ghost knock problem, need help

Hey guys, we finally got our truck up to the dyno to have our tuner work on it and we ran into an issue with it pulling serious timing due to knock, at the time we had 87 octane in it which we thought would be fine for 10:1 compression, well we came home and filled it with 92 and it still seems to be pulling timing, well tonight I put about 5 gallons of 110 mixed with 3 gallons of 92 in it and still the same thing.

Here is my question, our truck is a 408 and has brand new Delphi knock sensors, but the wiring harness is from the old junkyard 6.0. Has anyone had issues with that wiring harness causing ghost knock?

What headers, are they hitting anything, are you positive the header bolts are tight?

American Racing 1 & 7/8ths stainless steal headers, bolts are tight and no they aren't hitting anything.

Knock sensors may be bad.

Hmm, well they are brand new, I talked to a GM tech and he said the sub harness on the knock sensors get replaced a lot, were gonna start there and see what happens.

True story. Check it out.

yup its always recommended to replace the harness with knock sensors

and if that doesn't fix it you may need to lower the sensitivity of the knock sensors in the tune.

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Originally Posted by RedHeartbeat

and if that doesn't fix it you may need to lower the sensitivity of the knock sensors in the tune.

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He did that actually and we picked up like 80rwhp just from that, he said it was still dropping the timing down to about 8 degrees under load due to knock.

I've seen the engine harness shorted before. This causes knock. Replace the knock sensor harness first.
There r tables u can edit to limit timing and knock, but that's not going to fix the issue. It will just limit the amount of knock and how low timing can go.
When I run into this I try another calibration and PCM. If there's nothing wrong with the truck mechanically, it'll fix it


Good luck.

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Originally Posted by mr2cam

He did that actually and we picked up like 80rwhp just from that, he said it was still dropping the timing down to about 8 degrees under load due to knock.

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Your ghost knocks might be coming from your pistons. I remember reading about somewhere. Forged pistons can cause the ghost knocks. Are our spark plugs showing any sign of actual preignition?

I had a similar issue on my 08 with Burst Knock. I don't know if any of the Gen III had burst knock, but it seems to be fairly common on NNBS trucks with headers (from what I've read on HPT). I lowered my knock sensors sensitivity by like 5% at a time until it was almost eliminated. For some reason, I was only getting knock on the shifts, which I assume was possibly my headers hitting the frame because they were super close. I eventually disabled burst knock in my tune and had no issues. I know you've already lowered the sensitivity, so I'm just throwing this information out there for people who may stumble across this thread and have an issue like mine.

just for those reading with similar problems

something that i have run into is a loose timing chain. granted, i seriously doubt that is the case with OP (my truck is pushing 200k) but i had a knocking start up, and slowly get worse. first just light pinging under load, all the way to noticeable drops in power and pretty serious knocking even under light acceleration, in spite of new knock sensors and harnesses, new intake and exhaust manifold gaskets, 91 octane, etc. never once thought to check the timing chain. when it finally donned on me to check it, it was definitely loose. replaced it and no problems since.

I had the exact same problem and new spark plugs fixed mine

Thanks for all the responses, we replaced the harness and 2 new knock sensors and it didn't fix it, after doing some research and noticing a noisy valve train we found out that the comp 1.82 roller rockers can cause this problem, we have not seen any signs of actual detonation though.

What were gonna do is switch back to the stock roller rockers and see if the knock goes away at all, if it does we might put the 1.82's back in and just turn down the knock sensitivity as much as possible.

Anyone have any ideas or thoughts on our new developments?

sounds like a sensitive calibration.

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Originally Posted by farmtruc

sounds like a sensitive calibration.

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+2

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Originally Posted by 2boostedSilverado

+2

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you would know!! didn't you change over to a newer pcm

Yeap :)