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Go Back   Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum > 2002 - 2009 TrailBlazer/Envoy Tech > 02-09 Vortec 4.2L I-6 Engine and Drivetrain > OEM Issues

OEM Issues Original Equipment Repairs and/or Problems

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  #1  
Old 05-20-2008, 08:11 AM
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05LT 05LT is offline
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Normal Engine Noise?

GM did their homework. I think the 4.2L I-6 is a good match for the Trailblazer/Envoy – plenty of pep with acceptable fuel consumption. One of the first things I noticed on the test drive was the powerful but yet smooth and quiet engine – even at idle. I was impressed. Despite this, it was shortly after my purchase when I noticed an engine noise. I’m not sure what it is. I’m not familiar with the engine design of the 4.2L I-6 as I have mainly been around OHV (pushrod type) engines, but it sounds like a slight taping or ticking noise coming from the engine. The sound is not very loud so it’s not immediately noticeable; although, it does seem to be most noticeable during warm up, but once the engine reaches operating temperature the sound becomes almost undetectable. Call me crazy, but during the first couple minutes of driving it almost sounds like a (very quiet) diesel engine – best way I can describe it. Can anyone tell me the exact source and cause of this particular noise? Has anyone else noticed a similar noise? Is this considered “normal?” Fluid levels are good. Dealer performed an oil/filter change just before I took delivery. Should I be concerned?
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  #2  
Old 05-20-2008, 12:08 PM
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sounds like 2 possible causes.......
a cold Inline 6 may produce the ticking/knocking sound when cold.
it may be related to piston slap......... the other is Cold carbon knock.
I get cold carbon knock after lets say maybe 10,000 miles of use or so......
this can be quieted down by running a bottle of techron to get rid of that carbon build up. It helps mine quiet down.

Once the engine is warm- the I6 purrs and makes very little noise.
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Old 05-20-2008, 01:24 PM
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Most likely it is the fan clutch. The design (up until about '07 models) had a system for opening and closing a valve to allow the silicone based fluid to move around. The ticking is the valve opening & closing when cold.

As you noticed, it goes away when the engine warms up.
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Old 05-20-2008, 05:21 PM
simoniz simoniz is offline
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Hey guys, How much is a clutch fan? I have a 02 TB with 73k on it. I just bought it nd when the motor is cold it sounds like a 15 years old motor but once it's warms up... the motor sounds fine. Does this sound like a clutch fan?

Thanks for your input.

Mark
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Old 05-20-2008, 08:59 PM
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My 02 does the noise thing on cold start. It is piston slap and goes away when there is heat in the motor. The ticking noise is most likely the fan clutch. Start the engine cold and open the hood. Listen for the ticking. Looking from the front, there is a weathertight plug on the right side of the fan housing cowl. Unplug it and the ticking should stop. BE CAREFUL, the fan is very close to the plug. Mine started ticking 6 months before the fan clutch died.
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Old 05-21-2008, 12:16 AM
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the ticking fan clutch and the piston slap or cold carbon knock are different tick sounds.

the fan clutch- you can leave the hood closed and crouch next to the front drivers wheelwell area... and you will hear a very distinct ticking sound-

the piston slap can be heard inside the vehicle- while the fan clutch- I have to open window or door and listen carefully....

hope that helps
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Old 05-21-2008, 06:16 PM
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Well I can definitely hear the noise I described from inside the vehicle – doors and windows closed. Is this a bad thing? Piston slap doesn’t sound like a good thing! Should I take my Trailblazer to the dealership? What should I tell them?
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Old 05-21-2008, 07:04 PM
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in severe cases- the V8's had some work done on them.
but I have never heard of a I6 being repaired for piston slap.
they will most likely tell you its normal.
Many post on other websites say its not going to damage the engine.
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Old 05-21-2008, 07:15 PM
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Ok. What exactly is piston slap? What causes this to happen? Is it just simply a design flaw? Will this cause any type of reliability issues later down the road?

Also, what is carbon knock?

Thanks.
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Old 05-22-2008, 12:45 AM
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piston slap in short is caused by a piston that doesnt exactly fit in the combustion chamber. so it rattles and knocks as it moves up and down. as it heats up and expands and the movement stops.

cold carbon knock is built up carbon on the piston....


carbon buildup- in older cars- would cause run on. meaning that when the engine was turned off....there was enough HOT carbon in the piston to continues to ignite fuel and the engine would run on... and then sputter to a stop...

cold carbon knock- - I cant find a definition of it at the moment....
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