Cracked Exhaust Manifold Affecting FE?
 

I had the "doughnut" gasket between the down pipe and the catalytic convertor replace a few months ago, and very soon thereafter the car developed a similar sounding exhaust leak. Turns out it is the manifold.

That would explain the slight "ticking" sound that it makes, and the brief whiffs of exhaust I catch when I restart the engine.

Could the cracked manifold affect the fuel economy?

I am going to have it fixed - ~$400 in parts and ~2.5 hours of labor if no studs break, etc. This doesn't rise to the level of the cost of new clutch, so I figure it is worth fixing on my ~167,000 mile xA. The clutch and the starter are original and have held up fine.

It never affected my economy, but an exhaust crack DID make my idle go crazy. It would stumble along at 300 rpm and then surge to 2000 rpm. It's a common failure on the 90's Civics, so parts were plentiful and I replaced the whole thing.

In my case, when I had a bad gasket, it did... but I guess it depends on where the crack is and how it affects flow.

Before or after O2 sensor will determine FE effects.

Right - I don't know where the first O₂ sensor is. I have noticed a idle stumble lately - you can feel it shake and the needle drops and it seems like it will stall; but it has recovered, so far.

I just want a quiet(er) car again - with my new aero mods not painted yet, people notice my car, and to have it be rough sounding doesn't help.

In the cars I've been over its either at a common point where all 4 runners meet in the manifold or before the cat converter under the car.

If there's a crack or leak, an effect exists which draws more air in giving lean appearance, as velocity increases more goes out the leak, but volume increases reducing the effect at higher RpMs.

I'd guess your sensor is before cat if you are noticing the change in rpm at idle/low rpm.

Is the manifold cast iron or stainless tubing?

regards
mech

The manifold is a forged steel part. It had two welds that had cracked - #1 and #4, an the flange rim had also rusted away on at least one area. They had to fight tooth and nail to get one bolt out - it was between #1 and #2 under the header tube and down behind the head near the firewall. The hex bolt head was very shallow and it got very hacked up. They managed to cut the washer out and that relieved the pressure, and the bolt finally came out.

I also have a leaking rear muffler (which was overshadowed by the header leak) and I failed the emissions test - because the CPU was "Not Ready" due to the erroneous O2 sensor reading. I'll drive it 100+ miles and it will pass, I think.

I'll find out if my FE gets better. I have to wait until I get the sticker to put my video mirrors back on, but so far the coasting is better than it was before with my beat up skirts and strakes.