Exhaust Manifold - '98 TJ of course

A cracked exhaust header should be replaced since it can damage the valves by letting cold air in through the crack.

Jerry

please explain to me the science behind this 'fact'.

Do your own research if you want to argue against what I said that I stand behind...which on the surface sure seems like is you're itching to do.

-- Jerry Bransford To email, remove 'me' from my email address N6TAY, PP-ASEL See the Geezer Jeep at

Yeah ... I'd also like to know , first of all how "cold air", or any air for that matter, could possibly enter a (pressurized by the engine firing) exhaust manifold ... geez .. that's like saying cold air can enter via the tailpipe, make it's way through the exhaust sytem to the exhaust manifold wher it will ... and this I'd really like to understand ... damage "the valves". Now, in my youth I have reduced back pressure on a few autos and run them hard thereby burning exhaust valves ... maybe this is what you are thinking? Or maybe you are thinking that the O2 sensor might somehow be misled by the cracked manifold thereby changing the mixture? Look there is no argument here ... just a question as to how this works ... I am always eager to learn more about the jeep ...

bowgus did pass the time by typing:

Think of it this way. Each time one of the cylinders exhausts it sends a pulse through the pipe. All those pulses merge at the collector and form a flow of exhaust all the way to the tailpipe. Once flowing, exhaust like any fluid has a mass. In other words the flow wants to continue to move in the direction it is moving. This is also why having a huge 3" exhaust pipe can hurt. The flow becomes too slow and you loose any scavenging effects. Larger pipes do not equal more power.

Since the exhaust pulses stop/start they also form a slight vacuum behind the pulse. That is where air can get into the manifold. Air getting in fools the O2 sensor into thinking your running too lean, then bad-things happen.

Sounds wonky doesn't it? :/ Leaks past the collector don't tend to suck much if any air into the system, leaks at the exhaust flange cause the most problems.

I let mine go on WAY too long:

As Jerry mentioned, yes, it can cause damage (especially if it is as bad as mine was). I *think* this is because a crack that far up the exhaust manifold/header (like in the second picture) creates a sort of vacuum leak, allowing too much air into the mixture, and causes excessive heat. Not sure of the specific details on this, but people whose opinions I trust/respect have told me this is 'bad and may cause damage', and I believe them. :) Not sure if I did cause any damage by letting mine deteriorate for so long, but everything *seems* to be fine. If your crack is further down the pipes (like in the first picture), closer to the collector area where all the pipes converge, I doubt you have much to worry about hurting anything at this time.

As for replacement, I found a Thorley header at a price too good to pass up (I was orig>Other than noise, is there any harm in letting this go unfixed for an