6.6 Direct Injection Reliability Concerns

Khoa Pham · January 18, 2021

Hello Friends,

My dad is in the market for a new 2500 Express van since his 2014 Express with the 4.8L V8 gave out at only 130,000 miles. Naturally, I wanted to find him a 2020 van with the 6.0L V8 but he is drifting towards the newer 6.6 gas.

For 2021, Chevy did put the 6.6 L8T motor into their 2500/3500 Express Vans. I am sure that this powertrain combo is the same as the HD Silverado pickups.

Since I am unable to find much information about this motor, does anyone know what sort of engineering GM has done to prevent carbon buildup on the intake valves? This is an issue with DI motors and I am concerned with longevity and increased maintenance costs if one has to take their truck in every 30,000 miles to have the carbon buildup sandblasted.

If Toyota decided that adding both port and direct injection to their motors would resolve this issue, what sort of magic did GM come up with?

Ray Pickle · January 20, 2021

There are chemical treatments to address the your valve concerns. B.G. among others have suitable products.

spm1us · January 20, 2021

From what I have researched it appears that reducing the oil change interval has a dramatic effect on the quantity of carbon deposits. I plan on using Shell Gas and Truck 5W30 oil and Baldwin B7449 filters on a 4500-5000 mile OCI. The use of a GM Dexos 2 approved oil would also keep your warranty in effect. GM has also designed the valve covers to catch the oil vapor that contributes to creating these deposits. Some owners are also installing Catch Cans to trap the oil vapor. With Direct injection, there is no longer any fuel wash to keep the intake valve clean.

-David- · January 24, 2021

I am a prospect buyer and am also very interested in this subject. I like to keep vehicles for a really long time. As of now I have not see anyone show borescope photos or any other evidence of how well these engines mitigate the buildup over time or miles or if they suffer from fuel dilution in the engine oil. A recent thread showed a catch can installed that was collecting oil/water mixture in the 6.6 which shows that the OEM separator is not 100%. Fuel type (top tier), oil type, oil change intervals, type of usage, etc. are all contributors. In general I do not like the idea of adding catch can in that the water/moisture caught can freeze and *might* inhibit gas flow of which can cause other issues. It surprises me that GM will not run a port and GDI system like Toyota and Ford are now using in lieu of straight GDI.

Khoa Pham · January 26, 2021

On 1/24/2021 at 9:28 AM, -David- said:

I am a prospect buyer and am also very interested in this subject. I like to keep vehicles for a really long time. As of now I have not see anyone show borescope photos or any other evidence of how well these engines mitigate the buildup over time or miles or if they suffer from fuel dilution in the engine oil. A recent thread showed a catch can installed that was collecting oil/water mixture in the 6.6 which shows that the OEM separator is not 100%. Fuel type (top tier), oil type, oil change intervals, type of usage, etc. are all contributors. In general I do not like the idea of adding catch can in that the water/moisture caught can freeze and *might* inhibit gas flow of which can cause other issues. It surprises me that GM will not run a port and GDI system like Toyota and Ford are now using in lieu of straight GDI.

Agreed! Would love to see how these hold up since the Chevy promo videos are so proud of having the first DI motor in the HD segment. I have my doubts since the engineers made questionable choices on the new 3.0 duramax too.

CamGTP · January 26, 2021

If the engine ran long enough moisture is not a concern to have. I have 109,000 miles on my DI 5.3 engine, I took pictures of my valves at 104,000 miles and have zero concerns with them at that mileage. There is a thread on it I made.

As a previous post said, they make cleaners that anyone can use. If you are that concerned you can buy a can of cleaner and run it through the intake every 20-30k miles just to make you feel better.

Carl Brothers · February 9, 2021

I've ordered a 2021 Sierra 2500 HD with the 6.6 liter GDI motor. It'll be here around Feb. 27th. Don't know how well it'll work, but, I'm installing a water injector set up between the MAFS in the air inlet tube and hat on the intake manifold. I used to run water injectors back in the late 70's and 80's and aside from letting me get away with running street gas in a high compression engines, it also kept the cylinder heads and piston tops totally carbon free after many miles between teardowns to fresh these motors up with new rings and bearings. Still not sure if it'll help with carbon build up on backside of intake valves, but, I'm going to give it a shot.

HondaHawkGT · February 11, 2021

On 1/24/2021 at 9:28 AM, -David- said:

I am a prospect buyer and am also very interested in this subject. I like to keep vehicles for a really long time. As of now I have not see anyone show borescope photos or any other evidence of how well these engines mitigate the buildup over time or miles or if they suffer from fuel dilution in the engine oil. A recent thread showed a catch can installed that was collecting oil/water mixture in the 6.6 which shows that the OEM separator is not 100%. Fuel type (top tier), oil type, oil change intervals, type of usage, etc. are all contributors. In general I do not like the idea of adding catch can in that the water/moisture caught can freeze and *might* inhibit gas flow of which can cause other issues. It surprises me that GM will not run a port and GDI system like Toyota and Ford are now using in lieu of straight GDI.

