Oil Temp: The Cold TDI Myth Explained

Hello everyone,

I intentionally chose a somewhat provocative title .

We all know that our TDI cars take a long time to warm up in the winter, and it's often said that even after longer highway drives, the oil doesn't even come close to reaching operating temperature, and the car feels like it's the interior of an igloo for half the winter.

I recently got an oil thermometer and was very curious to see what actual values it would show, because until now, I've almost always managed to get the water to 90 degrees Celsius in the winter. However, that doesn't really tell us much, because the display has this nice plateau function that averages everything between 75 and 105 degrees and displays it as 90.

Regarding the oil temperature, things are different there. I know how it's measured, namely using a self-built dipstick sensor (see technical article) directly in the oil pan, without the temperature being affected by the cooling of the sensor, because it only touches the oil (and the plastic guide of the dipstick above). Also, the oil-water separator, which is located before most of the standard temperature sensors, does not interfere with the measurement.

My results surprised me quite a bit, considering what I had expected (see above).

Currently, the temperature here is between -2 and 0 degrees Celsius, and I drive 10 km to work. This morning, the oil temperature was 0.2 degrees (I drove yesterday evening, so it apparently didn't cool down completely). After about 7 km, it reached 62 degrees (that's when the water temperature gauge started to move), and after about 8-9 km, it was 72-73 degrees, which is the point where the water temperature reaches the (corrected) 90 degrees. When I arrived at work, the oil temperature was 76 degrees. The driving conditions were mostly slow city traffic (average fuel consumption of about 5.3 liters, max 1500 RPM, and only briefly reaching 2500 RPM at the end).

"The second experiment was yesterday. After a long period of idling with the oil at 40 degrees Celsius while the outside temperature was -2 degrees Celsius, I started the engine and drove gently through the city for about 10 km (after which the oil temperature reached approximately 90 degrees). Then, I drove on the city highway for about 15 km at speeds of 80 to 100 km/h, with occasional braking and acceleration. The temperature stabilized around 97 degrees Celsius. At the end, when I drove a bit more erratically without using cruise control (intentionally, by letting the speed drop to 80 km/h, accelerating a bit, etc.), the temperature reached 100 degrees Celsius, which then dropped to 92-95 degrees Celsius over the one kilometer between the exit and my home."

Third experiment: After a longer period of driving at approximately 70 km/h with an outside temperature of 5 degrees Celsius, the oil temperature was 92 degrees. Then, I accelerated to 190 km/h in one go with full throttle. During this acceleration, the oil temperature rose to 102 degrees. However, after decelerating before a construction site and exiting, the oil temperature dropped back down to 94 degrees.

The sensor responds extremely quickly. Regarding accuracy: When I built this device, I conducted tests that, at room temperature, yielded an error of perhaps 1-2 degrees (my reference thermometer wasn't more precise), and when immersed in boiling water, it displayed a reading of 100.2.

What do you think about all of this?

Is my TDI particularly well-insulated in terms of uncontrolled heat dissipation, or are all those that don't get warm broken, or is the truth somewhere in between?

What are your measurements (please always specify the measurement method!)?

Regarding the measurements: I always drove with the climate control set to 23 degrees Celsius, which causes the heating system to open very widely as soon as the oil temperature exceeds approximately 30 degrees. Therefore, the cooling effect of the heating system is also included in the measurements.
And one more thing: My ASV does NOT have a glow plug heater installed from the factory!


Best regards,

Jan.

I live quite close to the highway, so it's only about 3 kilometers to get onto it. Then, I drive about 3 kilometers until a rather long hill, which is about 3 kilometers long, starts. The speed limit is 120 km/h, which I always adhere to.

This morning: -4°C.
Before I get onto the highway, I drive for 100 kilometers (the first 3 kilometers). I shift up to 5th gear relatively early, at around 1800 RPM.
Water: no change in reading.
Oil (MFA): no complaint.
Consumption (Mfa): 10 liters

Highway, 120 km/h, slightly downhill or flat (for the next 3 km), all in 5th gear.
Water: The gauge is moving slowly, reaching just below 70°C.
Oil: approximately 58°C at the end.
Consumption: increases to a significant 7.5 liters.

