5th Gen Mods, Maintenance

Rotten Eggs/ Sulfur Smell – Exhaust Fix

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Side Exit Exhaust – Rotten Egg Smell Fix on 5th Gen

Side Exit Exhaust 5th Gen 4Runner

Sulfur Smell (Rotten Egg Smell) Exhaust fix on 5th Gen 4Runner

Exhaust systems that smell like rotten eggs or sulfur are usually due to an exhaust leak. You may have a hole in the muffler, tailpipe or exhaust pipe. With most late model 4Runners, there is probably not an issue of an exhaust leak, although this could potentially be your problem. Always check your exhaust for leaks, holes, and punctures before jumping to any conclusions.

After driving the 5th Gen, I noticed the smell at one freeway onramp almost every time. I had always wondered whether it was just that spot getting on the freeway or whether it was actually my 4Runner. Turns out, it was the 4Runner.

It is common to smell that rotten egg stench (hydrogen sulfide) in the 4Runner when you have your rear hatch window down or even cracked and you throttle hard. This can happen when your 4Runner downshifts or you are going up long grades. For me, it always happened going up long grades and almost always punching it when getting on the freeway.

Whats causing the rotten egg smell?

When your engine starts to suddenly run richer than normal, the sulfur compounds in the fuel break down into simpler compounds.

When the compounds change within the catalytic converter, it creates a buildup of sulfates on the catalyst and then becomes unstable.

The sulfates on the catalyst will then react to create hydrogen sulfide which is then pushed through and out the exhaust system. The hydrogen sulfide that is expelled through the exhaust is creating the rotten egg smell.

Our catalytic converter is designed to turn the hydrocarbons and carbon monoxide into carbon dioxide. The catalytic converter usually does a really good job of doing this, but when the engine starts to run richer, that’s when we experience the rotten egg stench (hydrogen sulfide).

Why does it smell on the 4Runner?

This problem is common for many generations of the 4Runner, not just the 5th Gen. The 4Runner, like most SUVs and some cars create a vacuum effect when in motion. The exhaust gases are similar to leaves following a car down the road, you have seen that right?

The vacuum whirlwind of gases behind the 4Runner is just kinda following us down the road. As this vacuum effect starts pulling the gases up and behind the 4Runner, we often smell the rotten egg fumes through the open rear hatch window. Keep in mind, this is only when our engine starts to run richer.

The spoiler on the 5th Gen might even be disturbing that airflow and forcing some of the fumes to enter the cabin. If you know more about this or can explain it better, please comment.

How to Fix Rotten Egg Smell on 4Runner?

Side Exit Exhaust on 4Runner (Close Up)

How do you fix the rotten egg smell on your 4Runner?

Simple, reroute your exhaust to the side. With a side exit exhaust, you will remove the vacuum effect that is created by the factory exhaust location. Side-exit exhaust will force the hydrogen sulfide fumes out and away from the 4Runner, letting the wind take it down the road, and ultimately out of your cabin.

Call your local muffler shop and have them give you a hand. Or, do it yourself.

Trimming Plastics?

In the image above, you can see above the tailpipe where we had to trim the plastics. The tailpipe would have been about 1/8″ away from the plastics causing the plastics to melt. If you are rerouting your exhaust system, just keep in mind that you may need to trim off some factory plastics in order for the system not to cause more damage, depending on how and where you route the tailpipe.

Does it work? 

Yes and No. In short, it minimalizes the smell better than the factory location.

We have been driving with the side exit exhaust system on the 4Runner for a couple weeks now and I did not smell the rotten egg stench until this one specific grade on the freeway (where it was always the worst). If the factory location was a level 10, the side exit exhaust location would be a level 2.5… very minimal in comparison.

In my opinion, it looks much better than the factory location but it does come with its limitations off-road.


Another point to consider is when you are off-road. If you take your 4Runner on moderate trails and come off the occasional rock, you may hit your exhaust tip. I have a handful of times, and it’s not a great feeling.

Whether your exhaust is in the factory location or side-exit, it is something to think about. Both locations are prone to get hit when you are off-road. If you are not the off-road type, then this might work for you.

If you plan on an aftermarket bumper down the road, you may want to cut your tailpipe off just after the rear axle and aim the exhaust down. This would almost eliminate the chance of hitting your exhaust tip/tailpipe on a rock and possibly solve the rotten egg smell as well.

We are going to cut the tailpipe back to the axle next. I just wanted to see if a side-exit exhaust would eliminate the rotten egg smell and it did. So, onto the next mod… cut more.

An inside look at the side-exit tailpipe and isolator hanger

An inside look at the side-exit tailpipe

The golden question: Do you smell it? Did you do something about it? Comments and Questions below!

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March 30, 2019 9:55 am

Wow, I’m glad I came here before ripping my exhaust tip off. I just installed the exhaust tip and since it’s been a nice, warm day today, it’s the first time I’ve driven with the rear hatch window open (got the 4Runner in the winter). I thought I messed up my exhaust with the install but it seems like a coincidence. I guess if I wanted to test I could take the exhaust tip off and drive around without it with the hatch window open, but I think this article explains enough that it’s just the way it is.

Steve Bond
Steve Bond
August 24, 2018 12:04 am

Hey Brenan. Great article! Since it’s been a few months since you posted it, how do you think the mod is doing keeping the exhaust out when the window is down? I am a surfer but also in the construction business and there are definitely times when it more convenient to let something long hang out the window than strap it up top. I like 4 runners and have owned a 3rd 4th and currently have a 5th Generation Trail. And I’m sure it doesn’t eliminate the exhaust entirely but anything better would be great! I’ve had some luck rolling… Read more »

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