Hey guys, been searching but coming up blank. '92 in my sig had two bad cats, so I cut the exhaust off before the first cat, which means I now have dual pipes that extend roughly 1.5" from the o2 sensor. Since then I've had a really rough idle that I'd assume is related, but thought I'd ask those more knowledgeable. Starts and runs fine, if you don't hit the gas. While driving it'll sometimes try to cut out/buck while at cruise, but acceleration is fine. Sometimes coming to a stop the idle will fluctuate and the engine dies, but sometimes it'll idle fine. Sound normal considering the cut pipes? I've got a set of Pacesetter longtubes that'll be going on it and I intend to run true duals w/o cats and Flowmaster Super 44's, though for the time being I'm just clamping the mufflers to the headers until I can fabricate the whole setup. Any issues? I thought maybe the o2 sensor was just getting false readings, as in being so close to the exit maybe it was seeing too much oxygen? If that's the case, can I cheat and just disconnect it for now? Thanks in advance!

 

the o2 sensor at the end of the pipe shouldn't be the cause of your issue, although the o2 sensor wont live long that close to the end of the pipe...


first thing, pull the codes.


second, what were wrong with your cats? clogged?


you could have all sorts of problems that caused the clogged cats in the first place. a bad cylinder, or low compression all around, can send all kinds of gunk down the exhaust and clog cats quickly. 300k miles on the engine?


check codes first, then let the investigation begin.


also, youll be wasting your time with "true duals"

 

Both cats clogged, severely from the original engine which threw a rod at 288k if that says anything about its condition. Replacement has around 70k miles and is in very good shape. Will be checking for codes as soon as my day starts. Curious as to why you say I'm wasting my time with true duals?

 

with duals, you lose the scavenging that is inherent with a y-pipe. that translates into a loss of low-mid range torque. duals should be reserved for applications that spin 6500+ rpm, or even more.


post up codes when you retrieve them.

 

So how long did you drive around with the replacement engine and clog converters?

As far as "true duals" are concerned...why pay for two of everything? I'm with the previous post from '89 on this one. A single exhaust, of sufficient size (and not TOO large), is a proven winner on these heavy trucks. You need torque wherever you can get it.

 

Roughly 7k miles. As far as the torque loss, given my special case here, none. I have a standard trans to take advantage of, if you get the RPM up in first then give it a hefty kick, it hurts. This truck hits the interstates a lot and usually spends a lot of time at high revs, routinely hitting 5 grand. Never used much for actual "truck" things. It was kind of a compromise when I bought it.

 

5 Grand. Your motor makes peak HP at 3500rpms and less HP above that, but whatever floats your boat. Open exhaust like you have will mess with the O2 sensors readings, put the longtubes on already and install the O2 in the passenger side collector, and then if you just gotta have duals don't use anything larger than 2.5" with slightly smaller being ideal. If you leave the O2 disconnected you are crippling the EFI system.. may as well rip it all off and install a carb.