I have a 99 Z28 Automatic. I have noticed a diesel sounding\pinging noise at times when I am between 1.5-2.0 RPM when I apply the throttle. If I really get on it I cannot hear it or it seems if I am above 2.0 RPM I do not hear it. I do not hear it when I take off from a complete stop either. Only between 1.5-2.0 RPM it seems is when I hear it. Sometimes below 1.5. And I only hear it for a second and then it goes away like something was loose and tightend up or stabilized or something. Can anyone help?? Thanks in advance!

 

Is it happening only on cold starts, and once it warms up it goes away? If so, then you have piston slap. It's not rare for the LS1. My SS sounds like I have forged pistons on a cold start, but once it's up to operating temp it's fine. The pistons are rattling in the cylinders because they haven't expanded yet. Once they do expand with heat, then it should go away.

Trevor - LGM

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No it happens even after the car is warmed up. If anything it seems to not happend while the care is still cool.

 

What mods do you have? Is it under WOT? Doesn't sound like it is, so it's likely not detonation.

Likely one of your belts. There is a TSB out for them on pre-2000 models. The idle tensioner bracket and pulley cause a tick tick tick noise. Goes away under a heavy load because slack is picked up and the engine drowns it out.

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It is possible to have audible detonation at low rpm acceleration if you are light on the pedal and you are not using the premium fuel required per our owners manual. This is especially true if there is carbon build-up in the combustion chamber.
99s are not known for piston slap.
I haven't heard about the idler pulley, that is an interesting thought.
If I had to guess though, I'd say this person is not hard on the pedal when it happens, they use 87 octane fuel, they have some miles but never had a top end cleaner done, and the problem is worst on hills.
Dave

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I use 91 and 92 octane fuel plus I run octane booster on occasion. I have used some fuel injector cleaner thinking it might be carbon build up. I have had my tensioer and pulley replaced as well as the belt. I would not be suprised if that was what it is as I have had constant problems in the past with squeaky belts. I have had the belt replaced 3 times, the tensioer twice and the pulley once. The problme is not different on hills.

 

Well, I think you pretty much ruled out detonation... unless you have some pretty nasty mods you haven't told us about.
Dave

 

I run only 92 octane, and started to have the same pinging noise symptoms earlier this year. The only change was where I was buying my gas. Instead of Chevron it was Costco; it was so convenient to stop there on my way home and I stuck to 92 octane, but the pinging continued. Even the wife's Taurus was running rough (on 87). Switched back to Chevron, all is well.

DaveH

 

Maybe I should add that I only hear this noise when I am in gear. I also here a high-pitched noise, kinda like a pulley squeek or a vaccuumm leak. The high pithced noise gets higher as I go through the gears. I cannot get the "Diesel" sound when I am park and throttling the engine from under the hood or in neutral. Thanks again in advance!

 

An engine will not detonate unless under load.
Just throwing that out for consideration...
Dave

 

Well I did switch gas when I moved. I went from Amoco which sold 92 Octane to Phillips 66 which has 91. I guess I never really stopped to consider that. I will go back to Amoco and see what happens. Also it seemd like when they replaced the tensioner, pulley and belt (3rd time for belt) that it started but I can't really remember for sure. Thanks again for everyone's help.