87 tune spark knock

[Murphy406]

I have a 2008 Silverado with a 5.3 that I installed the 87 tune on. I disabled the AFM as well. Everything else is stock. With hard acceleration I have terrible spark knock. I went back and install the modified factory tune with removal of the AFM. No spark knock even at WOT. Any ideas?

[Murphy406]

I tried another tank of gas to make sure it wasn't bad gas as I have seen people have had this issue. So I used a better brand of gas 87 octane and no improvements. I then ran some 93 octane with the 87 octane tune and the spark knock is gone. I don't really want to run 93 octane with a 87 octane tune. At the moment I am using the modified stock tune to remove the AFM. If anyone has any ideas to eliminate the knock I am all ears...

[TuckerW]

Seeing that changing the fuel to a higher octane resolved the issue I am going to say that it is just bad quality 87 fuel. Different companies use different additives in their fuel. Mobil, Exxon, Sunoco, Amoco, BP, Chevron are best brands to use, with Mobil and Exxon being the best. Try another tank of 87 from one of the listed providers and see what your results are.

[Murphy406]

The 87 I used was from Chevron. There is nowhere else around me that uses any of the other brand fuels. Most seem to be tier 3 fuel.

[gregory]

Question for ya. Does the issue only occur when engine is up in temperature or all times? I had issues with detonation and the problem was with the t-stat was getting engine up to 220 which was causing detonation, just a thought.

[Murphy406]

I am not sure about that. I will put the 87 tune back on and give it a try. Thanks for the input.

[Murphy406]

I contacted tech support and was told to retard the timing on WOT. SO I believe I put 2 degrees retard on all 3 WOT settings and still the spark knock is pretty bad. If I did this right I attached a log file.

[techntrek]

Question for ya. Does the issue only occur when engine is up in temperature or all times? I had issues with detonation and the problem was with the t-stat was getting engine up to 220 which was causing detonation, just a thought.

Just a point of info. With the factory tune and the 93/diablo tune, when towing my engine will cycle up to 235 (coolant) before the fans kick on high, then back down to 210. Repeat. This is "normal". I say this because I haven't had any problem with knocking at that temperature. Every gas supplier under the sun (multiple trips all over the US).

I don't like the cycling so I modified the fan settings and it holds at 210 while towing, now.

[Murphy406]

Seeing that changing the fuel to a higher octane resolved the issue I am going to say that it is just bad quality 87 fuel. Different companies use different additives in their fuel. Mobil, Exxon, Sunoco, Amoco, BP, Chevron are best brands to use, with Mobil and Exxon being the best. Try another tank of 87 from one of the listed providers and see what your results are.

I tried another tank of fuel. I have the log posted above. I believe I am seeing 6 degrees of timing retard due to knock. That is with 2 degrees retard at all three WOT settings available. It seems fine under light loads. If I accelerate moderately while going uphill I am fine. Once I get over 75% throttle sounds like a coffee can with a handfull of rocks in it. I don't want to pull 8 degrees of timing out. Sounds like that will hurt my performance as well.

[gregory]

93 octane is the reason no detonation at 235. 87 octane has lower flash point and will pre fire at those temps for sure. he is running 87 which engine temps can cause these issues. But I am leaning more towards bad fuel or something in gas tank that don't belong there.

[Murphy406]

It looks like my highest temperature was less than 200 degrees unless I am looking at it wrong. It looks like the total timing retard due to knock is almost 10 degrees. I think that is excessive.

[Murphy406]

I also have a 1999 Sierra running the 87 tune from a Predator U7198x. Same gas. No spark knock. Seems to me they habe the same engine and transmission for the most part. If it was gas shouldn't the 1999 have the same issues?

[gregory]

You would think so. Maybe mike can look at your files and try to figure this one out. be sure and list any after market items like CAI etc.

[Murphy406]

Everything is stock. I haven't changed anything other than the tune.

[mikel]

log the stock tune and see if it knocks