Workshops and Skills: Learn & Improve Your Abilities

Today I picked up my A4 Avant V6 TDI Quattro (AFB) from the workshop after taking it in for an inspection on Monday (it had 150,000 km on the odometer, but the engine itself has only done 55,000 km).

For quite some time now, I've been dealing with two issues that are really driving me crazy.

'The engine starts well when cold, but around a coolant temperature of 20-30°C, it starts to misfire and run roughly, as if something is wrong. Around 40°C, the misfiring completely stops.'
After a longer drive (engine properly warmed up), and a short stop at the bakery, the engine is difficult to start; it takes quite a while of cranking before it finally ignites.

I've been complaining to the workshop (a friendly dealership in Hürth) about these two problems for a long time, and all their repair attempts have been unsuccessful.
Included were items such as: 1) the classics like coolant temperature sensors, disconnecting the EGR valve, new vacuum hoses, fuel injectors, control unit, etc.
Regarding point 2), we then replaced the battery, the coolant temperature sensor, the crankshaft position sensor, the fuel injectors, and the throttle position sensor. Eventually, we also replaced the turbocharger valves, among other things.

All of that effort yielded no results, so I ended up spending countless hours on the internet, using a diagnostic tester, and employing an immense amount of patience to try and figure things out myself.

On Monday, when I took the car in for its inspection, I suggested to the mechanic that he check the starter motor's RPM and also test the voltage drop between the battery and the starter motor, as well as the grounding connection. (See Michael II's forum post for more details).
I don't know exactly what they did, but on Thursday they replaced the starter motor AND a ground strap. At the same time. Now nobody knows if it was the starter or the cable that was causing the problem.
The fact is: the engine starts immediately even when it's hot, and the starter motor spins noticeably faster than the old one.

So, problem number 2 is solved (after about 10 attempts, each of which probably cost around 6000 euros). I still can't say exactly what the cause was, though.

Problem 1 persists, and I'm truly at a loss; all the measurements are optimal, there are no errors in the memory, it's a complete mystery.

For me, it's once again clear that if you go to a technician, you absolutely have to tell him exactly what he needs to do, otherwise he might find the problem, but not the root cause.

Stefan.

Hi Stefan,

Is there a clear piece of fuel hose in front of your VP44 injection pump on your AFB system?
Is there air in the diesel filter? Are the oil supply lines to the injectors okay?

Have you checked the article yet?
/viewtopic.php?t=3687&highlight=afb

Best regards, Rainer.

Hello Rainer,

I've already read the article; that's why I had the idea about using a measuring tape. And the starting problems are now really gone (after extensive testing) thanks to the new starter motor.

'When the engine is cold, it starts perfectly fine. However, after about 100 meters of driving, it starts to misfire; THIS is currently the main problem that needs to be solved (in addition to the acceleration vibrations, but all possible measures have already been taken there, including replacing the engine, transmission, turbocharger, drive shafts, etc.).'

Everything related to the engine's operation is new: from the fuel tank vent solenoid valve to the water pump, timing belt, fuel injection pump, turbocharger, injectors, various sensors, transmission, etc.

Believe me, EVERYTHING underneath is new; even the control unit has already been replaced. So far, this whole ordeal has cost the VVG insurance company approximately €40,000 (!!!). That's how much the entire car cost new just 5 years ago...

Okay, so there's only the issue of the engine 'sawing' sound after a cold start, and the vibrations when accelerating in higher gears at around 1400-2100 RPM.

NO, it's not the timing belt tensioner, nor the fuel injectors, nor air bubbles in the fuel, nor a seized water pump, nor...

The vibration was originally the reason the car came to the workshop, even with the first engine. Now it's on its second engine and third transmission, and it still vibrates.

HELP!

Maybe someone else has an idea of what we could try.

Stefan.

Hello,

You mention that the coolant temperature fluctuates between 20 and 30 degrees Celsius, which indicates regular oscillations. Have you tried tricking the temperature sensor by simulating a higher or lower temperature? You could try replacing the sensor with a potentiometer or connecting a potentiometer in parallel or series with it. At least this will help you determine whether the control system is operating close to its stability limit or if there's another problem. Once you find the cause, the solution is usually not far off.

