Need some help understanding STFT and LTFT

[Ma6A6]

Hello everyone!!!

Still working on my car and now I'm trying to understand fuel trim because I have P2177 code (lean off idle)

My Long Term Fuel Trim is now at 20.3% which, as far as I understand indicates a vacuum leak right?? (some times, randomly, my idle goes up to 1000 with MAF readings of ~5.5g/s)

Fuel pressure at rail is moving from 750 to 1700 psi with RPMs

Now, what I don't understand is my Short Term Fuel Trim... if I press the gas pedal it goes up to 10%, the when cruising it moves around 0% and when released it moves to -12%. I understand it should fluctuate but is -12 to +10 normal??

I know I have to replace my valve cover but i don't know if this will help with fuel trim or should I look somewhere else.

Any help/comments will be really appreciated.

Regards
Mauricio

[EvolutionArmory]

http://wiki.ross-tech.com/wiki/index.php/Fuel_Trim_Info

http://wiki.ross-tech.com/wiki/index.php/18609/P2177/008567

[EvolutionArmory]

Some 2.0 FSI specific things I would look into first is a bad PCV valve.

Get the intake system smoke tested and repair any air leaks.

Maybe look into the G247 rail pressure sensor.

Look for any major oil leaks near the transmission. Having a bad PCV valve for too long can do some damage to the rear main seal and can induce an air leak there. Seems odd but it happens.

[Ma6A6]

Some 2.0 FSI specific things I would look into first is a bad PCV valve.

Get the intake system smoke tested and repair any air leaks.

Maybe look into the G247 rail pressure sensor.

Look for any major oil leaks near the transmission. Having a bad PCV valve for too long can do some damage to the rear main seal and can induce an air leak there. Seems odd but it happens.


Thank you so much!!!

I don't have a PCV, I'm using a pcv delete plate and the fuel rail pressure sensor is the original one and seems to be reading OK. Does this sensors fail sending wrong values???

I have read the Wiki and will start to do some tests using my VagCom (i have been using a generic OBD Reader and my phone) and get the intake smoke tested. I do have an oil leak between engine and transmission

I have a lot of things to check...

Thanks again!!
Mauricio

[EvolutionArmory]

Ditch that PCV delete. They usually cause more harm than good. When the OEM system is working, it’s actually pretty efficient.

You don’t have a PCV valve now but that doesn’t mean that it wasn’t an issue that took out the rear main previously.

Having an oil leak between the engine and transmission on top of a lean code would really make me suspect a bad rear main. Throwing a bottle of oil leak detection dye in would really help to isolate the source of the leak. The vacuum pump oil seals can leak oil down the back of the engine and will pool at the bottom. If there’s no dye up there then it’s safe to assume it’s the rear main that is leaking.

Also, keep in mind that all of these cars are old now so it’s possible that you could have a leak coming from the vac pump AND the rear main too 😞

[Ma6A6]

Thanks!!! Few more things for the ToDo list...

[Older_not_Wiser]

Do the diagnostic procedures outlined by EA, but ultimately I believe you will find the issue is with clogged injectors that need to be cleaned or replaced. LTFT of +20 would have to be a huge vacuum leak. Check timing pull by cylinder. That will likely point to which are not flowing per spec.

[Ma6A6]

Do the diagnostic procedures outlined by EA, but ultimately I believe you will find the issue is with clogged injectors that need to be cleaned or replaced. LTFT of +20 would have to be a huge vacuum leak. Check timing pull by cylinder. That will likely point to which are not flowing per spec.

Thanks!!!!

Do you know how do I check the Timing Pull by cylinder??? I can't find any info about this in gasoline cars (have found some info on diesel)

Thanks in advance

[EvolutionArmory]

Run a log of 020 in MVB. If one cylinder is pulling a lot of correction factor VS the others, you could have a problem with that cylinder.

[Ma6A6]

Run a log of 020 in MVB. If one cylinder is pulling a lot of correction factor VS the others, you could have a problem with that cylinder.

Thanks!!! I did check that today and pistons 2 and 4 move to -3 and -5 kw respectively while 1 and 3 remain at 0. (don't have a log, checked in screen) Is that a lot of correction factor??

Thinking on the rear main seal... would that be a huge vacuum leak as Older_not_Wiser said???

[EvolutionArmory]

It could cause a large air leak, yes. Put the leak detection dye in and isolate the source.

-3 and -5 isn’t a lot of CF but since the other 2 are zero he could be on to something with the injectors. Ideally you want zero across the board but there are a few things that can influence it. Fuel quality, spark plugs, false knock from bad or improperly torqued knock sensors, etc. Like I said earlier, old cars can have multiple issues.

And put your PCV system back to stock with a new PCV valve. Nothing sucks more than trying to diagnose something with an unknown variable like an aftermarket part that is known to cause issues with some cars. Those block off plates can make the car run worse than with the part they are intended to upgrade. I would never put one of those on my car unless I was running a turbo that wouldn’t allow me to run the stock PCV system.

