Common underboost causes?

[leshkin]

On a recent, 300mile round trip, I started getting some hesitation under heavy acceleration, which happened about 3-4 times in a space of about a week. As a result I started playing about with VCDS and doing some logging on the car as I was mainly interested in checking how well the MAF is working or if I had some serious boost leaks or engine controller errors.

I found no errors, but while doing this, I found that actual v's requested boost is consistently lower over the rev range, with the MAF readings showing air flow maxing out at 198g/s, which is a bit poor for a mapped AMK S3 (correct me if I'm wrong please).

So, for all you turbo wizards out there - What are possible causes of the missing boost?

My thoughts are that it's a boost leak or whichever valve that controls the boost level is stuck open, which prevents the turbo to deliver the requested boost. Could it be a limping turbo on it's way out? It's not smoking or showing any other signs of going.

I am running a stock DV and have a Forge 007P knocking about from the old car, so could try that. But before I start trying things out blindly I want to get people's opinion on other possible causes and if additional logging is in order to confirm the root cause. If the latter is true, I'd love to know the blocks I need to log as the latest version of VCDS does not have a lable file for my engine controller, which makes it a little tricky to decipher the meanings of some values

Here is a sample of the log I took earlier today:
RPM .Req ...Act. ..Mass Flow
.........Press Press
/min .mbar .mbar .g/s
1400 1820 1080 23.06
1520 1830 1110 26.19
1640 1840 1150 28.69
1760 1880 1200 32.61
1920 1980 1250 38.5
2080 2150 1320 48.28
2280 2320 1440 61
2480 2400 1620 78.06
2760 2440 1930 109.14
3080 2480 2400 130.08
3440 2520 2430 140.28
3800 2550 2350 148.72
4120 2550 2320 163.86
4480 2520 2330 169.47
4800 2450 2210 174.06
5120 2430 2130 178.69
5440 2410 2090 185.33
5720 2370 2040 190.53
6000 2330 2030 193
6240 2340 1990 194.25
6440 2330 1960 194.11
6640 2320 1920 198.06
6840 2300 2030 197.42
7000 2290 1950 126.08
6760 990 1760 5.22

Full log is here

Tell me if I'm going on a wild goose chase and the car is fine

Cheers for the help.

 

[MintyS3]

I would of thought boost leak, dodgy n75 or weak actuator for boost issues such as that. Sure someone else will add.

198g/s isnt a poor reading.

 

[sportstractor]

198 isnt poor, its about average. Hesitating could be boost/vac leak.

 

[Welly]

Could be a boost leak. Could be that the turbo is incapable of producing that boost any more due to being tired. Could be a spongy actuator, but it's still making reasonable boost.

Unlikely to be the N75 as it is performing as required.

Tuffty is reading this ATM so I won't go into logging block 032 - he will already be writing it I'm sure, so why re-invent the wheel? :thumbsup:

And as said above, 198 isn't a poor reading at all. Thats what you should be expecting from a stage 1 map to be fair. Puts you at about 247.5 BHP. Normally 250 is expected out of a Stage 1, so sounds good to me.

 

[Welly]

He will already be writing it I'm sure, so why re-invent the wheel? :thumbsup:

Disregard, he's gone.

Log block 032 in VAG COM and have a look at the values.

The value on the left is the one we are interested in. It should be within +/- 5%. If any more either way, it's likely to have a leak somewhere.

What you are looking at there is short term fuel adaptations. i.e. how much fuel is being added or taken away at idle by the ecu to compensate for missing air.

 

[leshkin]

Cheers Welly!

Will log some more tonight as advised. At the end of the day - if the turbo is not in top shape, then perhaps it's a good excuse to start planning a hybrid upgrade

 

[finesse]

leshkin, is the under boost intermittent? I have the same issue just started this weekend, one minute boost, next minute weak and lazy.

 

[leshkin]

finesse, no, the issue that prompted me to look at this was very short and very, very intermittent. It happened about 7 times under heavy acceleration and I have not had it for over a week now. Last time it happened, I was gunning it up a hill and it happened about 3 times in quick succession (that pesky A3 got away because of this ).

If I was to describe how it feels, I would say that someone/thing is squeezing the tip for a split second and quickly releasing it. It was not harsh, but very noticeable. Not sure if that makes sense.

This reminds me - got to log block 032...

 

[leshkin]

So, logged block 032 and the left lambda % value never changed from 0.6 throughout.

Does this mean I'm chasing non-existent issue? Thing is, something caused the power/boost/whatever to drop... could it be the tip collapsing? I heard that this happens on higher boost set-ups, not with basic remaps. I wouldn't know how a collapsing TIP feels, so maybe someone experienced this?

 

[Morgan-VAGCHECK]

Hiya,

Have you made any logs of N75 duty/boost together.. 118 for example ?

If you need me to review these hesitation/boost issues just let me know.

