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Old 05-07-2011, 06:14 PM   #1
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chevy 350 valve covers off, "sneezing" noise coming from engine

Hey guys. My brother recently got his 350 back together and fired it up to break in the cam. Ran it for about 10-15 mins at 2400 rpm and shut it down. Valves were a little noisy so we re-adjusted them and fired it up again afterwards. It was running nice and smooth at this point. Started to adjust timing and suddenly a loud noise was coming from the pass side valve cover. We took the cover off and unplugged the distributor so we could crank it over. All rockers are still adjusted fine yet it is making a "sneezing" sound only when cylinder 6 is on the compression stroke. I think it might be releasing comression from somewhere but I have never heard this before. Anyone have any ideas?

Motor specs are 350, procomp heads, 10.7:1 compression, howards 234/244 cam, comp pushrods and roller tip rockers. I'm kinda stumped at this point!
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Old 05-07-2011, 06:22 PM   #2
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Re: chevy 350 valve covers off, "sneezing" noise coming from engine

Yeah it's allergy season. My engine was doing that too till I replaced the air filter.



Sorry couldn't resist! Check the plug on that cylinder, might be a little loose. Try a new plug, old one may have cracks that release compression.
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Old 05-07-2011, 06:34 PM   #3
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Re: chevy 350 valve covers off, "sneezing" noise coming from engine

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Originally Posted by oamhmad View Post
Yeah it's allergy season. My engine was doing that too till I replaced the air filter.



Sorry couldn't resist! Check the plug on that cylinder, might be a little loose. Try a new plug, old one may have cracks that release compression.
Plug was tight and they are brand new. Held my fingers by the plug and I can't feel any compression escaping through there.
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Old 05-07-2011, 06:49 PM   #4
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Re: chevy 350 valve covers off, "sneezing" noise coming from engine

Sneezing like a sudden release of air pressure? Does it happen at the same point in the stroke every time?
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Old 05-07-2011, 08:07 PM   #5
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Re: chevy 350 valve covers off, "sneezing" noise coming from engine

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Sneezing like a sudden release of air pressure? Does it happen at the same point in the stroke every time?
Yes thats exactly what it sounds like, and it does it on que at the same point every rotation as it's turning over. Didn't pay attention to where the valves are as it makes the noise but I want to say it was when the intake valve started to move on cylinder 6.

EDIT: I pulled the plug out of cylinder 6 and it still makes the noise so it may not be cylinder 6 that is the cause. Tomorrow I can try to pull a few more plugs one at a time and see if the noise goes away on a specific cylinder. They are tough SOB's to get to though with hooker longtubes!
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Old 05-08-2011, 09:37 PM   #6
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Re: chevy 350 valve covers off, "sneezing" noise coming from engine

Just to let you guys know, we found the problem. Felt above the carb and found out it was puffing compression through there. Originally it sounded like it was coming from the pass side but we pulled the drivers side valve cover off and found that a rocker arm had fallen off a spring and was hung up. This is the 2nd time it's happened so I think the locknuts are junk. We tightened the rocker back down and the compression noise went away. My brother will be ordering nice brand new roller rockers to put on since the old ones were only roller tip and he got them used. Anyways, thanks for the help guys.
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