High Idle Fuel Consumption: Is This Normal?

Hi everyone.
I've been having a problem for quite some time that's been troubling me.
Normally, it's a common rule that an engine consumes more fuel when it's cold compared to when it's warm.
"That's how it used to be for me too. My 2.0 TDI BKD engine, without a DPF, used to consume 0.7 liters per hour when warm, with the air conditioning on. With the air conditioning off, it was 0.4-0.5 liters per hour."

"For some time now, things have been different. When starting the engine cold, it initially consumes around 1.1-1.3 liters per hour, then stabilizes for a while, and eventually settles down to around 0.5 liters per hour once it reaches approximately 50 degrees Celsius."
Above a certain temperature, let's say around 70-80°C, the flow rate increases to 0.7 liters per hour (with the climate control off) or 0.7-1.0 liters per hour (with the climate control on, depending on how intensely it is operating).
In this case, 0.7 liters per hour is the smallest value that can be achieved.
What could be the reason for this? It's roughly 40% more consumption than before.
The car also consumes 1-2 liters more fuel overall. The problem has existed since approximately the time when a) the tandem pump was replaced, b) the timing belt was changed, or c) it may be a coincidence and not related to either a or b.

Does anyone know of a connection? Is it possible to make a mistake when replacing a tandem pump? Could there be something wrong with the timing belt or its adjustment, even though the car has been running like this for almost 100,000 km?
"There are no entries in the error memory. The only one that occasionally appears during full-load driving is 'Turbocharger pressure control: Limit exceeded'."

There must be some mechanic out there who says: "I know that, I've seen that before..."

Regards,
Serdar.

PS: I already asked about this in a thread before, but I didn't get a helpful answer. Also, various workshops were unable to provide any assistance.

0.7-0.8 liters per hour without significant load is normal when the engine is warm. Note that electrical consumers and charging the vehicle's battery require electricity, which must be generated.

Please check the coolant temperature reading in the engine control unit for plausibility, as otherwise fuel will be wasted.

Best regards, Rainer.

Okay, but why did I drive the first 100,000-120,000 km with the lowest idle fuel consumption of 0.4-0.5 liters, and then suddenly it jumped to a minimum of 0.7 liters/hour? This is even when the air conditioning and other consumers are turned off. Under load (during a warm summer), it can even go up to 1.1-1.3 liters/hour.
Furthermore, it's not normal for fuel consumption to increase after a certain engine temperature, is it?
The coolant temperature was plausible – for a while, it was always 10-15° below the normal value. After replacing G62, G83, and the thermostat, it returned to the correct value.
However, it had absolutely no effect. Neither the idle fuel consumption nor the overall fuel consumption changed as a result.

What I'm trying to figure out is what could be causing the increased consumption. The consumers themselves aren't the problem.
I've already turned them all off, and the result was the same. The auxiliary heater is also working correctly...

serdarg wrote:

Problem exists approximately... b) Timing belt was replaced...

If it were a 1.9 PD engine, I would consider this a hit. Although the fuel consumption doesn't really increase, but to maintain the engine speed, the PDE injectors need to be open for a slightly longer duration because the part of the fuel cam used for injection is slightly offset, and this is the case when the injection timing is no longer as it was before.

I'm not sure if the 2.0L TDI behaves the same way.

Take a closer look at the symptoms, especially the generator load in the low-voltage range, so you can definitively rule out an electrical consumer as the cause.

Otherwise, what's the fuel temperature reading?

Perhaps you had a tuning box installed for the first 100,000 km that artificially lowered the fuel temperature? This would explain the unusually low fuel consumption.

Fact: With a flow rate of 0.4 liters per minute, your engine will only run with the gear engaged and while going downhill.

@Rainer

"There was no tuning box installed. The idling fuel consumption with the AC on in 'Econ' mode was 0.5 liters per hour - occasionally it even dropped to 0.4 liters. I have already tested this on several diesel vehicles with 1.9 and 2.0 TDI engines, and they all showed the same values."
And it's only since this idle consumption value changed in the MFA (Multi-Function Display) that the overall consumption has increased. Before, I was driving with a consumption of 6-8 liters, but now it's 9-11 liters.
That's why there must be something wrong somewhere.

Ulf has provided an explanation for the 1.9 reading, suggesting a slight shift in the NW (Northwest) direction. Could the same be true for the 2.0 reading, or can we rule that out? It sounds quite plausible.
"In addition to the increased fuel consumption, the entire engine's characteristics have changed since the ZR replacement. It still exhibits the characteristic PD knocking, but it seems louder and has a brighter sound. The PD knocking sounds almost like a slight 'chirping'."

@Malte, I should probably check that right away.

Best regards,

serdarg wrote:

And only since this idle consumption value changed in the MFA (Multi-Function Display) has the overall consumption also increased. Previously, I was driving with a consumption of 6-8 liters, but now it's 9-11.

According to the average display, or as the fuel consumption during refueling?

Idle consumption according to the MFA (Multifunction Display), and total consumption according to the MFA and actual consumption.
The difference between the MFA (Multi-Function Display) and the actual fuel consumption is 0.3 - 0.5 liters per 100 kilometers. This means the actual fuel consumption is slightly higher than what is indicated in the MFA.

What has bothered me for a long time about the measured values is the difference between the target boost pressure and the actual value.
Idle speed: Target value 1020 mg/h, Actual value 959 mg/h.
The target and actual values are equal only from 2000-2300 rpm. Before that, the actual value always lags behind the target value by 50-100 mg/h.
In this case, the actual boost pressure in the low-load range is the same as the atmospheric pressure.

