Pre ignition

1st Vette

1st Vette

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The dreaded rattle. I have access to 91 octane fuel only in my area . I have been using only Shell Top Tier 91 since new mostly except while traveling, but always at least 91 and always Top Tier . Yesterday while going to move to the right hand lane while in 4th gear at about 1400 pm I noticed the blind spot lights come on in my right mirror and gave it some gas to quickly get ahead of the vehicle beside me . And then heard very distinctly the dreaded "rattle" . First and only time it has happened.
Has anyone else experienced this ? And is anyone aware of an octane additive that is " approved " for use in the LT2 engine ? And if so have any of you used one . Thanks .
I should add that I was in auto mode which I do infrequently, usually always in manual and generally keeping the RPM up . So , although I didn't notice I am pretty sure it would have been in 4 cylinder mode at the time .
 
cdnz said:
DRIVE IN MANUAL MODE. LOL....
4cly at 1400 rpms. 😬
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Believe me , 98% of the time I do . I never let the engine lug . Don't forget that supposedly the ECM won't allow the engine to lug either according to what the engineers/programmers feel is too low . Next time you are in manual mode at 1500 RPM try to up shift .... it won't. Same as trying to downshift at higher RPM , it won't let you.
 
I suspect you fooled the tranny. It's happened to me occasionally where the car was in the wrong gear when I went to pass. I had been slowing down behind a slow car and the tranny obviously was down a gear, but I sped up a little to check for the pass and the alternate clutch went up a gear and I bogged when I gave it gas. I didn't notice a rattle though.
 
Murray20c8 said:
I suspect you fooled the tranny. It's happened to me occasionally where the car was in the wrong gear when I went to pass. I had been slowing down behind a slow car and the tranny obviously was down a gear, but I sped up a little to check for the pass and the alternate clutch went up a gear and I bogged when I gave it gas. I didn't notice a rattle though.
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They will sometimes do that if you are in manual and you happen to pull the left paddle just as the ECM decided it's time to go down a gear like from third to second and you end up in first if the revs are low enough, because of the timing . In manual as you have probably noticed the TX will down shift when the revs drop low enough whether you pull the paddle or not . This though was a definite pre ignition rattle. Very distinct . Not odd shift point related I'm afraid.
 
1st Vette said:
They will sometimes do that if you are in manual and you happen to pull the left paddle just as the ECM decided it's time to go down a gear like from third to second and you end up in first if the revs are low enough, because of the timing . In manual as you have probably noticed the TX will down shift when the revs drop low enough whether you pull the paddle or not . This though was a definite pre ignition rattle. Very distinct . Not odd shift point related I'm afraid.
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I meant that when the car bogs you can get piston rattle
 
Sounds like a pretty obscure set of conditions before you could possibly hear this noise again. I have only ever run 91 octane, and always in automatic mode with no issues in that regard. Trust the engineering, it's not perfect but I think you have nothing worth worrying about.
 
vh04x said:
You can do a compression test, but you're probably overreacting. ECU has maps for all of these octanes
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Hard to tell obviously but perhaps the last batch of fuel the station received wasn't up to snuff or they put 89 in the 91 tanks . Without having it lab tested there is no way to know. But I know for sure what I heard, it's a very distinctive sound I've heard before in older vehicles when the timing is out . As for compression it's too high a compression that is likely to cause it and I don't see how the compression in any of the cylinders could increase...... and yes , the ECU/M does adjust for different grades . It just dials back the timing with lower octane. At any rate it's not something you want happening under any circumstances. I guessing a bad batch of fuel .
 
1st Vette said:
Hard to tell obviously but perhaps the last batch of fuel the station received wasn't up to snuff or they put 89 in the 91 tanks . Without having it lab tested there is no way to know. But I know for sure what I heard, it's a very distinctive sound I've heard before in older vehicles when the timing is out . As for compression it's too high a compression that is likely to cause it and I don't see how the compression in any of the cylinders could increase...... and yes , the ECU/M does adjust for different grades . It just dials back the timing with lower octane. At any rate it's not something you want happening under any circumstances. I guessing a bad batch of fuel .
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We used to say "Sounds like a plymouth". lol.
I got bad gas once and it made the car run like shlit, but I don't remember any valve train rattle. Add some octane boost or seafoam if you think it's the gas.
 
Murray20c8 said:
We used to say "Sounds like a plymouth". lol.
I got bad gas once and it made the car run like shlit, but I don't remember any valve train rattle. Add some octane boost or seafoam if you think it's the gas.
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We have another station here with Top Tier so next fill I will try that . Down to about a quarter tank thankfully if thats the issue which I hope it is. I just wish we got 93 here but not many snow machines burn it , lol. Could always add some 100 octane I suppose but the price is crazy and hard to say how much to add ....
 
I used to burn a mix of 100 low lead and regular in a 68' 350 and no knock . I'll get fuel at a different station in a couple of days as it's getting low . Only twice now have I heard the dreaded rattle but would rather not hear it at all obviously. Thanks for the reply. :)
 

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