Thanks for the help MK378. I will inspect it like you said. I haven't researched it much, but looking down at the header area, it looks like I just need to disconnect the O2 sensor and remove the heat shield and look around as well as I can from above and below the car... sound about right?

So the header definitely has a 3" crack right next to the O2 sensor, as seen in the picture below. So two questions at this point.

1) Could this be causing the symptoms I've been having? (For instance, very poor gas mileage especially in city driving, poor acceleration, occasional exhaust smell in cabin, slight sputter when shutting the car off.)

2) What is the best option to repair this, besides buying an OEM replacement header? I have read a little bit about complications using aftermarket parts, such as troubles with the length of lower O2 sensor wire reaching the replacement catalytic converter. If I understand correctly, the OEM header has a built in cat...

Any thoughts would be awesome. I hope this is the solution to some of my problems, particularly the gas mpg.

Attachment 5562

I keep finding articles that suggest getting the existing header welded (which often fails), buying aftermarket parts (which usually fails emission tests), or buying OEM (which costs an arm and a leg). Can anyone weigh in on this?

Edit: Btw, I read this thread: https://www.hondacivicforum.com/foru...version-73237/

While very helpful, the suggested items add up a cost that is well over OEM pricing. Ugh... :(

So, apologies for being a pain in the ass here. I called around to see if anyone would weld this thing. No local welders wanted to do it, but referred me to one place that would. When I called them, they said, "oh yeah... it's the exhaust manifold with the catalytic converter attached. I know of it. We have one here that we repaired and the guy never picked it up... been sitting here for years. You can have it for $175." The guy has no idea how many miles were put on it, and said he'd charge even more to fix mine. So, I don't know if I should buy the repaired piece, or try my best to piece together an LX to EX conversion through eBay for a similar price.

I guess the tough thing for me is I don't know what fixing this thing is going to do for my car performance wise. I don't care about racing the thing... it's all stock. I'd just like to get it running a little closer to stock performance. From what I understand, a crack like the one seen in the picture above could throw off the O2 sensor and mess with the amount of fuel being injected into the engine, causing it to run rich. As I mentioned earlier, it's not throwing a CEL, but the car does seem to run crappy nonetheless. So, is the repair likely worth the money? Which route would you guys go if not trying to build a race car?

I hope this doesn't sound too redneck but has anyone used a super tough epoxy like JB Weld or Marine Tex to fix a crack?.

My son's 98 Civic had a crack where you have your crack. That crack was repaired by welding. My welder used welding rod for cast iron. This was last summer. I think it is still holding. I don't remember exactly how much I paid for this repair, but it was less than $200USD.

There is no performance loss or gain with this repair.

I would talk to the guy at your shop making sure the work was done properly with cast iron rod for the repair. Also, see if he can offer you some warranty.

Hey, thanks for the replies. I tried JB Weld for a quick patch to keep the fumes out of the car. Even though I let it cure for about 16 hours, the stuff went up in smoke and filled the car with a hell of a smell... I don't recommend it!

I am settling for a Dorman exhaust manifold replacement. I'm hoping this thing pans out well. I was kind of hoping this would be related to my mpg, but as you guys are saying, this wouldn't affect mpg. I've tried almost everything I can think of. The only thing I'm left wondering about is the O2 sensor. The previous owner spliced the upper O2 sensor for some random reason. I wonder if this could be causing problems. I don't have a CEL, so maybe I'll just have to settle with 20 mpg. Meh @ D16Y7.

20 mpg is awful. Get that manifold fixed, reset the ECU, and check mpg again. If still bad, make up a thread in the fuel econ section.

hey, thanks cvcrcr99. When I put the new manifold on, how do I reset the ECU? Just leave the battery unplugged?

Edit: nevermind, I used the power of Google to figure it out. (From what I see, drive car til its warmed up; turn car off, disconnect battery or pull FI ECU fuse out; put fuse back, start car, close hood; make sure all accessories are off, do not press any gas or brake; just let the car idle for 10 mins by itself; turn car off, then back on, and go for a drive like you normally do. it will take usually one tank of gas to completely relearn the fuel trims... as quoted from another site.)

Thanks all.

Hey, so I swapped out the manifold and a new upstream O2 sensor (Denso 234-4099). I unplugged the battery, let it sit for a few to clear the ECU, and then started it and let it run for ten minutes. The idle is now getting so low that the car shakes. Also, when I put the new manifold on, a little amount of steam / smoke came off it for the first few minutes as it heated, though there is a sticker on the thing... Any thoughts?