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Discussion Starter · #1 ·
I have a 2008 GMC 5.3 flex fuel with 101,618 miles. Running a diablo intune on it. Currently I have the AFM turned off and I am running the MPG booster canned tune. I was running the canned 87 octane tune but switched to the MPG booster a little but ago. I've only ever put 87 octane unleaded fuel in the truck

Had my truck serviced today, front differential, rear differential, transfer case fluids replaced, dropped transmission pan and filter, and replaced plugs and wires. Replaced plugs with AC-Delco 41-110 and Napa Belden premium wires. Got it all done for $360.

Mechanic took it for a test drive, tested it pretty hard and said he heard a preignition tap/knock. I've never heard of it nor heard a knocking before. Mechanic said it is hard to hear. He mentioned that unless I am driving the truck hard or under heavy load, the preignition knock should not be an issue. I rarely tow and I don't drive the truck hard.

He did recommend using 93 octane fuel if I am running any tune on the motor. He also said to run a data log with the intune and if there is a data point that lists KR or knock, if it is bouncing around then I have an issue. I am not too inclined on the intune beyond AFM delete and canned tunes so I don't know how to review the logs.

So I ask, what is a preigintion knock as google wasn't too helpful in explaining it and providing examples of what it sounds like.
Do I have an issue I need to address or am I fine?

Any information and advice is most appreciated.
 

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I have some. Only happens when it is up to operating temp, and when it happens it sounds like a slight metallic rattling/tapping. I know what to listen for now so I hear it, but nobody else does and if there is any other noise like a window open or the radio on you can't hear it at all, so it is nothing terrible and "relatively harmless" is what I've been told. But, I still don't like it. I think it is a result of carboned up pistons and such, since it doesn't do it until it is warmed up enough that leads me to believe the carbon buildup gunk eventually gets hot enough to trigger that under some loads.

I tried seafoam, it helped slightly, but not much.

I now have some MCCC that I'm going to try when I have some time and some help again.

Using higher octane may help, but if you're having to use higher octane to avoid knock when nothing is modified to require it, then it is an issue that needs fixed, and higher octane gas is just masking it, IMO. FWIW, mine would still do it, tune or not.
 

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Pre-ignition or ping is when the fuel ignites from heat and pressure prematurely and puts pressure pushing against the piston as it's still moving up. So it wasted power and it is hard on the engine. The degree can vary. Higher octane fuel is less flammable and less susceptible to pre-ignition. In other words it can handle a little more heat and pressure without self ignition. The problem is probably the alcohol maybe the octane is not correct. Retarding the time will help to a certain extent. Higher octane will correct the problem. Retarding the time is more masking the problem in my opinion because your walking to fine of a line. lol
 

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I'm gonna say that the mpg booster tune is putting the motor right on the edge of lean. That's how it works.
When running a tune like that you need to drive aware of that and don't overload the engine. Drive it with a soft foot.
Otherwise go back to the 87 tune and drive how ever you like.
Just my humble opinion of coarse since I don't have that tuner.
 

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May be time to do a induction service. With these newer motors they are increasing the compression ratio. I know my wifes car runs nearly close to my diesel. But because of this the timing has to be just so or will cause issues. Also because of this higher compression it is causing carbon build up on the back of the valves. This also causes inadvertent issues. F*rd has been having a lot of these issues on their ecoboost motors. Amongst everything else that seems to fail on them. a little detonation wouldn't concern me too much. But if noticeable at idle then I would be more inclined to have it addressed. No need to wash cylinder walls, bend connecting rods burn bearings and break wrist pins if you don't have to.
 

