Diesel Temperature: Causes, Symptoms & Fixes | Posts 48+

Klausel wrote:

Regarding fuel temperature, anyone who has a nice, cool underground garage should be able to contribute by doing a test on a hot day - I'm curious.

Hi.

I don't think that would be very informative.
"In my opinion, the temperature inside the pump is likely the most important factor, as heat can also enter there when the engine is warm."

I don't believe that the relatively slow-flowing (and cooling) fuel significantly affects the pump's temperature, but rather that it is primarily determined by the air flowing through the radiator.

However, to (allegedly?) compensate for this, the fuel temperature sensor is integrated into the VP 37, and the EDC uses the signal from this sensor to compensate for changes in the fuel density.

Hi Ulf,

Regarding the diesel temperature, I agree with you; this is also evident from my initial test run.

Start:

Fuel: 22.5 degrees, charge air: 21.6, coolant: 27.9.

After 8 kilometers and 11 minutes:

Fuel: 31.5, Charge air: 29.7, Coolant: 87.3

The outside temperature was around 22 degrees Celsius. The measurements were taken while the engine was idling without a heavy load, and in the second case (the 8 km distance), the driving was in the city with a maximum engine speed of 1500-1800 RPM.

That likely comes almost entirely from the engine's heat, as you mentioned, and other factors shouldn't play a significant role.

Best regards,

Jan.

Jan6K wrote:

Fuel: 22.5 degrees, charge air: 21.6, coolant: 27.9. After 8 kilometers and 11 minutes: Fuel: 31.5, Charge air: 29.7, Coolant: 87.3 The outside temperature was around 22 degrees Celsius. The measurements were taken while the engine was idling, without putting it under heavy load. In the second case (the 8 km distance), the driving was in the city with a maximum engine speed of 1500-1800 RPM. That is likely almost entirely due to the warming caused by the engine, as you say, and other effects should not play a significant role.

Hi Jan,

Friction within the pump is likely also contributing "somewhat" to the heating.
What was the temperature of the fuel in the pump according to your measurements?

@all: Engine temperature and intake air temperature.

I once experimented with adding a 100-ohm resistor in series with the engine temperature sensor. This reduced the "apparent" engine temperature in the range of 90°C by about 10°C.
The differences are likely not much greater when the engine is warm, compared to summer and winter conditions; in other words, I've essentially tricked the engine control unit into thinking it's winter.
Success in the 2000-4000 rpm test: no issues detected.

The same applies to adding a 470-ohm resistor in series with the charge air temperature sensor (which seemingly lowers the temperature by 50°C, to approximately 38°C): this also didn't result in any increase in power. If there's a difference, it's more likely a minimal loss in performance, but this is uncertain because there was a strong headwind today.

It seems something can be done about the fuel temperature sensor, but it often backfires because a faked higher temperature increases lower temperatures more than higher temperatures, meaning the performance gain would be greater in winter than in summer.
Within the engine control unit itself, a seemingly uniform shift of the entire temperature spectrum could only be achieved through resistor substitution, for example, by mapping 0°C to 20°C and 80°C to 100°C.

The desired countermeasure against summer sluggishness would be, for example:
0°C becomes 2°C, and 80°C becomes 100°C.
But I don't think it's possible to achieve that with an NTC sensor, because you would have to make its characteristic curve "steeper" .

Hi Ulf,

Unfortunately, I can't tell you, as I stopped running that measurement block because of the resolution. Every additional block significantly reduces the sampling rate.

I checked the water temperature again, but I didn't record the values. If I remember correctly, it was below 40 degrees, but I can't be sure because I was only looking at the cooling water temperature, and the log was already turned off.

If I take measurements again, I'll pay close attention to that specific aspect, or I'll take measurements specifically for that purpose.

Best regards,

Jan.