I've got a stock 97 Civic EX that has a nasty gain in engine/exhaust noise and vibration from 3500rpm-3800rpm through all gears.

A shop put on a new exhaust manifold to replace a cracked one, but really hosed up my car doing it. I'm still not sure if it was right. It was run for a bit with the O2 sensor bad in it. Is it possible that I could have fouled my cat with all the fuel-rich exhaust going through it? The entire rest of the stock exhaust is less than two years old, so it can't just be that it's an old system resonating badly on its last legs. The cat is the only original part on it besides the downpipe from the manifold. Any ideas?

Also, I've got some strange phantom problem with the power in the car. I'm getting an occasional draw on the system that causes all the lights to flicker in intensity. I can't find anything wrong with it...new battery, headlight bulbs, and plug wires. Nothing fixed it. Alternator tests fine. Any ideas for me, or am I going to have to spend hours and hours with my Hayne's manual checking every ground on the engine.

 

still nothing? I posted this a whole day ago!!

 

I would check that manifold one more time, it might be warped.
With the other prob, if the alternator is fine then the only other thing
i can think of is a ground.
Are you the first owner of this car?
How many Miles are on the clock?

 

I'm the third owner, other two didn't really do anything bad to it, bad mods and stuff like that. 136xxx...

 

I checked through the manifold again, and I can't find any leaks. I could just be missing it as well, especially if it was in the lower downpipe, near where it bolts to the cat. I'm pretty sure that it is the cat going bad though.

I think it's still doing it's job enough to keep from getting a secondary o2 sensor light, but it might be resonating wrong. I have no idea how much gunk could have been built up in there with the fuel that got into it. I might be wrong about this, but I understand that when there was a nasty leak in the exhaust after they replaced the header wrong, the engine had to compensate for the lack of balance in the exhaust readings from the emissions sensors by making the air/fuel ratio more fuel rich. Too much fuel means that it has to come out somewhere. Therefore, it comes out with the exhaust and sits in the cat or elsewhere in the pipe causing damage to the cat and the exhaust in general. Please correct me if I'm wrong about that.

 

I agree, but unfortunately you will need to fork out the big bucks to replace the cat.

 

if your chasing an exaust leak, take an old piece of garden hose, or heater hose, or whatever you have around thats at least 1/4 inch... about 2-3 feet will be plenty.. hold one end NEAR(NOT IN) your ear, with the engine running, move the other end around the engine and exaust.. be carefull of moving parts, hot stuff, and you will find the leak easily, if there is one...

 

....new exhaust.
I dunno man, I don't think I would know.

 

that garden hose idea is clutch! I'll do that next time I'm back home from school...

The entire system is new except for the cat and downpipe. That's why I'm really not wanting to spend the money. I'm hoping that it's just a leak on one of the seals of on an older patch I know about. If it's the cat, then I might as well put on a new aftermarket exhaust and sell the good stock stuff on ebay.

 

If its a rattling vibration noise...I would say check the heat shield on the cat