Best regards,
Christian.

Hi,

I think Chris's idea about manipulating the temperature sensor signals is good.

If that's not it, let's try supplying the ESP with fuel from a large jar and see if the pump is working.


Place a jar of vinegar and a container of hot water next to each other; this can be used to adjust the fuel temperature to a specific value.

An additional fuel pump wouldn't be a bad idea; as you know, the VP 44 is sensitive to air in the fuel. With an extra pump, you could 'prime' the system before this test.

Thank you for the tips!

Unfortunately, I have to say it again: I've already tried that.

The temperature sensors for water and oil have been checked for the validity of their readings and both have been replaced.
The water temperature sensor has already been replaced with a potentiometer by the workshop.

What can I say... NO insights gained, the vehicle remains completely unresponsive.

The fuel injection pump is already the third one (I forgot to mention that?), so it can also be ruled out as a cause of the problem.
Fuel preheating, fuel tank valve, priming pump; all already checked; it's not due to a lack of fuel, clogged filters, or air in the line; this has been checked multiple times.

'Nevertheless, thank you very much for the tips. I expected this high level of quality in ideas, and it really is helpful! (no sarcasm!)'
Unfortunately, unfortunately, unfortunately... Not this time.

However, I am very interested in further tips and ideas!

Stefan.

Hello for the first time,
Have you ever tried replacing the mass airflow sensor because of that rough running? Some people have experienced problems with it, especially after a certain mileage. That could also explain the jerking. Oh, and one more tip: they are now available as exchange units.




Beware of storms and wind, and cars made by Audi.

'Yes, the mass airflow sensors were replaced during every inspection; that is, every six months or every 15,000 km; some were even replaced more frequently. In all cases, they consistently showed a reading below 750.'

Again: the ENGINE has only approximately 55,000 km on it, while the body has 150,000 km; therefore, the mileage should not be taken as a reliable indicator. Even 50,000 km shouldn't be a problem, especially since it's a brand new engine manufactured in 2002.

I'm sorry, but a mass airflow sensor probably won't work either.

Stefan.

Hello Stefan,

Does it only idle roughly, or does it also run rough under load? Could you please describe the problem in more detail?
So wie ich das verstande habe hatte der Wagen neu das Problem nicht. Nun hat er es trotz Neuen Motors den meisten Hilfsaggregaten und zwei neuen Getrieben. Correct?

Best regards,
Christian.

The engine is idling and making a sawing sound while stationary.
Even the slightest press on the accelerator or an increase in idle speed while the vehicle is rolling will eliminate the sawing sound.
This prompted the workshop to either increase or decrease the idle speed.
However, this didn't improve the situation; it only shifted the sawing action to a different speed range around the idle speed.

By the way, it doesn't matter whether the air conditioner is on or off, or whether many or few appliances are running; the sawing motion is absolutely consistent, both in terms of timing and amplitude.

As described, the temperature curves of the sensors would be recorded for both water temperature and oil temperature. They are virtually identical, differing only by extremely small tolerances.

By the way, the engine always runs, regardless of whether it's a cold start at -10°C or after 3 hours of inactivity in a warm garage at 15°C. What really matters is the engine temperature.

It reminds me that when it's really cold, the idle speed is automatically increased. Once the initial warm-up stage is over (and the idle speed drops), it starts to misfire. Then, it seems there's another cold start phase where the misfiring suddenly stops and doesn't reappear until a certain temperature is reached.

So:
- Level 1: Increased idle speed, usually only in winter when the temperature is below 5°C.
- Level 2: Appears to have a normal speed, but it's cutting very aggressively.
- Level 3: Everything is completely normal, idle speed is stable and at the target value.

...

Unfortunately, my reference materials do not provide details on everything that happens during a cold start, the specific stages involved, or which parameters are relevant at each point in time.
Apparently, the workshop doesn't have this information either; otherwise, it might be easier to investigate the cause of the problem.

Stefan.