And you should do a proper full throttle log of 020, foot to the floor the whole time from 1500 to redline or as high up in the RPM range as you feel comfortable. If 3rd gear is too fast, do it in 2nd. That should only get you to about 60 MPH. Doing a proper log will give you a better idea of what the car is doing than just looking at the values live.

[Ma6A6]

It could cause a large air leak, yes. Put the leak detection dye in and isolate the source.

-3 and -5 isn’t a lot of CF but since the other 2 are zero he could be on to something with the injectors. Ideally you want zero across the board but there are a few things that can influence it. Fuel quality, spark plugs, false knock from bad or improperly torqued knock sensors, etc. Like I said earlier, old cars can have multiple issues.

And put your PCV system back to stock with a new PCV valve. Nothing sucks more than trying to diagnose something with an unknown variable like an aftermarket part that is known to cause issues with some cars. Those block off plates can make the car run worse than with the part they are intended to upgrade. I would never put one of those on my car unless I was running a turbo that wouldn’t allow me to run the stock PCV system.

And you should do a proper full throttle log of 020, foot to the floor the whole time from 1500 to redline or as high up in the RPM range as you feel comfortable. If 3rd gear is too fast, do it in 2nd. That should only get you to about 60 MPH. Doing a proper log will give you a better idea of what the car is doing than just looking at the values live.

Thank you so much EA. Valve cover and PCV are on their way here. Hopefully next Sunday I can work on the car and will report back. I will do the log (yesterday I forgot to "start recording") and i am looking for the DYE.

I really appreciate all your help!!
Have a nice Day!!!

[EvolutionArmory]

Make sure you buy the right PCV valve.

If your car was built before 12/05 it needs the R part number and a blank breather tube.

After 12/05 takes the P part number and has a check valve in the breather tube.

Mixing up the part numbers will cause you to either have 2 check valves or none.

[Ma6A6]

Make sure you buy the right PCV valve.

If your car was built before 12/05 it needs the R part number and a blank breather tube.

After 12/05 takes the P part number and has a check valve in the breather tube.

Mixing up the part numbers will cause you to either have 2 check valves or none.

Thanks
I have the R PVC and the blank tube.

[LRS/GSR]

If he bought a new valve cover it would be black. Doesn't that mean he needs the newest revision or only based on engine build date?

[EvolutionArmory]

If he bought a new valve cover it would be black. Doesn't that mean he needs the newest revision or only based on engine build date?

Valve cover color doesn’t matter.

Your combo of PCV and breather tube does. You can use either revision valve as long as you use the corresponding breather tube with it too.

An example. Say you have a 2005.5 that uses the R valve and blank tube. You can use the P valve as long as you use the breather tube that has a valve in it that goes with the P valve. You have to swap both parts.

If you don’t use the right PCV valve with the right breather tube, you’ll either have 2 check valves or none.

[Ma6A6]

Well, today I did some work on my car that hopefully lowers the LTFT.

New Valve Cover Installed
New PCV Installed
Cleaned Throttle Body
Cleaned MAP
Cleaned MAF
Replaced Turbo-CatConverter gasket

I decided to leave the PCV venting to air as well as the Breather. I made a plate to seal the breather pre turbo inlet from the tube. If I understand it well if the LTFT begin to come down this may indicate the air leak came from the crankcase. This is temporal just to try to identify the air leak source (haven't found the DYE at a reasonable price)

For the smoke test I think i will DIY my smoke machine since there is no one near me.

Will report back once I know if mi idea worked.

Regards

[EvolutionArmory]

Put your PCV system back to stock 100%

Why would you buy a new valve just to hook it up wrong? Why make things more difficult than they need to be?

[Ma6A6]

Put your PCV system back to stock 100%

Why would you buy a new valve just to hook it up wrong? Why make things more difficult than they need to be?

It will be just 1 day... to see if the air leak comes from the crankcase. Tomorrow night I will check LTFT and connect everything the way it should be.

I am trying to isolate the crank case from the intake... is this correct??

[EvolutionArmory]

Well, you already just bought and installed new parts. Usually you do tests before you swap parts. 😉

You don’t need to separate the intake from the crankcase to do a smoke test. You would block off the PCV hose going from intake manifold to the PCV valve to do a boost leak test so you don’t pressurize the crankcase but that’s not necessary for smoke.

The whole point of a test is to do it with broken parts to find the fault....

[Ma6A6]

With isolated crankcase the LTFT went up so I think the air leak shouldn't be in the crankcase. Still looking for a Smoke Test Machine around me.

Everything is stock and connected now... Also calibrated the throttle body and cleared codes as well as fuel trim values.... Will do a log on Fuel Trim and Injectors as soon as I can

[LRS/GSR]

What's your MAF reading at idle? Does it idle down to 800 rpm after cold start? What's your STFT at idle? STFT will always move around while driving in closed loop. It goes to closed loop correct? Do you put good quality premium fuel in the tank?

Winging parts at these cars gets expensive fast eventhough they are old.

