1.4 TSI CTHA - Fuel consumption figures (Solution)

Hello everyone,

My daughter is complaining about the fuel consumption of her Alhambra. I ignored it because she drives mostly short distances and it's currently winter.

Last week, I drove 2700 km across flat terrain in Austria, Slovenia, and Hungary. At the gas station, the average fuel consumption came out to 11.6 liters per 100 km. I drove 85% of the distance at 130 km/h using cruise control.

I've obviously tried a lot of things, logged some of them, and I'm running out of ideas.

- Car has approximately 200,000 km (miles).
- There are no errors!
- Seals for the air ducting were replaced.
- The air intake system was thoroughly checked and only minimal leaks were found, and these only occurred at pressures that the engine does not reach during operation. Up to 2.5 bar, everything is sealed/pressure-tight.
- The throttle valve and control valve were removed for cleaning, but this was unnecessary as they were not clogged.
- New spark plugs.
- Timing chain replaced and perfectly adjusted 15,000 km ago.
- N75, new.
- Wastegate is common, no hang-ups.
- Charging pressure is easily achieved and maintained across all operating conditions!
- Lambda 1 is well-regulated outside of extremely transient states.
- Runs smoothly, but with slightly less elasticity than usual, starting at 2500 RPM.

Now, let's talk about the minor details that I've noticed and that are making me feel uncertain.

"In partial load conditions (e.g., cruise control at 130 km/h), the actual boost pressure is consistently significantly higher than the target boost pressure. The target boost pressure only adjusts when, for example, driving uphill or when accelerating." Is this intentional?

2. The engine coolant temperature remains stubbornly fixed at 88-90°C. The "target" coolant outlet temperature in VCDS is usually almost always 90°C. However, the actual catalytic converter temperature drops below 10°C on the highway, especially in freezing temperatures. This seems logical to me, though. Should it be different?

"The most significant issue I've noticed is something that occurs during longer trips." After approximately 1.5 to 2 hours on the flat road, the "absolute load value" drops from an unhealthy 95-105% to the normal 75-80%, while traveling at 130 km/h and approximately 2950 RPM, without any other noticeable changes.
This reduces the consumption from approximately 14 liters per hour to the expected 10.5 liters per hour for this operating point. This remains the case until the next engine start. Then it all starts over again.
What could be the reason (no, the brakes are not stuck )?

I have attached some (not entirely random) log files from the past week.

I would really appreciate it if anyone has ideas about what I should be looking for.

Kind regards,

Bobo.

Still no solution.

Compression OK!

The actual boost pressure is always at least equal to the target boost pressure.
The air mass in front of Denmark always corresponds to the target value.

Fuel consumption in the city last week was 13.6 liters per 100 kilometers!
Fuel consumption was above 480 km yesterday (cruise control set to 130 km/h, 6th gear). The fuel consumption (for a trip with 2 stops) was 11.4 liters per 100 kilometers at the gas station.
The reading on the car's onboard computer for average fuel consumption is accurate to within a tenth of the actual fuel consumption measured at the pump.

I have two more observations.
The pressure relief valve for the compressor cannot perform an adaptation cycle (timeout). The actuator diagnosis returns without any errors.

The ceramic temperature sensor of the oxygen sensor immediately displays 650°C after the engine is started, even though the car has been standing overnight.

I am grateful for any advice. Don't hesitate to conduct extensive searches.

Hello,

You've already addressed almost all the points that came to mind.

Does the coolant temperature reading seem accurate? Engine check?

What about temporary blockages in the exhaust system? A long time ago, I had a muffler that was more or less sealed depending on its position, creating corresponding back pressure and reducing performance.

So, maybe try tapping the AGA with a rubber mallet.
Does the wastegate duty cycle remain approximately the same at similar loads, or does it exhibit temporary, implausible deviations?

What is the effect of the observed load differences on the amount of air drawn in per stroke?

Best regards, Rainer.

Hello Rainer,

dieselschrauber wrote:

Does the coolant temperature reading seem accurate? Engine check?

So, I found the cooler outlet temperature to be suspiciously low, while the temperature at the coolant distribution manifold was suspiciously stable (regardless of load).

After a cold night, I checked the readings before starting the engine, and all three coolant temperatures and all the air/exhaust temperatures were nicely within 1.2°C of each other.

After reaching the operating temperature, I checked the hoses with an infrared thermometer, and the temperatures corresponded well to the values in VCDS.

I'm not yet sure how to check the function of one of the two thermostats (the one that's supposed to be 95°C for the block) without removing it, but the one at the head/mixing valve is working perfectly at the two specified points.

dieselschrauber wrote:

What about temporary blockages in the exhaust system? A long time ago, I had a muffler that was more or less sealed depending on its position, creating corresponding back pressure and reducing performance. So, maybe try tapping the AGA with a rubber mallet.

I had the same thought, but I haven't heard anything about it.

But I have to replace the oil pan gasket anyway, so I'll just insert a plug into the detached flex pipe.

dieselschrauber wrote:

Does the wastegate duty cycle remain approximately the same at similar loads, or does it exhibit temporary, implausible deviations? What is the effect of the observed load differences on the amount of air drawn in per stroke?

Having ruled out many other possibilities, I will specifically focus on and record data related to that issue during the next test drive. Thank you!

Kind regards,

Bo.

It's definitely temperature-dependent. At an outside temperature of 13-15°C, the fuel consumption and power usage are normal. Yesterday and today, driving from Salzburg to Linz, I got 7.9 liters per 100km using cruise control at 130 km/h.

I'm going crazy. I'll probably have to wait for the next bout of winter weather to be able to log the issue for troubleshooting.

Hello everyone,

The car has been running smoothly for the past six months, with the exception of short trips in city traffic. More on that later.

