2021 Duramax L5P oil consumption cause and solution

wwillson

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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)

IMG_0860.JPG


The second pictures shows an EGT of 1213°F, 1531 RPMs, and 9.4 GPH fuel consumption in 9th gear (double overdrive)

IMG_0709.JPG


The third pictures shows an EGT of 997°F, 1725 RPMs, and 9.1 GPH fuel consumption in 8th gear (single overdrive)

IMG_0695.JPG


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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wwillson

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Not interested in towing at 80 mph?
No, but we sure see a lot of people who are. It's just not smart, in my opinion.

How does that battleship do in crosswinds?
Very well. You'll start to feel a 20 MPH crosswind and by 30MPH you want to get off the road. Rarely have we ever towed in a direct 30MPH crosswind, but have and we did get off the road. When I was younger, I looked at it as a challenge, now I look at it as a risk. I've seen many trailers in pieces in the ditch.

When it does get really windy, I always slow down a bit and it makes for much less energy in the buffeting. Towing at 50 on the interstate isn't idea, but it does get you safely to the next stopping point.
 
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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)

View attachment 106024

The second pictures shows an EGT of 1213°F, 1531 RPMs, and 9.4 GPH fuel consumption in 9th gear (double overdrive)

View attachment 106025

The third pictures shows an EGT of 997°F, 1725 RPMs, and 9.1 GPH fuel consumption in 8th gear (single overdrive)

View attachment 106026

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.

great info @wwillson ! I have a 2012 2500 with a Cummins and noticed that when I tow my 11K TT in 5th gear and lock out 6th gear my EGT's on my CST2 are much lower. On the 6.7L the torque curve is from 1,600 to 2,400RPM. I get better response and much lower EGT's in 5th gear. I use to tow 1,600 RPM and below in 6th gear and my EGT's would be 1300F+. Now I only see 1300F+ climbing a hefty grade at ~70MPH! I usually tow 72-75MPH w/ the wife on board and 68-72MPH alone. She hates to travel so I try to get there ASAP.

Good news my Cummins does not use any oil to speak of. I did have a spell where it seemed to use 1 quart over an oil change and I was pulling in a lot of headwinds, mostly in 6th gear. Now that I got that fixed, the consumption seems to be gone.

just my $0.02
 
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If you can somehow get your dealer to put you in touch with a zone rep I would pose the question to him/her and then ask for it to be kicked up the chain to current product engineering. You would probably need to vet whoever responds to make sure they arent just passing you off on some low level customer service functionary. Youre going to need to climb the food chain to get an actual answer from someone with the knowledge.
Anything less than that is anecdotal.

After using a scan gauge on my 2005 LLY D-max I found that the truck would often stay in 5th gear when it actually would have run more efficient with lower EGT's in 4th. After finding this info years ago I always felt the truck up-shifted quicker than I wanted when towing.
 

wwillson

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If you can somehow get your dealer to put you in touch with a zone rep I would pose the question to him/her and then ask for it to be kicked up the chain to current product engineering. You would probably need to vet whoever responds to make sure they arent just passing you off on some low level customer service functionary. Youre going to need to climb the food chain to get an actual answer from someone with the knowledge.
Anything less than that is anecdotal.

After using a scan gauge on my 2005 LLY D-max I found that the truck would often stay in 5th gear when it actually would have run more efficient with lower EGT's in 4th. After finding this info years ago I always felt the truck up-shifted quicker than I wanted when towing.
I appreciate your optimistic view of affecting change, but do you think GM is going to change shift mapping because I ask them to? Remember, any changes in the programming may affect emissions and fuel economy, so must be approved by a three letter agency. I don't think it's going to happen.
 
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Throttle opening ,more fuel , more heat. When driving semis decades ago, going up a mountain pass the Egts would rise so you dropped down a gear and the EGT would lower. What viscosity oil are you using?
 
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Throttle opening ,more fuel , more heat. When driving semis decades ago, going up a mountain pass the Egts would rise so you dropped down a gear and the EGT would lower. What viscosity oil are you using?
I use 15w40 and save the 5w40s for winter. Some people like 5w40 year round.
2006 LBZ, 2011 LML, 2018 L5P
 
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According to your data stream your EGR temp is what you are viewing? A rule of thumb with Aluminum pistons with a diesel is 1200* sustained is the maximum limit. You can start to melt aluminum at that temperature. Is your EGR temperature sensor in the up pipe pre turbo? Or is it in the EGR hot side pipe prior to EGR cooler?
 
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I run Mobil Delvac 1300 15w40 year round. I don’t drive my Cummins below 0F to avoid using 5w40. Next change is Mystik JT8 15w40.

Just my $0.02
 

wwillson

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According to your data stream your EGR temp is what you are viewing?
The data point is EGR1/1, which is the hottest exhaust sensor in the Duramax and my assumption the sensor is in or close to the manifold. I wish someone could tell me exactly where the sensor is, because I can't find any information, even googling. The rest of the exhaust sensors are post turbo (man is this interesting to watch) and at four other points in the after treatment system. All of the exhaust sensors are much cooler than EGR1/1.
A rule of thumb with Aluminum pistons with a diesel is 1200* sustained is the maximum limit.
I've read that too. The Duramax pulling in 10th will maintain > 1200°F all day long under the right conditions. Those sustained temps can't help longevity, but the marketing folks sure to like the torque war.
 
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I never mentioned change, only suggested that you might be able to get information.
I appreciate your optimistic view of affecting change, but do you think GM is going to change shift mapping because I ask them to? Remember, any changes in the programming may affect emissions and fuel economy, so must be approved by a three letter agency. I don't think it's going to happen.
 
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