A4 TDI 110 (AFN) Overboost caused by timing

[ini]

Lately, i have again been experiencing overboost in my B5 A4 TDI SE 110 'afn' engine. (boost cutting out under load/hills & motorway speeds, in 3rd gear and above around 3000rpm)

I had a clue as to the cause of the problem, because after towing a heavy trailer for several hours, my car would be much more powerful, and not overboost for around 2 weeks, gradually the power would drop and the overboost would start to return.

The sustained load was causing the injection timing to be advanced, resulting in my car running well for a couple of weeks until in adapted back.

I have just checked with VAG-COM, and surprise surprise, at running temps, my timing is nearly off the bottom of the chart.

The timing belt is slightly overdue for a change though. lol

[AutoTDI]

Hi ini,
I am interested in your comments on the AFN engine as I have same engine in my car. Please excuse my ignorance in these matters, but could you clarify the following for me:

Lately, i have again been experiencing overboost in my B5 A4 TDI SE 110 'afn' engine. (boost cutting out under load/hills & motorway speeds, in 3rd gear and above around 3000rpm)
Does "overboost" result in boost cutting out? Seems contradictory.

I had a clue as to the cause of the problem, because after towing a heavy trailer for several hours, my car would be much more powerful, and not overboost for around 2 weeks, gradually the power would drop and the overboost would start to return.
Why would towing a trailer for several hours prevent overboost for 2 weeks? How is this long time delay generated and for what purpose?

The sustained load was causing the injection timing to be advanced, resulting in my car running well for a couple of weeks until in adapted back.
Again, why should "dynamic" injection timing take a couple of weeks to adapt?

I have just checked with VAG-COM, and surprise surprise, at running temps, my timing is nearly off the bottom of the chart.
Do you have any figures for timing; ie "normal" and bottom of chart?

The timing belt is slightly overdue for a change though. lol

[ini]

I said 'overboost' when i really meant 'limp mode'.

I think the very retarded timing is underfueling the car, resulting in manifold pressure deviation, causing the car to vastly reduce boost n75 operation, until the ignition is cycled.

Why the adaptations last around 2 weeks i am not sure, i do not know what kind of algorithm's are used. I can only guess that the adaptations are weighted in favour of advancing the injection timing.

Around 6 or 7 times a year i tow cars for several hours on a heavy trailer.
For the first 50 or 60 miles the car will go into limp mode every so often under load, after that the car drives well.

When i finish towing, the car is always vastly more powerful for around two weeks, with the power slowly dropping off over the last week.

During this time the car does not go into limp mode, and behaves as it should, pulling strong throughout the gears at all revs.

I have not used VAG-COM on the car after i have towed, but my current timing measurements at temp with the 'TDI Timing Checker' are:

Fuel Temp - 133

Injection Advance - 23

This puts the timing well outside the specified range for my afn engine.

http://www.ross-tech.com/vag-com/cars/tdi.html

[AutoTDI]

Thanks for the info, ini.
I really don't understand that 2 week time delay. I can't believe that such a time constant would need to be programmed into the software. Could it be a sensor problem? Is it possible that something is overheating/expanding and giving a false reading and then taking a couple of weeks to recover. Or possibly a valve jamming - like the EGR valve?

[ini]

I dont understand the 2 week adaptation dropoff either, although if my timing is really mechanically retarded that far, i dont think there would be any requests to retard the timing, it must just be the waveform shape of the dropoff back to normal mechanical timing, if you catch my drift.

It is definitely not my EGR, but you may have a point about sensors.

If my mechanical pump timing was so far out, surely i would experience starting or performance/running problems (other than limp mode under load in higher gears)

I think i need to have another look with vag-com, this time not using the basic settings.

I have had some doubts about my fuel temp sensor for a while.

Could be time to replace all engine sensors.

Although after towing the car is great, and adaptation is usually 50 miles to take full effect.

I think the towing helps excercise the vnt mechanism too.

Anyone any ideas on what else would retard adaptation timing?

[ini]

In a bid to cure the manifold pressure deviation am going to try replacing the hose to the pressure sensor inside the ECU.

In the past i have replaced the hose inside the ECU plastic cover, but this apparently is the wrong hose.

The correct tube to replace is inside the metal body of the ECU, and joins the charge pressure sensor on the circuit board to the metal case.

I will also replace all control tubing and one-way valves.

[BrendanMC]

None of the above. Your turbo is sticking, sometimes in boost and sometimes not, bet it sometimes seems to do nothing then just shoot away, also if you slow down to go over a speed bump or such and come down a gear it will shoot away, run up hills without your foot on the pedal.
your turbo is sticking.
hell of a problem, mine's the same.

[deian3387]

I agree with BrendanMC, it could be the turbo pressure acctuator sticking, located on the turbo, when you go up hill and use the turbo under load the turbo gets hot. The hotter the turbo the more likley it is for the actuator to stick. common fault on these.

[ini]

Thought i had solved the problem a few times (N75,VNT,timing,replace control tubes), but it eventually turned out to be a partially collapsed CAT filling up with soot/crud.

Several hours of high temp running was burning off some of the offending crud and reducing the exhaust back pressure.

The exhaust restriction was throwing intermittent manifold pressure, and MAP error codes.

The same symptoms can be caused by vastly different hardware problems, and due to the adaptive nature of the management system, replacing one component can temporarily make the problem seem to disappear.

[AutoTDI]

The exhaust restriction was throwing intermittent manifold pressure, and MAP error codes.

In this thread: https://www.vwaudiforum.co.uk/forum/s...ad.php?t=18322 you reported that one of these MAP fault codes was: "short to B+", which looks very much like a MAP hardware problem. Is it possible that this code could be thrown as a result of an exhaust restriction? Of course, we don't know what the actual process is for generating this code so I suppose anything is possible. It would be nice to have a software flow chart showing what the effects of all these sensor measurements and diagnostics have on actual fuel delivery and turbo boost. I assume that the firms which re-map ECUs and/or sell power chips have all this info, but where do they get it from?