FSI - Partial Fuel Trim too high?

S

stew

Member
I'm making another attempt to diagnose the misfire on the FSI and have been looking at fuel trims.

Nothing obviously is pointing towards an air leak, upon clearing codes short term trim at idle fluctuates between 0% and -10%, long term trim at idle is -0.1%.

I've noticed that the partial long term fuel trim is sitting at 16.4%, does that seem too high?
 
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stew said:
I'm making another attempt to diagnose the misfire on the FSI and have been looking at fuel trims.

Nothing obviously is pointing towards an air leak, upon clearing codes short term trim at idle fluctuates between 0% and -10%, long term trim at idle is -0.1%.

I've noticed that the partial long term fuel trim is sitting at 16.4%, does that seem too high?
Click to expand...
What were the engine related DTCs, which you cleared?
Mac.
 
Good Evening,

Did you use VCDS to obtain these values? If so post screenshots of relevant VCDS pages and I could post the same for my FSI for comparison.

Andy
 
I've been reading up on fuel trims on Ross Tech. Looks like that may be too high (haven't been able to find BAD specific figures yet) and it could be a faulty MAF. Air mass flow value reads around 50m/s when going to red line smoothly in 2nd gear. Is erratic and peaks around 70m/s if accelerating hard. Not sure on what actual values should be, I haven't been able to find anything specific to the BAD engine code.

I'm guessing that if that partial long term fuel trim is out and it's running lean then its likely to be the MAF at fault.
 
Andrew said:
Good Evening,

Did you use VCDS to obtain these values? If so post screenshots of relevant VCDS pages and I could post the same for my FSI for comparison.

Andy
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Thanks Andy, I'd really appreciate that. Screenshot attached.
 

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  • Fuel trims.JPG
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stew said:
Andy, is there any chance you could also post the values for your pre cat lambda sensor too?
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No problem but to save me looking it up which measuring block do you also need? Cars gone for its MOT, depends on how late it comes back tomorrow if I can squeeze it in, but Wednesday no problem.

Andy
 
Andrew said:
No problem but to save me looking it up which measuring block do you also need? Cars gone for its MOT, depends on how late it comes back tomorrow if I can squeeze it in, but Wednesday no problem.

Andy
Click to expand...
I think 034 should do it. Thank you.
 
Bit more background.
FSI BAD engine, approx 121k. Well looked after and serviced regularly.

Its had a misfire for a while, very noticeable between approx 1500 and 3000 RPM, I believe that's 'stratified' condition. Doesn't always throw the EML.

VAGCOM usually shows code P0300 for random multiple misfire and then usually further codes for one or more cylinders. Most recently (and most regularly) P0301 for cylinder 1 however time before last it showed codes for cylinders 3 and 4.

My gut instinct based on experience with other cars, the way its driving and the data to hand makes me think its the mixture.

I had a search on Ross Tech and found suggestion that the high partial long term fuel trim (16.4%) that suggests lean running could be the MAF. I've attached a log of the live data from the MAF (measuring block 002). Unfortunately I can't find any BAD specific data to compare it with but using the calculation suggested in the article I found on the Ross Tech website (BHPx0.8) a peak figure around 85-90 g/s could be expected. In two runs mine peaked in the mid 70's. Maybe its a little too low?

I found elsewhere on the internet that the figure I have for the long tern partial fuel trim could be caused by a failing MAF, failing pre cat lambda sensor of a manifold air leak. I guess it could be any or all of these. But I really don't know what to make of this data. I've had a look at the pre cat lambda data in measuring block 030, this looks ok, it fluctuates between 00011 and 00111. Aging shows as 1.59, but I'm not 100% sure on how to run that.

Does anyone have any ideas?

I've also attached a log of the misfires made this afternoon, don't really think it will be much help though.

Any assistance would be greatly appreciated.
 

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  • LOG-01-014-015-016.pdf
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  • LOG-01-002-xxx-xxx.pdf
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stew said:
Bit more background.
FSI BAD engine, approx 121k. Well looked after and serviced regularly.

Its had a misfire for a while, very noticeable between approx 1500 and 3000 RPM, I believe that's 'stratified' condition. Doesn't always throw the EML.