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Sounds like OP never experiences it because he is driving it sensibly so it most likely isn't happening. The mechanic hammered on it and noticed it not OP. Yes octane is the fix, not the band aid cause it won't happen with 91 in it. 87 octane gas especially secondary rack gasoline is horrible. 87 octane is on the fine line of minimum octane necessary to begin with. So running at the bare minimum needed octane level and expect performance doesn't work well. Run only good name brand fuels to begin with like Sunoco or Mobile ect.. Consider 89 octane, even if it costs a few cents more. At least your not running at the bare minimum of what it requires. OP shouldn't worry too much as he isn't mashing the pedal causing the issue. What else do folks run in their trucks at the bare minimum?
 

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Rambodog said:
Sounds like OP never experiences it because he is driving it sensibly so it most likely isn't happening. The mechanic hammered on it and noticed it not OP. Yes octane is the fix, not the band aid cause it won't happen with 91 in it. 87 octane gas especially secondary rack gasoline is horrible. 87 octane is on the fine line of minimum octane necessary to begin with. So running at the bare minimum needed octane level and expect performance doesn't work well. Run only good name brand fuels to begin with like Sunoco or Mobile ect.. Consider 89 octane, even if it costs a few cents more. At least your not running at the bare minimum of what it requires. OP shouldn't worry too much as he isn't mashing the pedal causing the issue. What else do folks run in their trucks at the bare minimum?
All of this. Even my 15 does it with the 87 tune on it, if I hammer it some times. It almost never did it when I was running the 93 tune. And OP did you notice any more MPG with that MPG tune? I didn't, and the lazy shifts began to erk me.
 

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The 14s and up shouldn't have a problem with this with direct injection. Sounds like the injection is to early or not working as intended if it is getting preignition. Maybe they don't have it right yet or cannot get the injection timing right.
 

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Discussion Starter · #15 ·
ferraiolo1 said:
Rambodog said:
Sounds like OP never experiences it because he is driving it sensibly so it most likely isn't happening. The mechanic hammered on it and noticed it not OP. Yes octane is the fix, not the band aid cause it won't happen with 91 in it. 87 octane gas especially secondary rack gasoline is horrible. 87 octane is on the fine line of minimum octane necessary to begin with. So running at the bare minimum needed octane level and expect performance doesn't work well. Run only good name brand fuels to begin with like Sunoco or Mobile ect.. Consider 89 octane, even if it costs a few cents more. At least your not running at the bare minimum of what it requires. OP shouldn't worry too much as he isn't mashing the pedal causing the issue. What else do folks run in their trucks at the bare minimum?
All of this. Even my 15 does it with the 87 tune on it, if I hammer it some times. It almost never did it when I was running the 93 tune. And OP did you notice any more MPG with that MPG tune? I didn't, and the lazy shifts began to erk me.
So when I bought my truck back in 2013 when gas was $3-4 a gallon, I started recording the following metrics from the DIC at Every gas fill up: mileage, tripometer, average trip mpg, and fuel used. I also would log any changes I would make to the truck specifically installing tune, uninstalling it, service work, etc.
So with the 87 octane tune, I averaged 14.62 mpg over 11 months. Removing highway driving, I averaged 13.70 mpg.
I only have 5 months of MPG booster data but I averaged 13.75 mpg. Removing highway driving I average 12.85 mpg

Now I take these numbers with a grain of salt but they are a general idea of my fuel usage. As I cannot hear the sound of the metal tap, I will return it to 87 Octane tune and I will be turning firm shifts back on as I can't stand the lame transmission shifts. I initially turn shifts to hard but I did not like the jolting I was getting, dropped it to firm shifts, still had the jolts when it changed gears. Returned it to stock and it was ok but now it feels lame so I will put firm shift back on. I don't have the intune knowledge to adjust the shift points myself out of fear of damage so I stick to the canned tunes.
 

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Discussion Starter · #18 ·
apcnc--that's mostly local driving. If I do highway only driving, I've gotten between 15-17 mpg. I don't know what else to do to get the mpg up. I just replaced the plugs and wires, cleaned MAF sensor, throttle body, running airaid MIT tube, AFE pro dry s air filter, AFM off and the tune. I don't drive hard so not sure what else to do to get my local driving mpgs up
 
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