Edit (to add)

Collapsing TIP's in my experience can often feel quite violent when it happens, & are easy to diagnose when the fault/issue is replicable.

Regards
Morgan

 

[leshkin]

Thanks Morgan, I just put the little one to bed, so will try to log 118 shortly

Thanks for replying to my e-mail btw, I'm learning quite a bit about VCDS logging and interpreting the results at the moment, but will probably get in touch at some point regarding a checkup for the car. I have your map from some time ago, so depending on how many changes you guys made to it and the results of my logging, I might ask you guys to remap it with the latest code if it will benefit.

 

[leshkin]

Just did a few runs while logging 118 and here is the result of one of the runs (full results here):

Group A: '118
RPM Temperature Load Absolute Pres.
TIME
STAMP /min ∞C % mbar

150.86 1920 34 0 1000
151.17 1880 34 0 1000
151.46 1880 34 94.5 990
151.78 1960 33 94.5 1050
152.08 2040 31 94.5 1110
152.38 2120 30 94.5 1180
152.67 2200 28 94.5 1240
152.99 2280 27 94.5 1320
153.28 2360 26 94.5 1410
153.58 2480 25 94.5 1500
153.89 2600 24 94.5 1610
154.19 2720 24 94.1 1740
154.49 2840 24 93.3 1930
154.79 3000 23 91.8 2190
155.1 3160 23 85.1 2420
155.4 3320 22 83.9 2500
155.7 3520 21 82.7 2480
156 3680 21 80.4 2490
156.3 3880 21 83.5 2420
156.6 4080 21 83.5 2400
156.91 4240 21 86.7 2320
157.21 4400 21 89 2300
157.51 4600 21 88.2 2260
157.81 4760 21 90.2 2250
158.11 4920 22 92.5 2160
158.41 5040 23 93.3 2160
158.72 5240 23 94.5 2110
159.02 5360 24 94.5 2070
159.32 5520 24 94.5 2050
159.62 5680 24 94.5 2000
159.93 5800 24 94.5 2030
160.22 5960 26 94.5 2000
160.53 6080 26 94.5 2000
160.83 6200 27 94.5 1990
161.13 6280 27 94.5 1960
161.43 6400 29 94.5 1890
161.74 6520 29 94.5 1930
162.03 6640 30 94.5 1930
162.34 6720 31 94.5 1990
162.65 6800 32 94.5 1930
162.96 6920 33 94.5 1990
163.24 6960 33 94.5 1900
163.54 7080 34 94.5 1850
163.84 7120 34 94.5 1810
164.15 7200 35 94.5 1980
164.45 7280 36 89 2000
164.75 7280 36 0 2050
165.05 6800 34 0 1390
165.35 6320 33 0 1190
165.66 5880 33 0 1110
165.96 5480 33 0 1070

I assume that the 'load' value here is the state of N75, but I'm not sure what it means. The values change in the rev range, where I experienced the problem/hesitation - if my memory is not failing me. Every time this did happen, I didn't carry on with flooring it, so not sure if it recovers.

I googled the function of N75 and I think I get it, but I'm not quite sure why the N75 tries to open the waste gate before the turbo can barely manage the requested boost. Or am I reading this wrong?

Morgan, if you have a spare minute, then feel free to comment on the data that I collected so far. It's much appreciated.

 

[Morgan-VAGCHECK]

Hi,

It would be far easier & quicker given you are local if you just visit us so we can review the missing boost/hesitation , & we can explain N75 operation then.

Regards
Morgan

 

[leshkin]

I think I will Morgan - you have mail

 

[leshkin]

I thought I'd post the results of my visit to Vagcheck for completeness...

Morgan and Wak had a look at my S3 and found a PCV valve leaking and dirty air filter, so they were replaced. Spark plugs had a massive gap on all of them, which sounds like the main cause of the hesitation I was seeing. The misfires were not being detected by the ECU, hence not being logged as DTCs. According to Wak, misfire detection does not work well when it's the spark plugs that are the cause of it, so the plugs were replaced. Audi long life service is pants, is all I can say.

After some more map tweaking and logging, we saw airflow figures in the region of 205g/s and boost being held at the requested level over most of the rev range. The car is feeling like it got some power back and seems to pull much better/smoother. Looking at the airflow figures, I got about 5-10bhp back, which is great

On Morgan's/Wak's recommendations I have booked 4rings guys to replace all of the breather pipes, TIP and get a much needed service sorted as much of that pipework feels like it's made of jelly babies now.

I guess this goes to show that it pays to get a health-check done now and then as you get used to the way the car drives and don't realize it's full potential.

 

[sportstractor]

good work. I think the only thing I havent changed is the spark plugs on mine, what plugs did you fit? The Pcv was also an issue on mine when I got it and it made a great deal of difference when it was all sorted.

 

[leshkin]

The plugs were the NGK PFR6Q or Audi part # 101000063AA