Help diagnosing boost loss please

[s4dave]

VAG-COM cable arrived yesterday (fantastic service from Ross-Tech - thanks guys) so checked for Fault Codes (found none) and went out to check boost, etc. Logged a handful of WOT runs with Block 115 and home to check it all. First couple of runs look just fine, but the third run shows this:

270.10,3520,15.0,1000.0,1020.0
270.40,3520,15.0,1000.0,1030.0
270.70,3520,26.3,1000.0,1010.0
271.00,3560,69.2,1380.0,1070.0
271.31,3600,92.5,1740.0,1400.0
271.61,3720,112.8,1710.0,1710.0
271.91,3760,117.3,1690.0,1750.0
272.31,3880,115.0,1660.0,1690.0
272.61,3960,115.0,1640.0,1670.0
272.92,4040,113.5,1620.0,1670.0
273.22,4120,112.0,1600.0,1660.0
273.53,4200,112.0,1590.0,1650.0
273.84,4280,109.0,1580.0,1630.0
274.15,4360,107.5,1570.0,1580.0
274.47,4440,106.8,1560.0,1570.0
274.79,4520,106.0,1560.0,1570.0
275.11,4600,105.3,1550.0,1570.0
275.43,4680,104.5,1550.0,1560.0
275.75,4760,104.5,1540.0,1570.0
276.08,4800,103.0,1530.0,1550.0
276.41,4880,103.0,1530.0,1540.0
276.74,4960,103.0,1530.0,1530.0
277.08,5000,101.5,1520.0,1520.0 <<
277.41,5080,99.2,1520.0,1500.0 << *:-O
277.75,5120,91.7,1520.0,1340.0 <<
278.09,5200,90.2,1520.0,1330.0
278.43,5240,89.5,1520.0,1340.0
278.78,5320,89.5,1520.0,1330.0

Columns are Timestamp, revs, engine load, requested boost and actual boost.

Graphing it looks quite ugly - the actual boost (and engine load) collapses at timestamp 277.75. The rest of the log shows a similar gap between requested and actual.


Checking again for Fault Codes now shows this:

2 Faults Found:
17536 - Fuel Trim: Bank 1 (Mult): System too Lean
P1128 - 35-00 - -
17538 - Fuel Trim: Bank 2 (Mult): System too Lean
P1130 - 35-00 - -


Now my first guess from the boost drop would be a leak somewhere... but why would that make it run Lean? If I'm losing boost (air), wouldn't that make it run Rich?

The turbos are virtually silent and I've not noticed any smoke so (fingers crossed) it doesn't look like the Turbo Fairy.

Fortuitously, the car is booked into a dealer next week and is under Audi warranty - but any help here would be much appreciated.

Many thanks in advance,
Dave
P.S. This is a 2000 S4, 41K miles, stock power with recent TBB and DV.

[excursion]

Multiplicative trim is used to counter a fuelling problem throughout the whole rev range. This type of fuel trim usually indicates a problem that's worse at higher RPM. Injector timing is adjusted from the base MAP so you will be loosing power too.

Multiplicative trim usually points to a faulty MAF, especially so when lean. The MAF would be the first thing I'd suspect.

You can do a rough check for the MAF on VAG. Group 002 shows RPM and amongst other things the air mass. Do a full throttle run in a single gear and log the air mass, the peak air flow should be around 0.80 * HP. Anything significantly less usually points to a faulty MAF.

I would think you are on a Hitatchi MAF being a 2001, but even so these can fail.

[JonnyX]

Definitely sounds like the MAF sensor to me as well (having experienced the same).

[sitas3]

Dave, here is a block 002 log from my old stock S4 for comparisons: Logging was done in 4th gear, WOT from as low revs as possible!

0.68,800,85.7,9.18,21.33,
1.58,960,90.2,9.18,26.94,
2.49,1120,94.7,9.86,32.83,
3.39,1280,101.5,10.20,39.94,
4.30,1480,110.5,11.56,49.94,
5.30,1680,124.8,12.92,63.67,
6.21,1920,142.9,14.62,86.31,
7.11,2200,160.9,15.98,103.47,
8.13,2480,154.1,15.30,110.72,
9.03,2760,149.6,14.96,122.03,
9.94,3040,147.4,14.96,130.14,
10.94,3280,146.6,14.62,141.36,
11.85,3560,140.6,13.60,145.94,
12.75,3760,139.1,14.28,153.03,
13.66,4000,137.6,13.94,158.33,
14.58,4240,135.3,14.62,167.14,
15.52,4440,133.1,13.94,171.14,
16.48,4640,130.1,13.60,175.56,
17.45,4840,129.3,13.94,180.33,
18.45,5040,127.8,13.94,186.97,
19.47,5200,126.3,13.60,192.14,
20.50,5360,124.1,13.60,196.22,
21.54,5560,123.3,13.26,196.72,
22.60,5680,120.3,13.26,199.17,
23.67,5800,119.5,12.92,200.03,
24.77,5960,118.0,12.92,204.00,
25.86,6080,113.5,12.58,200.56,
26.97,6200,110.5,12.24,200.14,
28.10,6320,109.8,12.58,199.56,
29.24,6440,108.3,12.58,204.00,
30.35,6520,106.8,12.58,203.44,
31.50,6600,104.5,11.90,201.72,
32.63,6680,103.8,11.90,202.28,
33.75,6720,102.3,11.90,200.03,
34.89,6760,101.5,11.90,200.03,
36.03,6800,85.7,9.86,168.61,

Columns are:

time, revs,%, ms, g/s air flow

Agree with the others - probably MAF related.

