I originally had my cats replace and decided to go with magnaflow high flow cats and magnaflow muffler 2.25 dual and dual out due to my car smelling like sulfur if I pushed the car. After the exhaust was done the check engine light came on because on of the welds near the o2 sensor was leaking, took it back got it fix no problem. My check engine light just came back on and when I gun the gas pedal I feel a little hesitation then it goes but smells like sulfur after wards, sometimes i can be cruising and it will smell like sulfur. Any thoughts? I only use Chevron Premium Octane gas, and The car has 93k miles on it. When I pulled the codes before the exhaust leak was fixed it was saying something about the o2 sensor, and The egr flow. Any help would be greatly appreciated.

Have you pulled the codes after the exhaust job? Did you replace the O2 sensor?

I doubt the original cats where ever bad, that smell means they are working but the fuel/air mixture is off.

Two new O2s will take care of that. If you have a 96 to 98 there are four, only the two forward ones are needing replacement.

As for the hesitation, that could be O2 related but it could be a couple other things like wires, coil packs, etc...

Posting your year REALLY helps!

i have the same issue my 94 mark viii have 156K and all the original exhaust but smells like rottin eggs when i gun it

Same, on the 94 here.


I was running up I25 here in the springs, and got on it to run around a Semi before an exit, and got to smellin sulfur. O2 Sensor time.

mine too. only at wot and most noticibly in od in high speed runs? ive never smelled it at lower speeds. wierd.

its a 97 gen 2.

I doubt the original cats where ever bad, that smell means they are working but the fuel/air mixture is off.

Two new O2s will take care of that. If you have a 96 to 98 there are four, only the two forward ones are needing replacement.

As for the hesitation, that could be O2 related but it could be a couple other things like wires, coil packs, etc...

Posting your year REALLY helps!

Muffler bearings.

haha, you found the website too.

Muffler bearings.

its a 97 gen 2.

The you have 4. The rears just make sure the cats are working right. The fronts are the ones used to check emissions and correct the fuel/air mixture. If they go bad they send the wrong info, the mixture is off and you get the smell at the cats work over-time!

The you have 4. The rears just make sure the cats are working right. The fronts are the ones used to check emissions and correct the fuel/air mixture. If they go bad they send the wrong info, the mixture is off and you get the smell at the cats work over-time!


I will get the codes and replace those first. Thanks for the help.

my friends notice it when there in my car but i do not. I have no codes - so how do I know my 02's are bad? I got a 95 base, i also only had people notice it while my exhaust was off - well its off after the midpipe after the third cat. it is only after extended WOT.

i thought when o2 sensors will go bad youll get a CEL?

They can go bad enough to not throw a code. Eventually a code may pop up, but sometimes only for a short time at WOT. Basically the ECU realizes something is wrong at WOT when the O2s send what could only be bad info to adjust more than the ECU is programmed to do.

They can go bad enough to not throw a code. Eventually a code may pop up, but sometimes only for a short time at WOT. Basically the ECU realizes something is wrong at WOT when the O2s send what could only be bad info to adjust more than the ECU is programmed to do.

Thats what exactly happens to me. Today I raced a (he wanted to, not me) souped up civic (complete with soda can muffler AND sponsorship decals) and at about 120 (I wanted to embarass him, so I pretended that I couldn't catch him and let him gain 4-6 car lengths on me) and WOT it spits up the check engine, after I get back down to about 60 mph it goes away, its been doing this for months. I'm looking at 02s now? Oh well, something else to add to the list. I'm taking a whole week of vacation time from work and plan on correcting everything on it.

Yup, sounds like O2s to me. Replacing them really make a BIG difference if they are bad. Your butt-o-meter and gas tank will thank you.

well i've never got an cel ever... i dont wanna run into one of those things were it aint broke don't fix it. if i hook my car up to one of those autozone things to check for codes, and i dont have a CEL light up, will i notice anything? or does my chip limit the cel/codes from appearing?