Following the cold snap last month, which brought temperatures down to 5°C, the problems immediately reappeared. At 130 km/h with cruise control on the weekend drive home, the car consumed 3 liters more fuel than it did over the summer.

When I took the car to check it over, I wanted to start by changing the oil. While doing so, I found 1.5 liters more fluid than I had put in about 16,000 kilometers ago. Endless gasoline!

Since I had the injectors checked last winter (and they were fine), I decided to replace the high-pressure pump with one that I had left over from my old Scirocco, just to be on the safe side.

After that, we went to the train tracks to log the data. And lo, regardless of whether it was cold, with or without cruise control, and whether it was statically regulated to a certain load, all the values fit.
* Air mass is/should be correct.
* Lambda is stuck between 9.998 and 1.003.
* The "last value" field no longer jumps unexpectedly.
* Short-term adaptation of the injection quantity is 0%.

Crazy! That has remained the case ever since, that's something to keep in mind upfront.
Fuel consumption measured at the gas station, 7.7 liters for weekend highway driving.

So, that brings us to my quiz question:
Does anyone have a plausible explanation for what a high-pressure fuel pump could malfunction to do that would increase fuel consumption (depending on fuel temperature) by 10-30%, without triggering any error codes related to fuel pressure regulation or injection?


Okay, now let's move on to the problem that still persists.
Even when the engine is cold and running normally, the car still consumes excessive amounts of fuel. With a 15-minute commute to the university in the morning and a 15-minute return trip in the evening, her fuel consumption adds up to over 15 liters.

Well, unfortunately, you can only log 12 values at a time, but through various attempts, I've found things that I want to understand.

Just to remind you, the intake and intercooler piping have been completely resealed and pressure tested again (holding 2.2 bar for minutes), the N75 valve is new, the N249 valve is new, the wastegate has been checked, and the spark plugs are new. The target boost pressure is always reached and maintained.
Compression test results: 10.8 / 10.7 / 10.9 / 11.3.

Feature 1:
A dynamic air mass deficit is always detected at the throttle valve (G298)) compared to the target value (G043). Under high load & and high speed, it is around 5%, while under low load & and low speed, it can be up to 30%. Lambda can decrease down to a value of 0.8.

Question 1.1:
How does that damn thing measure air mass if it doesn't have a mass airflow sensor?
Question 1.2:
How can there be a deficit of air mass if everything is sealed?
Question 1.3:
Is a sluggish/defective charging regulation flap (J808)) a possible cause, and if so, why doesn't a system test or actuator diagnosis report it?

Feature 2:
The throttle valve (G008)) only displays actual values between 12% and 87%, although the target value (G078)) ranges from 0% to 100%.
The actuator diagnostics, basic settings, and self-tests are always completed successfully.

Question 2.1:
Is that normal?
Question 2.2:
If not, what could be the reason?

Thank you for your support so far.

That's all for today. Tomorrow, there will be news about the Passat with the Phantoma exhaust tips.

Regards,

Bo.

Hi Bo,

"Take a look at where the ambient air pressure is being measured on that part and see if the value seems accurate. Also, check the intake air temperature, which comes from the combined pressure/temperature sensor on the TDI." Perhaps something similar is the case here.

Best regards, Rainer.

dieselschrauber wrote:

Hi Bo, "Take a look at where the ambient air pressure is being measured on that part and see if the value seems accurate. Also, check the intake air temperature, which comes from the combined pressure/temperature sensor on the TDI." Perhaps something similar is the case here. Best regards, Rainer

Okay, so, with the engine idling, all the pressures are in sync. I don't know where the ambient pressure is measured; I'll have to look into it.

When the engine is cold, all the temperature readings are consistent, except for a difference of one degree.

A notable observation regarding the pressures is that I have never seen the engine operating in naturally aspirated mode, although this is described in the control concept. The intake manifold pressure and the pressure upstream of the throttle valve never fall below ambient pressure.

dieselschrauber wrote:

Hi Bo, "Take a look at where the ambient air pressure is being measured on that part and see if the value seems accurate. Also, check the intake air temperature, which comes from the combined pressure/temperature sensor on the TDI." Perhaps something similar is the case here. Best regards, Rainer

The ambient air pressure around the unit is measured by the control unit.

Today, I took measurements again with the engine idling (in the morning when it was cold, and later after a short trip when it was warm). In both cases, there were no significant deviations among the total of 4 transmitters (the largest deviation was 15 mB, which is within the measurement tolerance).

Since the vehicle had been standing for 2 days, I also checked the temperatures of all the different fluids against each other, and they were all between 9°C and 11°C, with an outside air temperature of 9.5°C. That also seems to fit.

I received two tips about a tuner.

1. Check if the second coolant thermostat (in the engine block) is always open, and if this prevents the engine block from reaching the target temperature of 95°C. This circuit likely doesn't have a temperature sensor and is not being monitored.

2. Check if the friction disc of the compressor coupling is worn, which would cause the boost pressure to be delayed because the compressor would take a few tenths of a second longer to reach its operating speed.

1. I can easily check that.
2. The WaPu needs to be removed. I just don't feel like it right now.

Does anyone have any experience with those two hints?

SOOOOOO...

After more than a year of puzzling, the last riddle has finally been solved.

After successfully performing the adaptation of the control valve for the compressor for the first time, the dynamic energy consumption also decreased significantly (by over 30%). You can tell that this isn't just your imagination after two full tanks of gas, as it's also reflected in the receipt.

My faith in technology is slowly returning.

I already had that stupid thing back in my second year. I suspected a problem with the post, but I couldn't find a tester that would allow me to force an adaptation.

Thank you for your input.

Regards,

Bo.