6.2L Chevy and odd knock story

[DriveHard]

Just going to throw this story out there and see what you guys think...

2011 Chevy Silverado with 6.2L
requires premium fuel or E85 from factory
I have owned it since 2012
Full length headers
dual exhaust with crossover
airraid custom intake
custom tune from Diablew

I run 99+% E85, even in the winter. I get what sounds like piston slap knock on cold starts almost all the time. Once per year we take a trip from Iowa out to Colorado where I have to run premium for the most part, as out of Iowa it is tough getting E85. By the time I get back I have the following observations:

Cold start is rougher than with E85
piston slap is TOTALLY GONE

I am now back to my second tank of E85, towed the boat a couple times. Cold starts are back to smooth as silk...but already starting to get the faintest hint of piston slap starting to raise its head again.

When warm, the engine is smooth as silk. It seems smoother on E85...especially at idle. Just seems a bit coarser on premium.

Power level seems equal between E85 and premium...am guessing that is due to the tune. I know Lew tunes only the gas maps, and leaves the E85 maps alone, so I am guessing it is running on "stock" maps on E85. Before the tune, power was noticeably higher on E85.

So any thoughts on why I get piston slap on E85? GM has a service bulletin that says it is due to carbon buildup on lightly driven engines...I can assure you, pulling a 25' boat several hours a week it is not babied. While I rarely redline it, I do accelerate with authority when warm. I find it odd that the carbon would disappear from what would normally be an easier duty cycle for my driving on a family trip out to the mountains. Maybe the high altitude eats carbon ;-)

Anyways...not looking to change anything, or really all that concerned, just wanted to share some observations.

 

[WyrTwister]

https://www.google.com/search?q=2011+Che...ent=firefox-b-1

 

[emmett442]

Any time there's an aftermarket tune involved, all bets are off when it comes to diagnosing your symptoms...

 

[tig1]

A friend sued GM over piston slap and lost.

 

[LeakySeals]

I can't speak to the change in octane influencing startup slap because I have a 6.0 lq9 max which requires premium. But I can say its very particular about the oil and filter used. What brand, weight, and filter are you using?

 

[JustN89]

Shoot, if I don't get piston slap on a cold start with my 3400, I start to worry! Doesn't seem to impact anything, though, so I don't give it too much thought.

 

[supton]

E85 would have to run quite rich at startup, right? Less volatile (and has to run richer to begin with). Wonder if that factors in somehow. Maybe it washes down the cylinder walls on startup, leading to slap; and premium doesn't. But why premium would run rougher is beyond me.... maybe it's running too rich on premium cold start but not enough?

Total shot in the dark here.

 

[mcrn]

Maybe its due to the driving style while on premium? Sounds like that is almost all highway? I do not know but just a shot in the dark. I know I drive mostly city but when I take my car on a long highway trip it runs differently. Can you set the tune back to factory and see if that changes anything?

 

[LeakySeals]

Originally Posted By: supton
E85 would have to run quite rich at startup, right? Less volatile (and has to run richer to begin with). Wonder if that factors in somehow. Maybe it washes down the cylinder walls on startup, leading to slap; and premium doesn't. But why premium would run rougher is beyond me.... maybe it's running too rich on premium cold start but not enough?

Total shot in the dark here.


The time of year could effect the rich mix at startup. I think his issue could also be winter oil flow at startup. Using same oil diff filter my truck slaps with a Fram Ultra. Less to none with a TG, wix, less efficient filters.

 

[DriveHard]

Originally Posted by LeakySeals
I can't speak to the change in octane influencing startup slap because I have a 6.0 lq9 max which requires premium. But I can say its very particular about the oil and filter used. What brand, weight, and filter are you using?



Fram Ultra
5w-40 PP in summer
5w-30 PP in winter

~130k miles

 

[LeakySeals]

Originally Posted by DriveHard
Originally Posted by LeakySeals
I can't speak to the change in octane influencing startup slap because I have a 6.0 lq9 max which requires premium. But I can say its very particular about the oil and filter used. What brand, weight, and filter are you using?



Fram Ultra
5w-40 PP in summer
5w-30 PP in winter

~130k miles


My 6.0 VortecMAX does not like the Ultra. I tried a few different Ultras, and they all made the engine slap and tick for 10-15 seconds. Long enough to feel uncomfortable. Its very quiet with the TG. Almost no ticking or slaps at startup. A few other on here have said the same. Its not all, but some engines. Suggest you switch to the TG or some other less efficient filter and see what happens. .Report back.