My 2021 L5P Duramax has been consuming oil since we started towing with it at about 2,500 miles on the odometer. The consumption wasn't regular and was as little as 1 quart in 6,000 miles and as much as 1 quart in 1,300 miles, all consumption is while towing. I wrote the consumption off to "a hard working diesel is going to consume some oil". However the variability in consumption has to be caused by something, so I started looking for why this could be. The highest consumption rate (1 quart in 1,300 miles) was pulling our 20,000 pound 5th wheel into awful headwinds. In that 1,300 miles we saw headwinds as high at 35 mph and were getting about 7 MPG. I tow at 60 MPH, because I'm not in a hurry and it's an easier drive than going faster. At 60 MPH the engine is running at 1400 RPMs in 10th gear. The TCU will shift to 9th going up hills, which is 1500 RPMs at 60 MPH and occasionally it will shift into 8th gear, which is 1880 RPMs at 60 MPH.
The one common theme when the engine consumes oil is when we are pulling into a wind and fuel consumption is high. Heat, has to be the cause of oil consumption, but how do I prove that with data? How about EGTs? How about a Banks iDash to measure the EGTs? Great idea! I bought one before our last trip and programmed the temperatures page with EGT, boost PSI, RPMs, engine oil temperature, engine coolant temperature, and fuel consumption in gallons per hour. The last gauge is the % DPF load, but it's not relevant to this test.
This is not science and was not done on a dyno in a test cell. This is an approximation of typical operating conditions observed with data from the ECM, displayed on the iDash. I'm glad we got that straight
I watched the iDash for hours while pulling about 2,400 miles on our trip to Colorado (no we didn't go the most direct route). I tried to take several pictures to demonstrate EGTs at a consistent fuel consumption rate. This is hard to do, but I got pretty close. These three pictures show the EGTs in 10th, 9th, and 8th gear at a constant speed of 60 MPH. All three were with a headwind, so the work being performed was higher than with zero wind. The baseline fuel consumption per hour with no wind and no grade is 6.2 GPH.
The first picture shows an EGT of 1256°F, 1403 RPMs, and 8.4 GPH fuel consumption in 10th gear (triple overdrive)
The second pictures shows an EGT of 1213°F, 1531 RPMs, and 9.4 GPH fuel consumption in 9th gear (double overdrive)
The third pictures shows an EGT of 997°F, 1725 RPMs, and 9.1 GPH fuel consumption in 8th gear (single overdrive)
The data varies by second, but these pictures are representative of pulling our trailer into a 10 MPH headwind.
I was able to observe the EGTs at different RPMs within the first 25 miles of our trip. When I saw how high the EGTs are in 10th gear at 1400 RPMs, I suspected this was the cause of the oil consumption.
Peak torque for the L5P Duramax is 1600 RPMs, so pulling at 1400 or 1500 RPMs is lugging, but the ECU/TCU mapping allows it to do this all day.
The Duramax allows you to limit the upper gear from the selector on the gear shifter. I decided to do an experiment and towed the entire way there and back, which was 2,300 miles of towing in 8th gear (roughly 1,880 RPMs). The EGTs averaged about 900°F for the trip (no wind), of course this varies from 300°F down a grade to 1350°F up a grade, but you get the general idea. Had I towed in 10th, then the EGTs would have averaged about 1150°F (no wind). Add in wind and it's very possible in 10th gear (1400 RPMs) the EGTs could have averaged 1300°F. I towed every trip previous in 10th gear.
What was the result of the oil consumption experiment?
2,300 miles of towing our 20,000 pound trailer in 8th gear at 1880 RPMs vs 10th gear at 1400 RPMs yielded ZERO oil consumption, none.
I think it's fair to conclude that the higher EGTs at lower RPMs heats the piston crowns hot enough that some oil vaporizes from the heat and that the lower EGTs from higher RPMs keeps the piston crown temperatures low enough that little oil vaporization takes place.
Don't tow heavy below the peak torque of your engine.
Again, this is not scientific, but there is some data to back up my conclusion.
The one common theme when the engine consumes oil is when we are pulling into a wind and fuel consumption is high. Heat, has to be the cause of oil consumption, but how do I prove that with data? How about EGTs? How about a Banks iDash to measure the EGTs? Great idea! I bought one before our last trip and programmed the temperatures page with EGT, boost PSI, RPMs, engine oil temperature, engine coolant temperature, and fuel consumption in gallons per hour. The last gauge is the % DPF load, but it's not relevant to this test.
This is not science and was not done on a dyno in a test cell. This is an approximation of typical operating conditions observed with data from the ECM, displayed on the iDash. I'm glad we got that straight
I watched the iDash for hours while pulling about 2,400 miles on our trip to Colorado (no we didn't go the most direct route). I tried to take several pictures to demonstrate EGTs at a consistent fuel consumption rate. This is hard to do, but I got pretty close. These three pictures show the EGTs in 10th, 9th, and 8th gear at a constant speed of 60 MPH. All three were with a headwind, so the work being performed was higher than with zero wind. The baseline fuel consumption per hour with no wind and no grade is 6.2 GPH.
The first picture shows an EGT of 1256°F, 1403 RPMs, and 8.4 GPH fuel consumption in 10th gear (triple overdrive)
The second pictures shows an EGT of 1213°F, 1531 RPMs, and 9.4 GPH fuel consumption in 9th gear (double overdrive)
The third pictures shows an EGT of 997°F, 1725 RPMs, and 9.1 GPH fuel consumption in 8th gear (single overdrive)
The data varies by second, but these pictures are representative of pulling our trailer into a 10 MPH headwind.
I was able to observe the EGTs at different RPMs within the first 25 miles of our trip. When I saw how high the EGTs are in 10th gear at 1400 RPMs, I suspected this was the cause of the oil consumption.
Peak torque for the L5P Duramax is 1600 RPMs, so pulling at 1400 or 1500 RPMs is lugging, but the ECU/TCU mapping allows it to do this all day.
The Duramax allows you to limit the upper gear from the selector on the gear shifter. I decided to do an experiment and towed the entire way there and back, which was 2,300 miles of towing in 8th gear (roughly 1,880 RPMs). The EGTs averaged about 900°F for the trip (no wind), of course this varies from 300°F down a grade to 1350°F up a grade, but you get the general idea. Had I towed in 10th, then the EGTs would have averaged about 1150°F (no wind). Add in wind and it's very possible in 10th gear (1400 RPMs) the EGTs could have averaged 1300°F. I towed every trip previous in 10th gear.
What was the result of the oil consumption experiment?
2,300 miles of towing our 20,000 pound trailer in 8th gear at 1880 RPMs vs 10th gear at 1400 RPMs yielded ZERO oil consumption, none.
I think it's fair to conclude that the higher EGTs at lower RPMs heats the piston crowns hot enough that some oil vaporizes from the heat and that the lower EGTs from higher RPMs keeps the piston crown temperatures low enough that little oil vaporization takes place.
Don't tow heavy below the peak torque of your engine.
Again, this is not scientific, but there is some data to back up my conclusion.
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