Strong smelling exhaust

[Brian83]

I've been chasing low MPG issues and believe I've hopefully gotten that fixed. But I'm still dealing with very strong smelling exhaust. Not like rotten eggs just very strong. Could a bad cat be the cause behind this? I believe I was running rich for awhile due to a broken coolant temp sensor. The cat looks like it was either been replaced or cut out, gutted and welded back in. It looks like someone took a hammer to it. Any way I'm at a loss and before I order a new exhaust pipe/convertor thought I'd ask you guys. Here are some pics of the data stream if you see anything weird.

 

[CharlesHS]

With your upstream O2 sensor reading 0.18 volts it indicates a somewhat lean condition and with the downstream O2 sensor reading 0.84 volts this indicates the catalytic converter isn't doing much. When the cat heats up and is working properly the voltage is supposed to be around 0.45 volts.

 

[Chris]

When is the last time you changed your O2 sensors? What about your cats?

I'm assuming both are original?

 

[CharlesHS]

When is the last time you changed your O2 sensors? What about your cats?

I'm assuming both are original?

Exactly what I was thinking....

 

[Chris]

Exactly what I was thinking....

Yep. I think a lot of people forget that these are general wear-and-tear maintenance items that need to be replaced on a schedule, regardless of whether they are throwing codes or not.

My cats failed on my previous TJ, and the symptoms were almost identical to the OPs, of course then mine became sluggish and driving poorly. I also replaced the O2 sensors at the same time, since it would be silly not to.

 

[Brian83]

O2 sensors are brand new NTK units.

 

[Brian83]

From the physical appearance of the cat I believe it may have been gutted and welded back in place

 

[CharlesHS]

From the physical appearance of the cat I believe it may have been gutted and welded back in place

I would say its time to have you cat tested, but with readings like that I would expect the cat to be trashed.

 

[Brian83]

Ordered new head pipe/cat assembly from RockAuto. Luckily as a 99 I only have the one downstream cat. Still surprised I didnt have a DTC for the CAT. I wonder what the threshold is for setting a code.

 

[Brian83]

When is the last time you changed your O2 sensors? What about your cats?

So I pulled my exhaust tonight in preparation of the new pieces showing up Friday. Cat is completely empty. Just a shell. Can't believe it wasn't setting codes.

 

[Chris]

So I pulled my exhaust tonight in preparation of the new pieces showing up Friday. Cat is completely empty. Just a shell. Can't believe it wasn't setting codes.

Well there you go, now you've found your issue. I am surprised it wasn't throwing codes as well!

 

[CharlesHS]

Good to hear you confirmed the cat was actually the problem although with all indications and readings; i would have been surprised if you found that it wasn't.

Apparently there is just enough of a difference in readings between the upstream and downstream O2 sensor otherwise there would have been a DTC.

 

[Brian83]

Good to hear you confirmed the cat was actually the problem although with all indications and readings; i would have been surprised if you found that it wasn't.

Apparently there is just enough of a difference in readings between the upstream and downstream O2 sensor otherwise there would have been a DTC.

So, now with the new Cat installed my rear O2 sensor is all over the place. Ranging from .15 to .75 and everything in between. It's not following the front O2 sensor. My first thought is I have a faulty convertor. I talked to the guy at the local jeep/off road shop and he thinks I may not have enough restriction in my exhaust. I'm running a straight through muffler dumped infront of the rear axle. Any thoughts? Also I've read in a few places that the early JTEC engines used the rear 02 sensor for fuel mapping? Any idea on that?

 

[CharlesHS]

What size exhaust system are you using...; the stock exhaust is around 2.25".
The larger exhaust piping allows the exhaust to flow out too quickly which will also lower torque the engine produces at lower RPM's.
As far as I know... the rear O2 sensor is just used as a comparison for the PCM to ensure the cat is doing its job...

 

[Goatman]

How many miles on the cat? It can take a few hours up to temp to cook off the assembly coatings