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2003 chevy trailblazer_lt
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Discussion Starter · #1 · (Edited)
I am new to this site and hoping someone can help.
I have a 2003 trailblazer 4.2L I6 with an upper engine knock. The noise is coming from the upper front of the engine.
Have ruled out the accessories on the front of the engine by running the engine without the serpentine belt, noise still there. Sounds like something with the valve train as the knock is going at half the rpm of the crank. With a piece of garden hose the noise is loudest at the front of the engine on top of the valve cover and up against the the part of the valve cover that goes over the timing gears. And on the side of the exhaust cam (passenger side of the engine). Also can hear the knock through the oil fill hole with the cap off.
The knock is most apparent when Idling warm and cold-- and at higher rpms when cold, when warms up you cant here it. It sounds like the familiar collapsed lifter tick but a bit deeper in sound. The noise goes away after 30 seconds when I plug the vent hole on top of the valve cover ?
Have youtube video that shows knock going away when plugging the vent hole if anyone wants to view it. No engine lights on either.
Any Ideas, Thanks
 

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2003 chevy trailblazer_ltz
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I've also posted this exact same issue with a link to the youtube video, I just found your Trailvoy post. I see your post is over a year old, did you find a solution? My knock has been there for a couple of years but would really like to fix it.
 

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2003 chevy trailblazer_lt
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Discussion Starter · #3 ·
TrialBlazer knock

Hi Red Dirt 4 x 4,

Thanks for you reply, no I haven't figured it out. Still have the truck and it runs well just has that annoying knock and it doesn't seem to be getting worse.
I have several older cars and anytime I have a problem and check the web sites I usually find loads of info on the exact problem, but not with this one. You are the only one that has responded and has the same issue.
I changed the air filter the other day and noticed its not dirty at all and its been in there for awhile, up to a year. Maybe 8,000 miles on it ?
Leads me to believe there may be an air leak in the air cleaner housing, air intake tubing or the black box on top of the engine. In my post I stated that when I plug the vacuum intake pipe coming out of the top of the valve cover the knock goes away. Could be not enough build up of vacuum in the crank case and maybe caused by a leak in the air cleaner housing/pipes/black box etc. When it warm up here I'm going to take a close look at all the plastic stuff leading to the throttle body. Please let me know if you find anything on your side. Thanks Jerry
 
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