Air-By-Pass valve?
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Cargo Master
Joined: Dec 2003
Posts: 2,735
Likes: 38
From: Sunny Florida
Air-By-Pass valve?
I have what sounds like an exhaust leak and I can only hear it when the truck warms up and it has been driven on the highway. Once the leak sound appears I can hear it every time I take off and while I am driving. If I shut the truck off and start it later the sound will be gone until I hit the highway again.
Today I stopped by my buddy's house and had him get under the hood while I gave the motor some gas and he showed me where he thinks the leak is coming from which was the Air-By-Pass valve.
Does this valve go bad and if it does will it make a exhaust leak sound? How can one tetst it? thanks
Today I stopped by my buddy's house and had him get under the hood while I gave the motor some gas and he showed me where he thinks the leak is coming from which was the Air-By-Pass valve.
Does this valve go bad and if it does will it make a exhaust leak sound? How can one tetst it? thanks
Laughing Gas
Joined: Mar 2003
Posts: 877
Likes: 1
From: Colorado, U.S.A!
Did this just start or has it always been like that? Year-model-engine?
The valve on my 89 makes some noise, but I can't hear it while driving. It has a hose that goes to the aircleaner to dump bypass air. If I disconnect that hose, it's louder.
As far as testing, (if yours is like mine) check the vacuum diaphram for leakage and when vacuum is applied, air should come out one place, when vacuum removed, it should come out the other. I learned that by using the air pump to inflate air mattresses! Works good.
Good luck.
The valve on my 89 makes some noise, but I can't hear it while driving. It has a hose that goes to the aircleaner to dump bypass air. If I disconnect that hose, it's louder.
As far as testing, (if yours is like mine) check the vacuum diaphram for leakage and when vacuum is applied, air should come out one place, when vacuum removed, it should come out the other. I learned that by using the air pump to inflate air mattresses! Works good.
Good luck.
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
Are we talking about the idle air bypass valve or the valve on the smog pump? The smog pump will definitely make a noise when it's venting to the atmosphere. Mine does that, but usually mine is more noticeable on cold starts, which is opposite of your condition...hmmm
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Cargo Master
Joined: Dec 2003
Posts: 2,735
Likes: 38
From: Sunny Florida
First off the truck is an 89 centurion with a 351W and I think EPNCS2006 that you have helped me out before so you may recall some earlier problems I had.
I may have the wrong description of the part as well. This is not the valve on the pipe that feeds the air to the back of the heads. This valve is before that. It's a black plastic valve with a yellow vacuum line and it is behind the intake by the PCV valve.
As far as this just starting, yes it just started a few weeks back. At one point when working on the truck last year I thought I could hear a leak from that area when I was under the hood but could never find the leak and it went away. It also never sounded like a real exhaust leak like it does now and again it only will leak or sound like it if the engine load is enough but once I get it to start it will stay with the truck until I shut it off. If the truck is in park and I get out and rev the motor I can't hear at all. There has to be a load for it to make the leaking sound.
I may have the wrong description of the part as well. This is not the valve on the pipe that feeds the air to the back of the heads. This valve is before that. It's a black plastic valve with a yellow vacuum line and it is behind the intake by the PCV valve.
As far as this just starting, yes it just started a few weeks back. At one point when working on the truck last year I thought I could hear a leak from that area when I was under the hood but could never find the leak and it went away. It also never sounded like a real exhaust leak like it does now and again it only will leak or sound like it if the engine load is enough but once I get it to start it will stay with the truck until I shut it off. If the truck is in park and I get out and rev the motor I can't hear at all. There has to be a load for it to make the leaking sound.
Laughing Gas
Joined: Mar 2003
Posts: 877
Likes: 1
From: Colorado, U.S.A!
Yeah, mine has that, I think it's the one that causes the air pump air to go either to the heads or the catalytic converter depending on engine load. I think it's the air management valve. You can check it by blowing into it and applying/removing vacuum. BTW, the check valve on the pipe that feeds the heads was bad on mine recently. I took it off and knocked most of the crud out then cleaned it with carb cleaner. It's fine, now. The one that goes to the convertor should be checked, too.
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