Mountain road, 120 km/h uphill (the last 3 km), using 5th gear for everything.
Water: The indicator in the middle of the mountain shows a temperature of just over 90°C and remains there.
Oil: reached a final temperature of 104 °C.
Consumption: Still at 7.5°C at the bottom of the hill.
Boost pressure: remains at 0.8 - 0.9 bar throughout the climb (so I have to accelerate quite a bit).

I always turn on the interior heating only halfway up the mountain, as the car was parked in an unheated garage overnight.

1Z with glow plug heating.

Here are three log entries:

http://www.ralfhandel.de/images/temperatur.gif

http://www.ralfhandel.de/images/ueberland-sommer.gif

http://www.ralfhandel.de/images/oel-wasser-mit-SH.gif

The waste heat from the preheated TDI engine comes from the automatic transmission, where 6 liters of oil are suddenly pumped through the internal heat exchanger.
The second dip at around the 7-minute mark indicates the point where I turned off the SH (presumably a device or system), which was just before it started heading towards the location.

The overland journey started after a 2-hour break and clearly leads through valleys and over mountains .
Where the even numbers are increasing uphill, the TDI engine also performs well going uphill.

Oh yeah... the first and last diagrams cover the same distance.

CU Gremlin.

@ Gremlin

From which measurement groups did you obtain these values?

m

Combination instrument panel, display group 003.

Hi,

The Gremlins diagrams are roughly in line with my values.

@Gremklin: Where is the sensor located that provides these values? Do you know? Is it near the oil filter, in the oil pan, or somewhere else entirely?

Best regards,

Jan.

The oil level sensor is located in the oil pan. So, it measures the oil level at the same point as your dipstick.

CU Gremlin.

Hi,

Great, since you're also reaching over 100 degrees, that's another sign that things are working exactly as I thought they would.

Best regards,

Jan.

Hm, that's some interesting news. Oil gets warm faster than water. Usually, one is always told that it's the other way around. That means that I can switch to the heating mode after about 10 km.

Best regards, WarLord.

Hi Warlord,

"Um... where are you reading that oil heats up faster than water?"

Both my measurements and the diagrams from "Gremlins" actually show the opposite. The oil only becomes warmer than the water further along, i.e., beyond 90 degrees, but not necessarily faster.

The fact that the two curves are so close together is likely also due to the fact that our TDI has this oil-water heat exchanger.

Best regards,

Jan.

Oops, I misread that. Hmm, so we can assume that the oil is roughly the same temperature as the water (as we can see from DocSnydor's explanations, this seems to be the case with my 1Z).

Best regards, WarLord.

Hi Warlord,

The fact that the two curves are so close together is likely also due to the fact that our TDI has this oil-water heat exchanger.




That's exactly it. These things not only provide several kilowatts of cooling power, but they also heat the oil simultaneously through the coolant.
In addition, the oil is used for cooling the piston underside and for the turbocharger, both of which get very hot (and quickly).

Is my TDI particularly well-insulated in terms of uncontrolled heat dissipation, or are all those that don't get warm broken, or is the truth somewhere in between?

What are your measurements (please always specify the measurement method!)?

Hello everyone!
Based on my own experience, I believe that TDI engines are definitely very well insulated from the factory, at least those with the large engine cover where you can't see the engine from below. On my 35i, 1Z, manufactured in 2/96, the large cover included an extra oil pan shield. The reason I say this is because I noticed the following about the oil temperature after I bought the car: After a brisk highway drive at an outside temperature of about 25°C, the oil temperature rose to almost 130° (!) over a distance of about 20 km. However, before that, I had to drive on the country road to warm up the engine to about 90°C. When I let off the gas and coasted at about 120 km/h, the oil temperature didn't really decrease, not even at the next traffic light or in the subsequent, very slow city driving. Long story short: The oil got very hot very quickly and took a long time to cool down.
After talking to a T3 turbodiesel driver who told me that he had removed the engine cover to lower the oil temperature, with success, I also tried this on my Eicher. Sure enough, even after driving for a long time, the MFA (Multi-Function Display) doesn't show temperatures exceeding 120°, even in the summer. However, I have to warm up my car for a bit longer after a cold start, even in the summer . With the current conditions here at the beautiful Starnberger See, around 4:45 AM, with a temperature of -3°, this means driving about 20 km to reach an oil temperature of 80°.
So, the truth about my car probably lies somewhere in between.

Best regards,
Dominique.