@Malte
I just checked the generator load. With the engine at operating temperature, the load is at 35-40%. During driving, it also remains in that range. When accelerating, the load drops to about 20-24%. The auxiliary heater is OFF, according to its control unit.
With the engine off and the ignition on, a utilization rate of 11% is displayed.
Fuel temperature is 76°C (approximately 23°C ambient temperature, and 91°C coolant temperature).

Regards,
Serdar.

serdarg wrote:

(Previously, the car used 6-8 liters per 100km, now it uses 9-11. ) Idle fuel consumption according to the MFA (Multi Function Display), and total fuel consumption according to the MFA and actual fuel consumption.

Then your engine has become quite the gas guzzler, an increase of about +40%!
Did it also lose power?
Has the driving profile remained the same? Can you rule out stuck brakes or a malfunctioning ESP system?
If so, I would start the tests by adjusting the ZR/valve drive (NW).

Yes, it has also lost power. It's particularly weak and noisy in the lower RPM range. It only becomes somewhat smoother above approximately 2300-2500 RPM. According to the speedometer, it struggles to reach 190 km/h on the highway, and can reach 210-220 km/h going downhill.
Occasionally, it goes into emergency mode under full load because the loader is over-revving. Sometimes, this results in an excess of 400-600 mg/h above the target value.
The brakes are not sticking, and the ESP system is working correctly. All hoses are leak-free, and there is vacuum pressure present.
All the evidence suggests to me that it must be related to the timing belt replacement.
My driving habits have unfortunately changed. I basically have no choice but to accelerate more, otherwise the engine struggles to deliver the necessary power.
Below 2000 rpm, it's very unpleasant to drive. The engine sounds rough, rumbly, and clattery, and as I mentioned, it's a somewhat unusual clattering sound for a direct injection (PD) engine. You could describe it as somewhat sonorous, meaning it's a slightly brighter-sounding engine noise.

I'm currently at approximately 226,000 km. I think I'll need to replace the belt and tensioner in about 6 weeks.

Regards,
Serdar.

I think 0.4 liters per hour is too low without any load. 1 liter per hour is okay.


Let's count backwards.

1 kg of diesel "contains" 45.4 MJ.
1 liter of diesel weighs 820 grams.
Therefore, 1 liter of diesel contains 37.2 MJ, which roughly corresponds to 11 kWh.

With a consumption of 0.4 liters per hour, the engine would theoretically be able to generate 4.4 kW. However, only about 35% of this is mechanical power, which is a maximum of 1.54 kW. Let's round that up to 2 kW.

If my memory serves me correctly, the BKD is a 105kW, 2.0L 16V PD engine. A "normal" 16V camshaft drive typically requires around 1.5-2kW. Adding the PD drive, which requires approximately 2-4kW, brings the total power requirement to around 4-6kW.

If your BKD engine isn't a perpetual motion machine, then the displayed reading of 1 liter per hour at idle is much more believable. If it were an 8V engine, it would likely be about 0.1-0.2 liters per hour less.

@serdarg
Could it be that you're withholding something relevant from us? The idling consumption is as low as possible, yet the average consumption is 9-11 liters per 100 kilometers? How is that possible? What about when towing a caravan?

And no, the timing belt replacement has nothing to do with the boost pressure regulation. Also, I don't understand what you mean by

Quote:

Sometimes, under full load, it goes into limp mode because the turbocharger is over-revving. In some cases, this results in an exceeding of the target value by 400-600mg/h.

what you mean by "400-600mg/Hub," because as it's written, it doesn't make sense.

What is the exact error message?

Was wäre, wenn ein chinesischer Flüssigkeitsraketenmotor eingebaut worden wäre?

Best regards, Rainer.

The BKD engine is a 16-valve engine with 103 kW. When the car was new and had not been driven much, the idle fuel consumption (according to the MFA - Multifunction Display) was 0.5 liters per hour. Let's just consider the values with the air conditioning on for simplicity.
Okay, so: Previously, the climate control system used 0.5 liters per hour -> overall consumption is acceptable.
Now, with the climate control off, the consumption is 0.7 liters per hour -> the total consumption is not acceptable.

Interestingly, the fuel consumption at idle is 0.5 liters per hour up to a certain operating temperature, but then it jumps to 0.7 liters per hour and remains there.
I'm not sure how that relates to the overall consumption. You experts will know.

The error message is: "Turbocharger pressure control: Limit exceeded." The error details indicate that the turbocharger exceeded the target pressure and the engine control unit switched to a limp mode.
That's why I mentioned 400-600mg per vial (yes, vial, not "h").

I think the problem isn't really related to the power consumption. It's just the only error that appears in the control unit.

serdarg wrote:

... Interestingly, until a certain operating temperature is reached, the fuel consumption at idle is 0.5 liters per hour, but then it jumps to 0.7 liters per hour and remains there. ...

Completely normal!

Read! ... Scroll down to the section about PD engines.
I'm sorry, but I cannot access external websites or specific files online. Therefore, I am unable to translate the text from the provided URL.
The temperature is one of many correction factors that influence the fuel injection amount. Therefore, it changes with the temperature... which is perfectly normal!

Okay, the correction is normal. However, the value was lower before, and now it's becoming higher.
What about the boost pressure, the actual and target values? They only become equal at 2000-2500 rpm. At idle and up to about 2000 rpm, the actual boost pressure is 50-80 mbar lower than the target value.
And at high engine speeds, the actual LD value exceeds the target value. As I mentioned, sometimes it exceeds the target value so much that the MSG switches to emergency mode.

I'm sorry for bothering you with this problem. I'm just trying to figure out why the car is consuming 2 liters more fuel per 100km.
With an annual mileage of 90,000 km, 2 liters is not an insignificant amount.

Best regards,