VAGCOM usually shows code P0300 for random multiple misfire and then usually further codes for one or more cylinders. Most recently (and most regularly) P0301 for cylinder 1 however time before last it showed codes for cylinders 3 and 4.

My gut instinct based on experience with other cars, the way its driving and the data to hand makes me think its the mixture.

I had a search on Ross Tech and found suggestion that the high partial long term fuel trim (16.4%) that suggests lean running could be the MAF. I've attached a log of the live data from the MAF (measuring block 002). Unfortunately I can't find any BAD specific data to compare it with but using the calculation suggested in the article I found on the Ross Tech website (BHPx0.8) a peak figure around 85-90 g/s could be expected. In two runs mine peaked in the mid 70's. Maybe its a little too low?

I found elsewhere on the internet that the figure I have for the long tern partial fuel trim could be caused by a failing MAF, failing pre cat lambda sensor of a manifold air leak. I guess it could be any or all of these. But I really don't know what to make of this data. I've had a look at the pre cat lambda data in measuring block 030, this looks ok, it fluctuates between 00011 and 00111. Aging shows as 1.59, but I'm not 100% sure on how to run that.

Does anyone have any ideas?

I've also attached a log of the misfires made this afternoon, don't really think it will be much help though.

Any assistance would be greatly appreciated.
Click to expand...
Hi

Sadly I have no experience with the FSI (@PlasticMac will hopefully chip in again) but looking at your scan it looks to me that the misfire is occurring on cylinder 1 by some margin with a bit on cylinder 3. Just to eliminate, it might be worth swapping things like spark plugs and or coil pack between 1 and 2 although you may have done this sort of thing already, so apologies if you have.

best wishes

Justin
 
birtyA2 said:
Hi

Sadly I have no experience with the FSI (@PlasticMac will hopefully chip in again) but looking at your scan it looks to me that the misfire is occurring on cylinder 1 by some margin with a bit on cylinder 3. Just to eliminate, it might be worth swapping things like spark plugs and or coil pack between 1 and 2 although you may have done this sort of thing already, so apologies if you have.

best wishes

Justin
Click to expand...
Thanks Justin but I've been down that road already. The fact that the latest data shows the misfire on number 1 is a little misleading, yesterday it logged the misfire on number 3 and number 4.

A few years ago I had a Boxster that had a bad misfire on start up and eventually reached the point of not starting. The cause was a split in the oil filler tube that was causing a loss of crank case vacuum. That only showed as an issue on one bank and predominantly one cylinder.

I had begun to suspect similar with this issue, however neither the short term or long term fuel trim at idle suggest this. I'm not ruling it out but I'm struggling to see how a vacuum leak on either the manifold or crankcase would only affect the partial long term fuel trim but not show up on the fuel trims at idle.
 
Two things I would carefully check or replace are the engine earth strap and the MAF sensor and check its wiring.
 
birtyA2 said:
Hi

Sadly I have no experience with the FSI (@PlasticMac will hopefully chip in again) but looking at your scan it looks to me that the misfire is occurring on cylinder 1 by some margin with a bit on cylinder 3. Just to eliminate, it might be worth swapping things like spark plugs and or coil pack between 1 and 2 although you may have done this sort of thing already, so apologies if you have.

best wishes

Justin
Click to expand...
I'd certainly check the brand, and date code for the coil packs. If the date code points to them being original, then change them, using Bosch, NGK, or Genuine Audi. If they are not original, but not one of those brands, again change them.
Your rev range covers the point at which the inlet flaps drop out of the airflow, (about 2,850 rpm). If it runs smoothly above 3,000, and up to 5,000, I'd suspect the flaps are not behaving, despite no codes to support that opinion. If so, fuelling will be off, below 3,000 ish.
It's possible for the flap actuator, and potentiometer, to move, but not the flaps. Do a suck test* to check the actuator and linkage.
Trying to think how to check if the spindle, and so flaps are actually moving though.
@Andrew has a inlet manifold in a partially disassembled state, perhaps he can see a way.
*The suck test is performed by attaching a rubber tube to the vacum port on the actuator, and sucking, which should see the linkage do it's thing.
Mac.
 