They can go bad enough to not throw a code. Eventually a code may pop up, but sometimes only for a short time at WOT. Basically the ECU realizes something is wrong at WOT when the O2s send what could only be bad info to adjust more than the ECU is programmed to do.

so as soon as you start smelling the sulfer smell, its your cats working over time because the fuel mixture is off. and its automatically the 02 sensor that usually is the culprate.

i know if you have bad 02 sensor, you can melt your cats pretty hard if you dont replace them, but no check engine... ? maybe its just stinky car because my exhaust is off after the midpipe? i think an A/F ratio and Fuel Pressure guage might be nice. Anyone got some guages in there car? where to put them?

Is it a good idea to change my o2's.. even if thier bad? I mean.. are they really expensive? Because sometimes I will smell sulfur.. not all the time.. just every once in a great moon.. so i'm guessing from reading this.. just go ahead and replace?

Edit: Okay.. I just read Abraham's post.. my car was only being smelly when my plugs were fouled.. I haven't smelt is since.. but maybe replacing them... will get my gas miles back.. every since I gapped my plugs .048 like OnebadMk said to do.. I've only been getting about 18 miles to a gallon.. weird..

Is it a good idea to change my o2's.. even if thier bad? I mean.. are they really expensive? Because sometimes I will smell sulfur.. not all the time.. just every once in a great moon.. so i'm guessing from reading this.. just go ahead and replace?

Edit: Okay.. I just read Abraham's post.. my car was only being smelly when my plugs were fouled.. I haven't smelt is since.. but maybe replacing them... will get my gas miles back.. every since I gapped my plugs .048 like OnebadMk said to do.. I've only been getting about 18 miles to a gallon.. weird..


im planning to do the 160 degree T stat and two stage colder plugs and some nology wires. if that doesnt do it, well then ill get into the 02 sensor.

about your plugs, something must have fouled up your plugs? that could be running to rich because of (tadaaaa) bad 02's

Quite possibly.. or they could have fouled because of my inability to read.. i was puttting supreme in.. thinking it was premium..then I was changing gas grades for my first week.. like I was changing underware.. I would go to reg.. sup.. pre... back to reg.. to pre.. My towns messed up.. 1 station will have premium.. 4 stations won't.. and I usually run my car down to about 1 gallon left till empty.. so all my additives can be fully injected into the motor... could be bad o2's as well.. i'll have to check into getting some new o2's...

but you guys think for sure bad o2's if you get the rotten egg smell? what about cat. converters themselves?

its because the fuel mixture is off...

Bad convertors dont cause the smell. A bad convertor will restrict exhaust flow, thats all.

As abe said, its only cause the mix is off.

im assuming because its too rich right?

so what kind of power loss can this cause(air fuel mix being off)? and can i change o2s myself?

O2s can be a little bit of a pain to replace, they are not exactly "easy" to get to but its doable.

Power loss = can be lots. It depends on how degraded they are. I never got a CE or the sulfur smell. I replaced them since I have over 100k. I still noticed big gains.

Fuel with a high sulphur content will cause this (cheap gas) or an intermittent rich mixture condition (Air filter plugged, WOT and the ECM going into or being in 'open loop' mode).

A good scanner can give you voltage readings of individual upstream O2 sensors and they should rapidly switch from about .1 volt to .9 with the overall average being .45 volts. I have a Davis CarChip E/X datalogger chip that plugs into a OBD II port and can graph the O2 sensors for voltage and switching rate. These can be found on the internet for $50 to $125 and are nice because you can selectivly monitor different things, like intake temperature, engine temp., fuel pressure, voltage, all kinds of sensor readings (TPS, all O2 sensors, 26 different readings in all).

If you are pre-'96 (OBDI), a good scanner will still give you this information, but it is recommended that O2 sensors be replaced at 60,000 miles for OBDI vehicles. For OBDII (some '95's and '96 up), it's supposed to be done at 100,000 miles. Some people go way beyond that with no problems-it just depends on how the O2's have held up over the years,

The smell is just a symptom of a rich mixture and it indicates that the cat is working as it was designed. If it only happens when cold or when running it really hard, it is probably not a major problem-yet. If you are noticing it all the time, you have to find out why it is running rich. If it is left in this condition, it will eventually cause problems with the cat, very expensive and a PITA to replace since they are actually part of the exhaust manifold. If incorrect gap is causing rough idle, or engine miss, it is causing a rich mixture-unburned fuel. A leaking injector, or possibly a bad coil could also cause a problem. Replacing the O2 sensors will not fix another problem that is causing the rich mixture, but it will try to make the ECM compensate for it.

