Exhaust Leak...cracked manifold?
 

I bought this '89 Firebird in October and when I test drove it, I could hear the exhaust leak. The PO could not find it. I thought, no big deal, I'll find it. Wrong...I can't find it either. It only does it under acceleration. It won't do it while idling and disappears altogether at highway speed. It won't even do it while holding the brake in drive and revving. It's driving me nuts. I've searched and searched and studied the whole exhaust and can't find anything. During the first month I ruled out the manifold gaskets knowing that if it was indeed one, I could burn a valve. Obviously they are not because I'm pretty sure it would have burnt a valve by now and the PO said it's been doing it for over a year before I bought it. I found one bolt holding the air tube onto the smog pump with the tube loose. Thought that was my exhaust leak sound, but after getting two new bolts and gasket, still had the same sound. Found a broken check valve on driver's side in the threads and thought....there it is. NOPE...fixed that and still makes the same sound. Those two problems weren't even making sounds apparently.

The other day, someone mentioned that it's possible that the manifold may be cracked under the heat shield which is why I can't find it. It definitely sounds like this leak is coming from the driver's side and if it is under the heat shield, that would explain why it hasn't burnt a valve I guess. I've never heard of a manifold cracking and I've never had it happen. Is this something that is common? Where else should I look for a leak like this that only appears under load? 89 Firebird 305 TBI. Thanks.

Re: Exhaust Leak...cracked manifold?
 

power brake the engine some and have a buddy help you find the leak. Cast iron manifolds can warp and crack although I have mostly have seen this in motorhomes and truck that tow alot and the passenger side manifold.

Re: Exhaust Leak...cracked manifold?
 

I bet its a manifold gasket leak. All to common. Normally, exhaust manifold gasket leaks wont burn valves. I would also check that the gaskets/connections where the y-pipe attaches to the manifolds. My iroc did the same thing yours is doing and it turned out to be the manifold gaskets. A leaking manifold gasket would be much more likely than a cracked manifold. Good idea to change them if they've never been changed.

Re: Exhaust Leak...cracked manifold?
 

Maybe I missed this in your post, but...

How much did it improve when you tightened all the bolts?

Re: Exhaust Leak...cracked manifold?
 

I found that my drivers side exhaust manifold had 2 broken bolts. It was the rear two and it was hard to see.

Re: Exhaust Leak...cracked manifold?
 

Yea, I didn't give great detail on my "checks." The bolts didn't tighten much, but I was able to crank on them a touch. I backed them all the way out, pulled back each manifold and noticed the gaskets were fairly new looking which made sense since the engine was replaced about 2 years ago according to the PO, however no paperwork to prove, but it doesn't smoke or have the tell-tale signs of a high mile thirdgen engine that I'm used to, so I buy it. Put the manifolds back on, tightened them down and no improvement whatsoever. Still had the exact same sound. The sound hasn't gotten any worse and has been consistent since I've had it. That's why I think it's a cracked manifold, but I could be wrong. I think it's under the heat shield, but I'll have to remove all the air tubes and the manifolds completely to get that off. I've toyed with the air tubes and it doesn't look like they are going to unscrew very easily if at all.

Re: Exhaust Leak...cracked manifold?
 

There's not supposed to be any "gaskets".

If you've got anything between the manifolds and the heads, take it out, and see where that takes you.

Re: Exhaust Leak...cracked manifold?
 

Also that burning valve is an old wivestale. Sure it's not good to run open manifolds, but your not going to burn a valve unless you take it out and hold a tourch to it LOL! Sounds like someone intalled gaskets where they do not belong or the manifold is indeed cracked-not always easy to see. Also make sure the Y-pipe to manifold flang bolts are sung.

Re: Exhaust Leak...cracked manifold?
 

No gaskets? Interesting. Every thirdgen I've owned (6 total) all had gaskets between the manifold and head. Although, I've never owned a brand new thirdgen, so I guess it's possible that at some point owners install gaskets. At any rate, I solved the problem. It turns out, I should have just replaced those "gaskets" the first time I had it apart. The things only cost 10.00, so I figured what the heck. I took out the old ones which looked pretty much like my new ones (maybe a tad older), put the new ones in, tighted it all back up and now the sound is gone. I threw the old ones away even though looking at them, there's nothing wrong with them, but for some reason part of or one of them was not sealing since brand new ones solved the problem.

I did clean both the manifold and head surfaces before putting the new ones on and noticed a little bit of crud/rust coming off. Maybe the previous installer didn't do this before putting the new ones on preventing one from sealing good. I don't know, but it's quiet as can be now and I'm glad that annoying sound is gone! Thanks for the tips.

Re: Exhaust Leak...cracked manifold?
 

Gald the issue is fixed, Exhaust gaskets are funny like that-they can look perfect, but still leak.

Re: Exhaust Leak...cracked manifold?
 

Doesn't mean the factory put them there.

There's no gaskets in there from the factory, no matter what wrinkles are in the sheet metal. Doesn't matter if it's a 3rd gen, truck, full-size car, G-body, or whatever; the factory doesn't use gaskets. The reason is, exactly what you're going through right now... they leak.