FFCam1991

Hey guys, I am currently struggling with horrible fuel economy and an assortment of other problems. One of them is that my cats (91 Formula 305 dual cats) are wickedly noisy. It isn't so much a "rattle" as a constant backfire. All exhaust is stock. Out the tailpipes it has a good deep rumble but at the cats it has a way different sound. Its especially bad when you downshift. Its a higher pitched sputtering, backfiring and rattling all in one! Are my cats blown? Should the exhaust at your cats have a different sound? I didn't think so but never really cared before. Finally, could wrecked cats play a role in fuel economy and performance? Thanks!

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DaveBerlinetta

It's unlikely that both cats went bad at the same time (but stranger things have happened). You didn't say how many miles on the car. Was the car in an accident that my have jarred the cats?

I would also check the heat riser valve. If it came loose from the actuator (rod rusted off) and is moving freely, it makes a hell of a racket.

92RS_Ttop

Bad cats will definitely kill your fuel economy and power, especially if they are plugged. I had a similar problem in my 94 K1500. The stuff inside the cat was all broken up, rattled constantly and I had no power and got horrible mileage(for a truck that is). Easiest thing to do if they aren't rusted out and you don't have to pass the sniffer is to remove them, clean out everything inside and replace them.

FFCam1991

Thanks guys,

Looks like I jumped the gun on the cats being bad. Upon further inspection I realized that the metal tube on the leftmost cat has rusted right off...last time I checked it was still connected but obviously not very securely. I believe this is the AIR tube?

I'm not sure what a heat riser valve is, could you enlighten me? All I see are the two small diameter tubes leading from each cat back up the exhaust manifold.

It is clear that this is the problem. I held the tube up against the bracket as it should be, and the noise almost vanished! What material is this and what kind of welding should be used to hook er back up?

Thanks!

rideon1200

My 87, when I did the exhaust I went back with a hi flow cat, sounded good and gave some HP. Like the previous post, we dont have sniff test, just visual. Then again, if the wanted to see my emissions, I would have had to take in my trash can.

92RS_Ttop

I'm not sure what it's made of, my car didn't have that hose when I got it. What my mechanic did when he inspected the car was use some hose and clamps and some pipe he had laying around to connect it back up. It's been working just fine for the last year.

FFCam1991

That's not a bad idea, but I have a feeling that it would still rattle around if I clamped a hose from one end to the other.

Any ideas on how to weld this?

92RS_Ttop

When he did it, he clamped the tube to the exhaust pipe at 2 or 3 places along its length. I think he used regular hose clamps, but I can't remember for sure.

As far as welding it, since my did Camaro did not have the tube at all when I got it, I don't know what it's made of and if it would be possible to weld it or not. Can you get a pic and post it?

FFCam1991

I'm taking it out to someone tonight who is gonna have a look and see if he can weld it. If the problem isn't solved, I'll try to take a pic and show you guys whats up.

Thanks

shadowknight

if all else fails JB weld it.... like 4 bucks at the zone.

DaveBerlinetta

It's a control valve between the exhaust manifold and the exhaust pipe on the passenger side. It stops the flow of gas and backs it up through the heat riser channel under the intake manifold, out the other side to the left side exhaust manifold, and back out the Y-pipe. It aids in heating the intake charge and speeds engine warm up.
On my '85, the valve is actuated by a vacuum motor (like the ones used in the heater box), controlled by a thermal vacuum switch.
If the valve is stuck open, no big deal. It will just take a bit longer to warm up the engine. If it's stuck closed, it's a restriction in the exhaust, lowering fuel economy and performance. Plus, all the extra heat can cause a bunch of other problems.
There should be an anti-rattle spring to keep it from, well, rattling when open. But it usually rusts off. Most of the time, the valve gets stuck open by the same rust, so we don't realize it's happened.

It sounds like you have your problem figure out with the broken air pipe. I thought it was made of stainless.