Catch cans will mostly capture water and a tiny amount of oil/combustion byproducts. Mine would freeze solid in the winter and it was extremely annoying. If I didn't drain it every 2 or 3 days, it would be so full of ice that it blocked path between the intake and PCV. IMO, if you live in a region that's cold, catch cans are waaay more of a hassle than they're worth. I'm thankful I didn't damage any seals driving around with blocked off PCV system.

If intake buildup is really that big of a concern, a PCV delete setup would be a better way to go. Plenty of sports car guys do it. Auto manufacturers would do it, but the government doesn't look kindly venting crankcase gases to atmosphere.

rileyc · March 26, 2023

I recently took delivery of a 2023 Chevy Silverado 3500HD with a 6.6L L8T gas engine after waiting 10 months from the start of order to delivery. A concern I have with the L8T is the build up of intake valve deposits. I have read that GM has an extension baffle system in the valve covers to mitigate the issue. I also have read about owners installing catch cans as a preventative measure. Additionally, many owners are doing shorter OCIs with the appropriate Dexos1 Gen 2 or Gen 3 oil and/or using top tier gasoline to help minimize intake valve deposits.

Since the L8T is only a few years old, I have not seen any borescope photos of the intake valves. Does anyone with the L8T have any actual data on intake valve deposits for this engine?

Dr1ft3r · March 28, 2023

Use top tier fuels and run cleaner through it every now and then.

I am using this every 6,000 miles:
https://www.amazon.com/Chevron-Techron-Concentrate-System-Cleaner/dp/B00092893E/

If doing that doesn't work for you, then just get the valves cleaned for ~$500 if you start noticing a loss of performance. If you don't notice any performance loss then there's nothing wrong with your valves.

It's not as big of a deal as some people make this out to be.

Ray Pickle · March 29, 2023

D.I. fuel delivery is employed in over 55% of powertrains presently produced. The real issue of intake valve carbonization is a simple matter of understanding the need to change old ways/habits as it relates to maintenance. Use appropriate chemicals at appropriate intervals and the likelihood of having to deal with worst case scenario of manual valve cleaning is minimized. Pretty much like everything else related to vehicle maintenance.

Jettech1 · April 6, 2023

On 3/27/2023 at 10:57 PM, Dr1ft3r said:

Use top tier fuels and run cleaner through it every now and then.

I am using this every 6,000 miles:
https://www.amazon.com/Chevron-Techron-Concentrate-System-Cleaner/dp/B00092893E/

If doing that doesn't work for you, then just get the valves cleaned for ~$500 if you start noticing a loss of performance. If you don't notice any performance loss then there's nothing wrong with your valves.

It's not as big of a deal as some people make this out to be.

Everything you put in your tank of fuel will only go through the injectors. It has nothing to do with the backside of your valves. That's where intake cleaners come into play. The best ones to use, I have no idea. But seafoam has always been good to me. Is it the best...who friggen knows...but it did help on my 2014 impala big time. I put my eyes on them before and after. So it worked amazing. I've read great reviews from Amsoil intake cleaning products and BG products as well. Any cleaner you want to introduce to the backside of the valves has to go through the intake, not the gas tank.

swathdiver · April 6, 2023

Get the engine blow by out of the intake, a catch can system properly designed does this.

Grumpy Bear · April 6, 2023

Blow-by gas is an interesting subject. Observations for what they are worth. All ventilation systems move gas by pressure differential. Manifold vacuum is the driver and until rings expire will be the major driver. Vacuum drawing fresh highly filtered air via the intake system on modern vehicles. The inside of a motor looks like a hurricane hit it. Oil expelled from rod journals whipped by the rotating assembly a major source of 'misted' oil. The baffles in the valve cover or some other device doing the bull work of separating that mist from the air draw. Some gas, blow-by gas, does get past the rings enter the sump. Mostly water vapor and hot combustion byproducts. If you motor seals well this is the smaller part. Mist the larger.

That isn't carbon on the backside of intake valves. Never gets hot enough even during the hot soak to reduce the hydrocarbons to carbon. It's the polymers, mostly VVI polymers glued together with the light ends of oil and Un combusted gasoline. That is the short and dirty version.

So, limit blow-by by getting a good ring seal and use an oil with low volatility and as little VVI as possible.

This is Cameron's motor mentioned above at 104K miles. GDI isn't an automatic death sentence.

Dr1ft3r · April 6, 2023

6 hours ago, Jettech1 said:

Everything you put in your tank of fuel will only go through the injectors. It has nothing to do with the backside of your valves. That's where intake cleaners come into play. The best ones to use, I have no idea. But seafoam has always been good to me. Is it the best...who friggen knows...but it did help on my 2014 impala big time. I put my eyes on them before and after. So it worked amazing. I've read great reviews from Amsoil intake cleaning products and BG products as well. Any cleaner you want to introduce to the backside of the valves has to go through the intake, not the gas tank.

If adding anything to the fuel has zero effect on carbon deposits, or any other type of deposit, then why would using top tier fuels as instructed by GM matter either? The only difference between top tier fuel and fuel not rated as top tier are the detergents used in the fuel.

Think on that.

Also, it is my understanding that changing the oil sooner has a positive effect on deposits accumulating in your engine.

Edited April 6, 2023 by Dr1ft3r