TDI RPM Fluctuations: Causes & Solutions

When accelerating, the engine speed fluctuates. What could be the cause?

Hello,

Does the engine sound change in parallel with the speed displayed on the speedometer? If so, I believe the clutch is slipping under certain load conditions. If not, it might be the instrument cluster.

Brösel wrote:

The VAG-Com test showed plausible mass airflow sensor and boost pressure values. Peak boost pressure occasionally reaches 2315 mbar, with a mass airflow rate of 845-850 at full throttle.

Hi,

"On my AXR, the maximum air mass values are consistently exceeding 1000 mg/h - the target value according to our specifications is 800 - 1100 mg/h."
http://www.dieselschrauber.de/Solldaten_4_Zylinder_PD.html
Yours is right at the very bottom of the tolerance range!

"Your boost pressure is already quite high (target value 1850..2250 mbar) - mine is just barely above 2000 mbar (maximum 2200 mbar)."

Hi,

Okay, let's go slowly, so you can write it down.

The car accelerates normally and smoothly, but the RPM (display) fluctuates significantly during acceleration.
If that's the case, I completely agree with Marco.
How about the process involving the engine noise? Does the engine noise change in parallel with the tachometer? If so, then the clutch is likely slipping. If not, then there might be a problem with your instrument cluster.

I was initially distracted by the target values for boost pressure and air mass (air mass was almost at its minimum, boost pressure was above its maximum). These values don't match my AXR engine at all.

Brösel wrote:

Hi Bertil, The TARGET value of the mass airflow (MAF) sensor is 850 at full throttle.

Hi Brösel,

According to the table and the workshop manual, the target value for the air mass at full load is between 800 and 1100 mg/H. So, yours, with 845-850 mg/H (as you stated in your first post), is at the lowest end of the scale! As mentioned above, mine easily exceeds 1000 mg/h (@Ulf that's why mine probably also runs like crazy ).

Hello,

Does the clutch still disengage properly? If, for example, a seal near the clutch has torn, you might have oil on the clutch. That could be the explanation.
It's likely due to occasional wheel slippage.

Based on how you're describing the problem, it's clearly related to the clutch. If the engine speed drops under full load, but the car continues to accelerate, it indicates a lack of proper power transfer.

The fact that the error occurs sporadically could be due to the "condition" of the clutch and external factors such as air pressure, outside temperature, fuel temperature/quality, which can result in different torque values from the engine.

-> Workshop!

Best regards,
Uwe

Bertil wrote:

So, yours, with 845-850 mg/H (as you stated in your first post), is at the very bottom end of the scale! As mentioned above, mine easily exceeds 1000 mg/H (@Ulf that's probably why mine is running like crazy ).

Hi Bertil,

If the functions of the air quantity limits in a PD (presumably referring to a specific engine control system) operate in the same way as in a VP-TDI (another engine control system), then "excessively measured air" should not lead to an increase in power output – because the torque limit would prevent it (see also LLK technical articles, etc.).

It's possible that when the turbocharger pressure increases rapidly (full throttle from partial load), the increased airflow could trigger a slightly larger fuel injection amount, but only for a brief moment, until the torque limiter kicks in again.

Or does the management of PDs fundamentally work differently ?

ulf wrote:

Or does the management of PDs operate fundamentally differently

... Good question ... next, please ...

Hi Ulf,

Okay, I can't give you a specific answer to that. But I've made the following observation. When I installed the faulty mass airflow sensor (MAF) from my brother's car (SEAT Toledo), the fuel injection was drastically reduced (which is understandable). After I reinstalled my own MAF, I experimented a bit.
Diode solution: No success - Emergency mode activated under high load - This worked with the original STG (control unit).
"I experimented with various resistors for 'age simulation,' and I noticed that even with excessively high mass airflow sensor (MAF) values (i.e., above the tolerance of 1100mg/H), the fuel quantity still increases. In my tenant's vehicle (original AXR engine), it didn't make any difference (as you described)."

Therefore, I suspect that my AT control unit might be programmed differently than the original one, or that the torque limitation is simply activated later.
I didn't notice or measure any performance differences between the two STGs.
Unfortunately, I can't make sense of these observations.


By the way, the acceleration from my tenant's car isn't any better (and is actually worse) than mine. Since it's a smaller car and a bit lighter than my "Pampersbomer" (a humorous nickname for a large car), I actually expected something different.

Bertil wrote:

Various resistors for "age simulation" - I noticed that even with excessively high mass airflow sensor (MAF) values (i.e., above the tolerance of 1100mg/H), the fuel quantity is still increased - in my tenant's vehicle (original AXR), it made no difference (as you described). Therefore, I suspect that my AT control unit might be programmed differently than the original one, or that the torque limitation is simply activated later. By the way, the AXR (acceleration) from my tenant's car isn't any better (and is actually worse) than mine - since it's a smaller car, and those are a bit lighter than my "Pampersbomer" (a nickname for my car), I actually expected something different.

Hi Bertil,

Perhaps it would provide some clarity if you occasionally logged the quantity limits of both vehicles during the 2000-4000 test.

ulf wrote:

Perhaps it might bring some clarity if you occasionally log the quantity limits of both vehicles during the 2000-4000 test.

Hi Ulf,

I would like to, but my tenant's car was the victim of a summer tire-wearing idiot tonight who refused to believe that the road has been icy in the mornings for the past 2-3 weeks -- a full-on collision (banana damage, about 20 cm dented - driver uninjured). I think the car is probably beyond repair, let alone that I can measure anything on it .

As far as I remember, the fuel injection volume was lower in the "Limo" model compared to mine.
During my last visit to the workshop (for an LLK replacement), I was asked if I had had any chip tuning done on the car. "Of course not" (which is actually the truth) was my reply. The mechanic then said, "...well, it must be something else... doesn't really matter..." Unfortunately, I didn't ask any further questions. However, since this company can use a VAG tester, I think they noticed the high fuel injection rates. Some time ago, we discussed the idle fuel consumption here, and mine was extremely high compared to others (1.2 L/h - warm). However, it was different with the old engine control unit (0.9 L/h). Since the new engine control unit was installed, the MFA display is also showing the correct values; previously, it was showing about 0.5 L/100km too low. It is no longer possible to correct the display (it wasn't possible with the old engine control unit either).