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2002 chevy trailblazer_lt
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Discussion Starter · #1 ·
Alright, so I bought my 02 with the 4.2 with 134,000 miles a few weeks ago, and noticed that in the mornings especially, my engine sounds like its knocking, only for it to go away after a few mins/miles of driving. I have a bunch of friends who have been auto mechanics for years, and my opinion on this "piston slap" condition is the exact same as theirs. Having experience myself going to aircraft school, and having learned about this, I believe I have somewhat of a decent explanation. This condition, in my opinion, is normal, as long as the sound disappears. A lot of piston powered aircraft can experience the same condition. They use "cam-ground" pistons, meaning the pistons are somewhat oval shaped at cold temperature, and the pistons will expand with heat, and provide a proper fit at normal operating temperature because of the high cylinder head temperatures encountered. This explains why the sound is prevalent when the engine is cold, and hardly noticed when warm. You'll hear different opinions on this, but I think the jury is still out. Let me know what you guys think. I know GM did this to save money, by not matching each piston to each cylinder, and this resulted in a sloppy fit. What's your opinion? Harmful or not?:)
 

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2005 chevy trailblazer_ls_ext
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The I6 engines are known to have "cold carbon knock". I notice it on mine when I use cheap gas. It's exactly like you say - a very pronounced knocking in the morning and goes away after driving for a few minutes.
A can or two of Seafoam in the tank (or Chevron Techron) seems to pretty much eliminate it.
I've learned the hard way to stick with top tier gasoline. Chevron and Shell seem to work the best in my truck.
 

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Discussion Starter · #3 ·
The I6 engines are known to have "cold carbon knock". I notice it on mine when I use cheap gas. It's exactly like you say - a very pronounced knocking in the morning and goes away after driving for a few minutes.
A can or two of Seafoam in the tank (or Chevron Techron) seems to pretty much eliminate it.
I've learned the hard way to stick with top tier gasoline. Chevron and Shell seem to work the best in my truck.
I've heard the same thing also. How many miles do you have on yours, and what's the longest distance you've traveled in it? I live in St. Pete, FL, and we're going to make a trip to Pigeon Forge, TN (sp?) and want to make sure it'll make the trip with no issues. Anyway, I'm pretty happy with this thing. Apart from the ****ty gas mileage, it's solid, holds everything we need, and has more than enough power to get around. So do you think piston slap is harmful?
 

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Is this noise a rattly-tapping noise? I noiced my 02 does makes this noise between 2500 and 2900 rpm or so. Done it since I got it at 112k miles (at almost 117k now). Below that, not really, and above that it seems to go away too. Although I make it a point to avoid much above 2500 rpm until it is warmed up (though hard to do when I need to merge on the highway). Aside from making a little noise, nothing else seems to happen, no power loss, no oil PSI wavering on the gauge (though it isn't too accurate anyway), nothing out of the ordinary, just the noise.
 

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Discussion Starter · #5 ·
Is this noise a rattly-tapping noise? I noiced my 02 does makes this noise between 2500 and 2900 rpm or so. Done it since I got it at 112k miles (at almost 117k now). Below that, not really, and above that it seems to go away too. Although I make it a point to avoid much above 2500 rpm until it is warmed up (though hard to do when I need to merge on the highway). Aside from making a little noise, nothing else seems to happen, no power loss, no oil PSI wavering on the gauge (though it isn't too accurate anyway), nothing out of the ordinary, just the noise.
The noise I'm describing is more like a metallic knock. Kinda like someone had a hammer :bonk: and it's really rhythmic, and is RPM dependent. That's piston slap.
 

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I've heard the same thing also. How many miles do you have on yours, and what's the longest distance you've traveled in it? I live in St. Pete, FL, and we're going to make a trip to Pigeon Forge, TN (sp?) and want to make sure it'll make the trip with no issues. Anyway, I'm pretty happy with this thing. Apart from the ****ty gas mileage, it's solid, holds everything we need, and has more than enough power to get around. So do you think piston slap is harmful?
I have about 72,000 miles on mine (bought it new) and I've made a couple of trips from Southern Ca. to Seattle, Washington (1200 miles each way) and numerous trips from Southern Ca. to Phoenix (about 400 miles each way).

From what I've read/heard the piston slap is not harmful. I have no proof though.
If you have some extra time - do a search here on cold carbon knock or piston slap. There is a lot of reading.
 

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Discussion Starter · #10 ·
I have about 72,000 miles on mine (bought it new) and I've made a couple of trips from Southern Ca. to Seattle, Washington (1200 miles each way) and numerous trips from Southern Ca. to Phoenix (about 400 miles each way).

From what I've read/heard the piston slap is not harmful. I have no proof though.
If you have some extra time - do a search here on cold carbon knock or piston slap. There is a lot of reading.
I agree with your opinion. Anything I need to look out for? Just a few days ago, the GF and I went from here to Sarasota, about an hours drive away on the highway, and the TB did great. Temp was anywhere from 180 or so all the way to 210 or a hair above. My worst fear is being in the middle of Georgia somewhere and being stranded. I know the fan clutches and the water pumps are points of focus for most.
 
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