Hope you get it sorted and please let us know if you do.

[s4dave]

Thanks guys, I hadn't suspected the MAF but will log block 002 tonight and see what gives.

It's a 2000, so I suspect I have the Bosch MAF.

Dave

[excursion]

Thanks guys, I hadn't suspected the MAF but will log block 002 tonight and see what gives.

It's a 2000, so I suspect I have the Bosch MAF.

Dave

I really don't know where I got 2001 from

[Dippy]

Dave, don't clear those DTC - make sure that the dealer sees them.

As others have noted it'll likely be the MAF and that's a minor changeout job (assuming they have one in stock). If the techs are any good, as soon as they see lean codes they'll try a new MAF.

[s4dave]

Thanks for all the input - I will leave the DTCs for the dealer, but it's looking like the MAF.

So, going on the 80% of BHP rule I should see air flow around 212g/s at peak. Logging Block 002 tonight gave me this:

STAMP, /min,%, ms, g/s
256.67, 3160, 92.5,12.58, 87.47
256.98, 3480, 92.5,11.90, 94.69
257.28, 3720, 91.0,10.88, 98.89
257.58, 4000, 89.5,10.88, 104.47
257.89, 4240, 89.5,11.22, 110.39
258.20, 4520, 90.2,11.56, 118.08
258.53, 4760, 89.5,11.22, 123.42
258.86, 5040, 88.0,11.56, 128.92
259.21, 5320, 86.5,11.22, 133.94
259.56, 5560, 83.5,10.88, 134.75
259.92, 5800, 82.7,11.22, 137.50
260.28, 6040, 78.2,10.20, 139.89
260.66, 6280, 77.4,10.54, 140.72

So that's obviously well down on what it should be!

Will let you know how I get on with the dealer Monday, but thanks again - everyones help much appreciated.

Dave
P.S. First car I've owned that I can debug!

[sitas3]

yes crap airflow figures! even my old S3 used to do a lot better than that...

[excursion]

Thanks for all the input - I will leave the DTCs for the dealer, but it's looking like the MAF.

So, going on the 80% of BHP rule I should see air flow around 212g/s at peak. Logging Block 002 tonight gave me this:

STAMP, /min,%, ms, g/s
256.67, 3160, 92.5,12.58, 87.47
256.98, 3480, 92.5,11.90, 94.69
257.28, 3720, 91.0,10.88, 98.89
257.58, 4000, 89.5,10.88, 104.47
257.89, 4240, 89.5,11.22, 110.39
258.20, 4520, 90.2,11.56, 118.08
258.53, 4760, 89.5,11.22, 123.42
258.86, 5040, 88.0,11.56, 128.92
259.21, 5320, 86.5,11.22, 133.94
259.56, 5560, 83.5,10.88, 134.75
259.92, 5800, 82.7,11.22, 137.50
260.28, 6040, 78.2,10.20, 139.89
260.66, 6280, 77.4,10.54, 140.72

So that's obviously well down on what it should be!

Will let you know how I get on with the dealer Monday, but thanks again - everyones help much appreciated.

Dave
P.S. First car I've owned that I can debug!

Look's like the MAF to me 1.8T's can put down higher numbers than that.

It's great to be able to diagnose problems such as these, before the advent of OBD it could be a nightmare!

[Dippy]

For reference (not just S4dave), the highest reading I have had is 280.83 g/s and that wasn't WOT.

[sitas3]

For reference (not just S4dave), the highest reading I have had is 280.83 g/s and that wasn't WOT.

Stock? Chipped? at what revs?

somehow it would be handy to try and get a set of comparisons under the same conditions for boost/MAF block readings - a plus would be for different remaps also. Same goes for the RS4. I've been doing some logging on my stock RS4 but there is nothing to compare against as most are now remapped.

[Dippy]

See my sig.

revs 6640
load 145.9
injection timing 21.08
MAF 280.83
Speed 148
Part Throttle

[Morpheous]

This code comes up when the mass meter is knackered.
Dont do too many power runs until its replaced or you will be doing your turbos too!!
Well down, stock A6 2.7T runs 220 g/s

[s4dave]

Yep, car is loitering with intent, at my friendly local Audi dealer, to have a new MAF meter... the diagnosis here was spot-on, so thanks all!

Dave