Being new, they will switch quicker (Os sensors get 'lazy' as they get older). I replaced mine about 2 months ago, was getting a CEL and a code that indicated my bank2 upstream sensor was 'lazy'-switching slow. I had no drivability problems, and I replaced both-saw no difference in gas mileage, and it really runs, to me, no different than it did before I replaced them.

Hope this helps.

Fuel with a high sulphur content will cause this (cheap gas) or an intermittent rich mixture condition (Air filter plugged, WOT and the ECM going into or being in 'open loop' mode).

A good scanner can give you voltage readings of individual upstream O2 sensors and they should rapidly switch from about .1 volt to .9 with the overall average being .45 volts. I have a Davis CarChip E/X datalogger chip that plugs into a OBD II port and can graph the O2 sensors for voltage and switching rate. These can be found on the internet for $50 to $125 and are nice because you can selectivly monitor different things, like intake temperature, engine temp., fuel pressure, voltage, all kinds of sensor readings (TPS, all O2 sensors, 26 different readings in all).

If you are pre-'96 (OBDI), a good scanner will still give you this information, but it is recommended that O2 sensors be replaced at 60,000 miles for OBDI vehicles. For OBDII (some '95's and '96 up), it's supposed to be done at 100,000 miles. Some people go way beyond that with no problems-it just depends on how the O2's have held up over the years,

The smell is just a symptom of a rich mixture and it indicates that the cat is working as it was designed. If it only happens when cold or when running it really hard, it is probably not a major problem-yet. If you are noticing it all the time, you have to find out why it is running rich. If it is left in this condition, it will eventually cause problems with the cat, very expensive and a PITA to replace since they are actually part of the exhaust manifold. If incorrect gap is causing rough idle, or engine miss, it is causing a rich mixture-unburned fuel. A leaking injector, or possibly a bad coil could also cause a problem. Replacing the O2 sensors will not fix another problem that is causing the rich mixture, but it will try to make the ECM compensate for it.

Being new, they will switch quicker (Os sensors get 'lazy' as they get older). I replaced mine about 2 months ago, was getting a CEL and a code that indicated my bank2 upstream sensor was 'lazy'-switching slow. I had no drivability problems, and I replaced both-saw no difference in gas mileage, and it really runs, to me, no different than it did before I replaced them.

Hope this helps.

The only gas I have ever ran through my mark is Chevron Premium gas, I no longer have original cats I Have Highflow magnaflow cats, 2.25". I think is time to replace the o2 sensor though. I have notice a power gain with the exhaust system but still a little sluggish for a 32v motor. I will keep you posted.

what brand of 02's sensors do you guys run?

02sensors.com has some good numbers for our car. like 38 bucks for one.

02sensors.com has some good numbers for our car. like 38 bucks for one.

Make sure they are heated. 96 and up use four, the front two are always heated - for all years - the rear ones are not heated.

Fuel with a high sulphur content will cause this (cheap gas) or an intermittent rich mixture condition (Air filter plugged, WOT and the ECM going into or being in 'open loop' mode).
Hope this helps.


This only happens at WOT and even then only in 3rd or 4th gear. so in actuallity it could be just fine?
I would think a semi-rich mixture might be a good safegaurd for the engine at 6000 rpm??

While were on the topic of o2's.. i'm asking the question that I usually see most people say ya!! and the others say.. NO!!!! Is it safe and not cause any codes to be thrown.. to remove my third cat.. along with the o2's there? Do I need to have the o2's put into the new exhaust pipe or just unplug them and go?

The third cat is not monitored, the car will have no clue its missing and there are no O2s associated with that one. Will be fine to remove if needed.

I's getting that rotten egg smell until I started using 94 octane. Now I don't even need 94, the 94 seems to have cleaned crap out. If my foot's not on the floor it's cause the cruise control's set.

The third cat is not monitored, the car will have no clue its missing and there are no O2s associated with that one. Will be fine to remove if needed.

then you can remove the speedo gear more easily ! bam twofer!