than the part throttle.
now depending on how much time you spend driving at one specific rpm and load this will cause the fuel trims to move to need to remove fuel via the lack of such vac seen under idle and high vac low rpm conditions.
did that make sense?
Yes, it absolutely does. Idle LTFTs are around +4 on al banks and this makes perfect sense. What does not make sense is the fact, that partial load LTFTs are great on 3 banks, being between -0,8 and -1,5% which as you stated wouldnot be affected by the leak that much, and bank IV is much richer showing -5%. The only reasons I feel are possible are faulty PCV leaking oil into intake manifold, despite being new, or maybe a failing plug or coilpack, but in this case I think the idle LTFT for bank IV should be noticably different than other 3, and it is not the case right here.
Anyway, I have found a used Y plenum, waiting for the seller to confirm that throttle body threads are not stripped or cracked. If it is ok, than i think I will replace it some time during the week and see if it fixed idle LTFTs, rpm fluctuation and maybe for whatever reason the bank IV LTFT at load.
So I have ordered new Y plenum, however it’s on backorder so I’ll have to wait for about a week. What tightening torque do you use for throttle bodies? Service manual states 9Nm. Is it correct? I don’t want to have brand new unit break the same way mine is right now, and I know that Y plenum tightening torque to IM is supossed to be 9Nm too, but the aluminium bolts break at 7.3 Nm from what I have measured.
Replaced Y plenum today. Maf readings seem to be between 4 and 4,5 g/s at idle. Engine load at idle is around 22-25% - is that a good reading? Also, idle LTFTs seem fine right now, and partial load LTFTs seem a bit lower now as well, the highest being bank IV with -3,9% reading.
However I have a new issue after having the throttle body sealed right now. Sometimes, when I put the car on park, the idle seems to go up, and fluctuates a little. What I noticed, is that suction jet pump hose connecting to the manifold seems to be quite loud. I posted a video on YT, and would love to know if it seems working properly: https://youtube.com/shorts/1dtQkeZbe3I?feature=share
Besides that, I have found no further intake leaks, including suction jet pump.
The video looked normal to me. The V10 pulls a TON of vac so when everything is working correctly you’ll hear a lot of vac noise. This is in part why even small leaks will cause a large issue.
Also the load sounds about right and so is the GPS that is usually what you should expect
Ok, so I think only think left is this: sometimes when I put in on park with ac off, The rpm go up to around 740-780 rpm, and it changes between this values. It is very stable, almost if I did it with the gas podał. No misfires or anything. It only started doing it after my throttle body was sealed, it never did it when it had a leak. Albo, restarting the car resolves the issue, but putting it back into drive and the back into park does not. No issues like that when I’m in gear. Any ideas?
Edit: So I just took my mother to work. Cold start, about 0,7 km and we stopped at a grocery shop for a second. When I put the car into park I had the worst misfires I have ever had with this car. I didn’t feel any while driving. I got my mother to work, came back home and the rpm did start to fluctuate in park. I did capture this on video which I will post a link to shortly.
Also, after that I did think that maybe for some reason throttle body aligment was the culprit, so I did realign both of the via VCDS. After that I did a little drive and the issue did not come back, however I feel it might. I will post about it as soon as it happens, or if I hit jackpot with it, I will post about this as well. However knowing, that throttle bodies should realign by itself makes my gut feel that it is just coincidence, and the issue is not resolved.
One more think. Would squishing that line from suction jet pump to the intake manifold have any effect on possibly getting the car to misfire just a little bit? I feel when I squished it and put my other hand on intake manifold that I felt some vibration.
Edit2: https://youtu.be/Kxvmm5hJCXw
This is the video I have captured. Forgot to mention earlier, I have also noticed, that maf readings on idle now seem to be around 2.7 g/s - is it still ok, or this would be too small amount?
Update: after throttle body adaptation both engine fluctuation when putting in park and misfires never came back. I still have very slight fluctuation, but not noticable in VCDS.
Also I have 3 questions:
- Is 2.9% idle LTFT indicative of a vacuum leak, or would it be considered normal?
- Is -5.5% partial load ok, or is it too rich?
- Is 2.72 g/s MAF reading with everyting off at 580rpm idle ok, or is it too low indicating a vacuum leak as well?
Edit: One more question. Would damaged smaller flex pipes affect anything concerning LTFTs?
Also I did about 30km after resetting LTFTs. Idle ones did not reach 1% all around. Bank 1 and 3 partial load seem to be around -4%, bank 2 is -2,2%, and bank 4 is -5,1%. Any ideas?
Edit2: I also did a maf logging, and at 7k rpm it was at 165,5 g/s. So twice that gives us 331 g/s. Is is too low? Rule of thumb, 80% of engine horsepower should give me about 350 g/s. Or is 331 g/s a good reading?
Edit3: Ok, one last question. Is 580 rpm idle a good value with everything off? With AC I have around 650 rpm