PlasticMac said:
I'd certainly check the brand, and date code for the coil packs. If the date code points to them being original, then change them, using Bosch, NGK, or Genuine Audi. If they are not original, but not one of those brands, again change them.
Your rev range covers the point at which the inlet flaps drop out of the airflow, (about 2,850 rpm). If it runs smoothly above 3,000, and up to 5,000, I'd suspect the flaps are not behaving, despite no codes to support that opinion. If so, fuelling will be off, below 3,000 ish.
It's possible for the flap actuator, and potentiometer, to move, but not the flaps. Do a suck test* to check the actuator and linkage.
Trying to think how to check if the spindle, and so flaps are actually moving though.
@Andrew has a inlet manifold in a partially disassembled state, perhaps he can see a way.
*The suck test is performed by attaching a rubber tube to the vacum port on the actuator, and sucking, which should see the linkage do it's thing.
Mac.
Click to expand...
If the flaps are down, but the ECU thinks they are up, (below 2850 ish), the fuelling will be lean, too much air.
Mac.
 
PlasticMac said:
I'd certainly check the brand, and date code for the coil packs. If the date code points to them being original, then change them, using Bosch, NGK, or Genuine Audi. If they are not original, but not one of those brands, again change them.
Your rev range covers the point at which the inlet flaps drop out of the airflow, (about 2,850 rpm). If it runs smoothly above 3,000, and up to 5,000, I'd suspect the flaps are not behaving, despite no codes to support that opinion. If so, fuelling will be off, below 3,000 ish.
It's possible for the flap actuator, and potentiometer, to move, but not the flaps. Do a suck test* to check the actuator and linkage.
Trying to think how to check if the spindle, and so flaps are actually moving though.
@Andrew has a inlet manifold in a partially disassembled state, perhaps he can see a way.
*The suck test is performed by attaching a rubber tube to the vacum port on the actuator, and sucking, which should see the linkage do it's thing.
Mac.
Click to expand...
Good Evening,

Q. Is there a simple check to guarantee all the actual flaps are opening and shutting correctly or at least moving?

Short answer no.

Longer Answer. This is perhaps extreme, but it would be possible after removal of the last (rightmost facing engine) bung (core plugs) that sits on the top of the lower inlet manifold (,there are 8 of them). You would then see the last flap move (or not) with the suck test. There will the be the problem of reinstating the bung air leak free and to my knowledge nobody ever has tried this rather drastic diagnostic technique. ( A place in Cheshire can probably provide a new bung!).

In reality if the first Block in Group 142 (whose input is from the potentiometer) changes from 100% to 0% as the revs approach 3000rpm then a >99% chance the flaps are moving correctly. The missing 1% is the case of a broken central rod on which the flaps sit but extremely rare, only ever heard of it once.

Misfires are not usually (but not always) associated with flap woes so I think forget it for now without evidence.

Andy
 
Thanks guys.

Coil packs are Bosch and less than 2 years old. Swapped around previously and code stayed on the same cylinder.

I did have the actuator for the flaps replace in 2018 due to a broken rod. At the time the flaps were quite clean and working correctly, but that was about 40k miles ago. I'll take a look a measuring block 142 when I next get time to pick this up again.

Should the flap be closed at lower rpm? I think this could be worth a look.

Another possible curveball, and something I forgot. A year or so ago I noticed that the EGR pipe had a leak. At the time I blanked this using a blanking plate for a Renault that allowed for movement of the EGR valve. I'm wondering whether I've got an air leak here, my thinking being EGR could be closed at idle but then opens under load allowing unmetered air in. Will also look more closely at that maybe see what there is in VAGCOM to get data for the EGR operation.
 
The flaps are held in the up, (active), position in stratified and homogenous lean mode, by engine vacuum via the actuator.
The flaps drop down, by return spring when vacuum is removed from the actuator, (controlled by a solenoid valve in the vac line).
In general, the flaps are no longer active above about 2,850rpm.
The flaps are binary, up or down, not variable.
It would be interesting to know if the engine runs without miss fires above 3,000 rpm.
Mac.
 
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