[Ma6A6]

What's your MAF reading at idle? Does it idle down to 800 rpm after cold start? What's your STFT at idle? STFT will always move around while driving in closed loop. It goes to closed loop correct? Do you put good quality premium fuel in the tank?

Winging parts at these cars gets expensive fast eventhough they are old.

MAF moves around 2.9 - 3.1at idle. (I am 7000ft / 2200m above sea level) I got 120 at 6000 rpm in 2nd gear.
Idle goes down to 750 after cold start (takes 1 minute)
After clearing codes STFT can go up to +20 but once the LTFT reaches 19-20 after a few days of use STFT moves around -7 to +9
Good quality gas yes. 92 octane

[EvolutionArmory]

Did you even put the leak detection dye in yet to isolate the source of your oil leak? Throwing all of these parts in will do nothing for you if you have a rear main leak.

I don’t mind offering you diagnosis advice but if you’re just going to throw parts at it and not follow any of the advice let me know now so I’ll stop helping with diagnosis.

[Ma6A6]

Did you even put the leak detection dye in yet to isolate the source of your oil leak? Throwing all of these parts in will do nothing for you if you have a rear main leak.

I don’t mind offering you diagnosis advice but if you’re just going to throw parts at it and not follow any of the advice let me know now so I’ll stop helping with diagnosis.

Hello EA!!! DYE is supposed to be here tomorrow. I haven’t put any more parts than the valve cover and PCV.

I really appreciate all your help and I am following your advice. It’s just that sometimes I can’t get things fast due to shipping times.

I still need to found some way to smoke test the intake (maybe a DIY with mineral oil and soldering iron) because no one near me has the machine.

Once again THANK YOU!!

[EvolutionArmory]

Apologies. I forget that not everyone can just go to a parts store down the street and buy a dye kit for 5 bucks. There’s 4 parts stores within 2 miles from me 😀

Definitely rule out the rear main, especially since you were running the block off plate. Do you know if the block off plate was used to correct a bad PCV valve or was put there just because. It would suck if it was put there just because and somehow contributed to the failure. I wouldn’t be surprised though.

[Ma6A6]

Apologies. I forget that not everyone can just go to a parts store down the street and buy a dye kit for 5 bucks. There’s 4 parts stores within 2 miles from me 😀

Definitely rule out the rear main, especially since you were running the block off plate. Do you know if the block off plate was used to correct a bad PCV valve or was put there just because. It would suck if it was put there just because and somehow contributed to the failure. I wouldn’t be surprised though.

No worries!!!

Next steps... check leaks with DYE, Log Injectors, MAF, Fuel trim... this will be for next week.

I'll keep reporting

[LRS/GSR]

I made a smoke machine with a pickle jar, Bic pen, old sock, bicycle pump, and baby oil. Oh and soldering iron.

Your MAF readings aren't far off but may have a little intake leak. I think it may just be part of the issue. I think EA is right.

Recently my car was 8% lean. I had 2 small boost leaks, smoke pouring out of dipstick tube seal, and upstream O2 not reading properly.

I bet if you smoked it would pour out of the effected area.

[Ma6A6]

I made a smoke machine with a pickle jar, Bic pen, old sock, bicycle pump, and baby oil. Oh and soldering iron.

Your MAF readings aren't far off but may have a little intake leak. I think it may just be part of the issue. I think EA is right.

Recently my car was 8% lean. I had 2 small boost leaks, smoke pouring out of dipstick tube seal, and upstream O2 not reading properly.

I bet if you smoked it would pour out of the effected area.

Thanks LRS! I think I will do my own smoke machine like you did.

I will get my DYE today and will see if there's is any problem in the crankcase. I'm thinking on pouring it when I leave the office and check once I get home. I also found another area with oil... Oil filter housing... and i think it will also create an air leak.

How should I connect the DIY smoke machine?? what should be plugged/disconnected/connected???

[Ma6A6]

Here is a table I made to check my MAF readings since I am at 7200ft / 2200m altitude. Maybe useful for someone else. It is for a stock car.

Its is based on some formulas I found so I can not guarantee this are exact but they helped me...

[EvolutionArmory]

MAF should be 1gs per liter of displacement at 500 rpm at sea level. Since our displacement is technically under 2.0 and our cars idle at 800 RPM, usually MAF at idle will be somewhere in the mid 2’s at sea level. It’s not an exact formula. Just a rule of thumb. I don’t know what multiplier you would use to account for altitude.

There’s actually a MAF value thread if you search for it. It shows elevation, flow at idle, max value during a pull for many different stages of tune, etc.

Mine idles at 2.5 ish and maxes out at 282.
Most people who live under a few hundred feet show mid 2’s at idle.

[Ma6A6]

Today I did the DYE test.... I thought it would be more evident but I think my oil leak comes from the Oil Filter Housing. I'm not sure about the real main seal... [confused]

The filter cap looks "green/brown" using the UV lamp, as well as the "wall" of the crankcase behind it.

I think the smoke test will confirm any of this leaks right???