DPF Clean on a LM2 3.0 Duramax?

cautious optimist · August 11, 2025

TL;DL: Seeking instructions on how to clean a DPF on a LM2 3.0 Duramax Diesel.

Long version: Hello all. I bought a 2021 GMC Sierra 1500 with the Duramax about 8 months ago. When I first purchased it, I was getting roughly 22-24 mpgs around town. About two months ago, I had to pound the gas to avoid getting hit by a redlight runner. Checking my rear view mirror I noticed my truck had belched out a significant amount of sooty exhaust. It had never done that before or since. Since that day, my mpg's have been dropping. First, I struggled to get it above 20mpg. This last tank it only reached 17.1mpgs. No check engine lights or anything indicating an issue. No major change in my driving habits or regular daily routes. I have no idea when it regens, so I don't know if it goes into regen more often or not. I topped up the DEF at 84800 miles (My first DEF Fill-up), then again at 87450. (2650 miles) Then yesterday at 90560 miles. (3110 miles) So it doesn't appear to be eating more DEF. I've wondered if somehow the DPF filter got clogged with that hard acceleration? I've read that there are cleaning kits for DPF's and I've seen videos on YouTube but I can't find any instructions specifically for how to do it on a LM2. Specifically I'm trying to find which port to use if it can be done. Any help or suggestions would be greatly appreciated.

newdude · August 11, 2025

11 minutes ago, cautious optimist said:

TL;DL: Seeking instructions on how to clean a DPF on a LM2 3.0 Duramax Diesel.

Long version: Hello all. I bought a 2021 GMC Sierra 1500 with the Duramax about 8 months ago. When I first purchased it, I was getting roughly 22-24 mpgs around town. About two months ago, I had to pound the gas to avoid getting hit by a redlight runner. Checking my rear view mirror I noticed my truck had belched out a significant amount of sooty exhaust. It had never done that before or since. Since that day, my mpg's have been dropping. First, I struggled to get it above 20mpg. This last tank it only reached 17.1mpgs. No check engine lights or anything indicating an issue. No major change in my driving habits or regular daily routes. I have no idea when it regens, so I don't know if it goes into regen more often or not. I topped up the DEF at 84800 miles (My first DEF Fill-up), then again at 87450. (2650 miles) Then yesterday at 90560 miles. (3110 miles) So it doesn't appear to be eating more DEF. I've wondered if somehow the DPF filter got clogged with that hard acceleration? I've read that there are cleaning kits for DPF's and I've seen videos on YouTube but I can't find any instructions specifically for how to do it on a LM2. Specifically I'm trying to find which port to use if it can be done. Any help or suggestions would be greatly appreciated.

Are you using the AC a lot? Idling to keep the cabin cool? You were 22-24mpg "around town" when you bought it 8 months ago. Big difference in weather from then.

Is the check engine light on? Any codes in the comupters?

Typically if you see smoke on a newer diesel, its either got a heavily sooted DPF and isn't regenerating enough to keep it clean (think excessive idling, lots of around town/stop and go driving), or there is a problem.

Edited August 11, 2025 by newdude

cautious optimist · August 11, 2025

7 minutes ago, newdude said:

Are you using the AC a lot? Idling to keep the cabin cool? You were 22-24mpg "around town" when you bought it 8 months ago. Big difference in weather from then.

Is the check engine light on? Any codes in the comupters?

Typically if you see smoke on a newer diesel, its either got a heavily sooted DPF and isn't regenerating enough to keep it clean (think excessive idling, lots of around town/stop and go driving), or there is a problem.

I guess I am using the A/C a lot more than Feb, March and April. I don't idle it to cool it down. No computer codes or check engine lights. I do have a lot of around town driving and shorter trips traveling from jobsite to jobsite. Many of these trips do include about 10-15 minutes on a freeway. A couple 500 mile round trips to visit a project in Colorado in the last few months. I thought to start with cleaning the DPF to see if it's that or another problem.

newdude · August 12, 2025

4 hours ago, cautious optimist said:

I guess I am using the A/C a lot more than Feb, March and April. I don't idle it to cool it down. No computer codes or check engine lights. I do have a lot of around town driving and shorter trips traveling from jobsite to jobsite. Many of these trips do include about 10-15 minutes on a freeway. A couple 500 mile round trips to visit a project in Colorado in the last few months. I thought to start with cleaning the DPF to see if it's that or another problem.

I guess I'd be looking at all the data stream for everything then and seeing things like DPF pressure differential, soot levels, fuel injector balance test, hydrocarbon injector performance, etc., instead of just jamming chemical products into an expensive piece of emissions equipment.

If everything is operating within specs per the GM service manual, and you don't have any codes right now like say P2002 particulate efficiency below threshold or any other particulate matter codes, its hard to just condemn a DPF like that.

silveradosid · August 12, 2025

have you checked the air filter, turbo charged engines suck a lot of air and require filter changes more frequently

Silverado4x4 · August 12, 2025

It's always a good idea to do a couple of full throttle acceleration on gas or diesel motors to blow out the crap in the exhaust or motor about once a week or 2 weeks.