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Fresh air to cat, yes or no? Modified system.

796 Views 4 Replies 4 Participants Last post by  Sully
Hi all,
88 YJ, 258, header, weber, HEI, exhaust air tubes all rotted / fell out while wheeling. New exhaust with header - do I want to remove the cap that is on the new catalytic converter, and supply it with fresh air from a new line under the hood - i.e., fresh air all the time to the cat, or do I want to leave it plugged? The whole system is plugged right now, both the line at the cat, and the one on the header, and both air tubes are removed. She seems to smell a little sulfurry at the exhaust. Would more air help the emissions process?


88YJ,4"susp,33"BFGMTs,9000#winch, hm swing out TC/HL mount ,258,999,4.10,weber32/36,GMHEI.
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I'm not totally sure about how it is on Jeeps, but more air to the cat would allow 1) the cat to get very hot, but it'd be doing it's job better, and 2) completely combust the unburned hydrocarbons, so I'd say, definitely, yes it should help the emissions.


/wwwthreads_images/icons/cool.gif '79 GMC Suburban 4x4 and '85 GMC S15 4x4
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The sulphur smell is bad news. You might be getting unburned fuel to the cat. I think that it would be best to reinstall the air injection system that was once attached to the exhaust manifold in order to burn any fuel in the exhaust completely. Fresh air directly to the cat may not help much. Be sure to install the air system exactly as it was because you don't want to inject air into the exhaust while decelerating. That would cause a backfire and could severly damage the exhaust components. Good Luck.

Not knowing the exact setup you had stock .... didn't either a computer or vacuum diverter switch the fresh air from the exhaust to the cat as "needed"? I got to watch how a GM did this with the computer display hooked up. Depending on whether it was in open loop or closed loop ...... O2 sensor would heat up and cool down and and the computer would switch the air ........ this would be pretty hard to "engineer out" and have the rest of the sytem work right ...... or pretty hard to "rig - in" without the air system and not have some type of "side effects".
Good luck!

The pulse air system for the stock 258 uses check valves(which are chrysler parts now $30) in series with the tubes to ensure the pressure in the cat, tubes, and whole system all remains in the correct direction (no backfires). Here is some links to the page on that describes the cat stuff and you can back up to the index page to get an idea of the other parts like check valves and where the tubes come from.
It also gives an idea of why and how the whole emissions air system works - hence you might see that the air WILL help but you should be cautious that the cat will run hotter and if the smell continues you might have it checked.
Good Luck!

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