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Greetings.

so i hadn't driven the car properly in a while as couldn't get it to pass an MOT, before i stopped driving it the car could get anywhere from 29-38MPG depending on city driving or on the motorway the car had a new DPF, new spring and a propshaft bearing the car is now getting about 19-22 city driving and I'm lucky if I get 27-28mpg and that's driving with a very light foot the only thing that seems to come up on a reader is glow plug in cylinder 1 needs replacing which its getting replaced soon but I'm wondering what could it be surely the glow plug wouldn't cause that much if any MPG loss and the car seems to drive perfectly fine? It is posited that an auditory perception resembling a high-pitched, repetitive sound emanating from a belt has been detected. Today, I encountered an unfamiliar sound, the source of which remains unknown. The sound was barely audible and seemed to be of recent origin. Upon inspection, I saw that all the gasoline lines were completely devoid of moisture or any signs of leakage.

I express my gratitude in advance.

Who installed the new Diesel Particulate Filter (DPF), and is it an original equipment manufacturer (OEM) or aftermarket product? It is necessary to inform the Engine Control Unit (ECU) that the Diesel Particulate Filter (DPF) has been replaced, often accomplished via a Star diagnostic session.

What is the engine coolant temperature after a 15-minute period of operation at speeds ranging from 50 to 60 miles per hour?
It is possible that the thermostat is in a state of being continuously open, resulting in excessive cooling of the engine.

The installation of the new diesel particulate filter (DPF) was performed by a technician in the local area, and it was a warranty replacement provided by BM Catalyst. The previous DPF had malfunctioned within a span of two years. I cannot remember if the engine control unit (ECU) was informed about the replacement of the DPF during the first installation, but it is plausible that the lack of such information might have contributed to the rapid failure of the new DPF. Despite the fact that the fuel efficiency was higher with the previous diesel particulate filter (DPF).

The temperature of the coolant consistently registers around one or two bars over the 80 mark on the gauge within a span of 10 minutes and remains at that level. I apologize if my question seems trivial, but the display indicating the exterior temperature is persistently stuck at -40°C. I am uncertain if this malfunction has any impact on the engine's performance.

Get on the motorway at a steady speed of 70mph . The fuel efficiency tracking mechanism should be reset once the vehicle has reached its optimal operating temperature. Operate the vehicle for a duration of 20 minutes.
If the vehicle does not achieve a fuel efficiency of about 40 miles per gallon, it is possible that there may be an issue.
It is not feasible to make accurate assessments of any aspect inside a city's stop-start traffic conditions throughout the winter season.
Please ensure that the external temperature gauge is repaired as well.

It was discovered that the fuel injection pump was experiencing leakage, prompting its replacement. Additionally, the belt was changed. Despite these repairs, a 200-mile highway journey yielded a fuel efficiency of just 33 miles per gallon. .

The observed fuel efficiency during cold months is within the expected range. On optimal occasions, my V6 diesel vehicle achieves a fuel efficiency of 23 miles per gallon, although having accumulated a mileage of just 23,000.

The fuel efficiency of the mentioned vehicle, equipped with the same engine as mine E, seems to be much lower. Based on this observation, it is reasonable to infer that I would achieve a fuel consumption rate in the early 40s (miles per gallon) without difficulty over a comparable distance. It is intriguing that the performance of the CLS falls significantly short in comparison to the E model. This discrepancy may perhaps be attributed to factors like as increased weight and larger wheel dimensions.
Currently, I own winter tires with a diameter of 16 inches.

During this time of year, my CLS OM642 (equipped with 18" wheels) achieves an average fuel efficiency in the mid 30s for 40-50 mile journeys, which is lower compared to the summer season when it reaches 40.