Reason to fix exhaust leak?

Are there any particular resons to fix a mild exhaust leak at the manifold besides for the noise? I have a cracked manifold on my 96 and it's not really worth the work to fix right now. It is a little noisy when cold, but quiets down when warm. I know going to a aftermarket header isn't much of a performance improvement, so I am not too excited about changing it.

Your gas mileage will go up when it's fixed.

A crack will suck in fresh O2, which fools the O2 sensor because it'll think your engine is running lean, so it'll dump more fuel than is needed, thus mileage will suffer.

mattbred said:

Your gas mileage will go up when it's fixed.

A crack will suck in fresh O2, which fools the O2 sensor because it'll think your engine is running lean, so it'll dump more fuel than is needed, thus mileage will suffer.

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x2 my header was cracked on my xj, I thought I could get away with it, but I suffered an avg. of about 20 less miles on the tank...significant enough for me with the price of fuel these days.

I ended up buying new gaskets (minimal cost), and jb welded where the hairline was which has held up fine. I DON'T KNOW IF THERE ARE ANY BAD SIDE EFFECTS OF USING JB WELD. If there are I haven't experienced any...but I'd get a second opinion.

HTH,
Ed

Thanks for the replies. My mileage has been pretty bad, but I attributed it to the lift and tires I put on. The cracked happened around the same time, probably from running through some puddles I shouldn't have and giving the manifold a cold bath. I was thinking about JB Welding it, as it is a hairline further down on the tube and not at the head or gaskets. I didn't know if the JB would withstand the heat, but I guess it's worth a shot.

Like Mattbred said--also, rich running will eventually kill your cat.

You'll fail your smog test.

I don't believe it.....oxygen will not enter a cracked exhaust manifold when the engine is running, because the gas pressure from the exhaust is higher than the air pressure. It just isn't going to happen. My 1990 has a small crack in the exhaust manifold, and I have not noticed a difference in gas consumption, as measured during highway driving.

It will also melt your motor mounts causing them to go bad prematurely.

x2 plus you'll have lower underhood temps, and you wont fall asleep at the wheel from CO poisoning.

1993 Red XJ said:

It will also melt your motor mounts causing them to go bad prematurely.

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Really?? Not intending to doubt you, just surprised and curious.
Ed

If it gets bad enough you will loose manifold pressure and not have enough back pressure on the valves to not burn one out

I would "assume" it would have an effect on the motor mount since the exhaust heat will basically blow right on the mount depending on where the crack is...

To clear something up also, you WILL notice a difference if you get a header for your 96. I think you may have been mistaking what you have been reading and applying it to your Jeep. The RENIX years (87-90) have a poor flowing head so getting a header for them is a waste. Your 96 has the best head (casting 7120 i believe from 91-96).

I have a 90, and had some pretty knarly cracks in my manifold. When i replaced it i also replaced motor mounts. The motor mount on mine was broken so when i took it off, the "cylinder" that the bolt goes through was completly seperated from the rubber. Im not sure exactly if that was caused by the cracks or not... sometimes the motor mounts going bad are what cause the manifold to crack in the first place. My gas mileage is better now, and temps are much more stable. I dont have smog here in Oregon, but if you have smog tests.. you may also fail because of this also.

Somebody once talk about the "venturi effect" about oxygen still getting in through the cracks, even when the motor is running.

It's worth looking into a replacement, or getting your current header welded up by a good welder.

since you are in CA you will fail the smog test during inspection.

depending on how long ago it got cracked, it will suck in fresh air etc etc. so check your cat as well.

mattbred said:

Your gas mileage will go up when it's fixed.

A crack will suck in fresh O2, which fools the O2 sensor because it'll think your engine is running lean, so it'll dump more fuel than is needed, thus mileage will suffer.

Click to expand...

Your absolutely correct. I have actually monitored short term fuel trim, air/fuel ratio and graphed front/rear o2's on OBDII vehicles before and after a cracked manifold repair. The un-metered air will enter the manifold and cause bank 1/sensor 1 to read lean, which will cause an over-compensation in fuel trim. This condition could not only make fuel mileage suffer, but also foul plugs, damage the o2s and eventually the cats. It's best to take care of it before it snowballs and causes other issues.