Rotten Egg or Sulfur Smell Coming from Diesel Exhaust

by Robert Moore

Diesel exhaust has always had a significant smell compared to the smell of a gasoline engine's exhaust, but in general it shouldn’t exhibit much of a sulfurous smell. A sulfur or rotten egg smell is exactly that, the presence of hydrogen sulfide in the exhaust system. There are a number of causes for the rotten egg smell you are experiencing, including a source of stench that may not even have anything to do with the exhaust or engine at all.

A Bit More About Diesel

Diesel fuel is often referred to as diesel oil because it is refined from crude oil, just like gasoline. Interestingly enough, diesel produces more energy than gasoline at 147,000 BTU per gallon as opposed to gasoline's 125,000 BTU. There is always going to be a certain amount of sulfur in diesel fuel, which depends largely on the quality of the crude oil it was made from. The different companies that produce diesel fuel are bound to certain specifications; however, the amount of sulfur in any given company's diesel fuel will vary.

Types of Diesel

There are two different types of diesel; Basic diesel and ULSD, or ultra low sulfur diesel. Regular diesel can have up to 5000 parts per million of sulfur to be legally sold within the United States, as to where ULSD-rated diesel can have a maximum of 15 parts per million. Technically, there are three different grades of diesel fuel rated for use with diesel engines: No. 1-D S15, or ULSD, which is rated at a maximum of 15 PPM, No.1-D S500 can have a maximum of 500 PPM and NO.1-D S5000, which can have a maximum rating of 5000 PPM.

Sulfur Smell in Exhaust

There are a couple of things than can cause a sulfur smell to be emitted with your exhaust gases. The first of which depends on the quality of the fuel available from your local gas station. If your gas station supplies the lowest grade of diesel, there could be up to 5000 PPM in sulfur content, which could cause a light-to-moderate sulfur or rotten egg smell in your exhaust. Pay attention to the grade of fuel you purchase, as a ULSD-grade diesel fuel -- noted on the pump -- has a lower sulfur content. ULSD-grade fuel can often be found at more popular or brand name gas stations. The only other likely cause for the sulfur smell, barring fuel quality, is either too much fuel being wasted and burning off in the catalytic converter or a clogged catalytic converter that releases too much sulfur. However, your vehicle may not be equipped with a catalytic converter, since diesel engines release lower amounts of noxious gases compared to gasoline. If your vehcile has a catalytic converter and ULSD grade fuel doesn’t solve your problem, you should probably look into replacing the converter.

The Battery

Often a noticeable sulfur smell in the cab can be confused for a rotten egg smell in the exhaust. The only thing under the hood that will likely emit a sulfur smell would be your battery or batteries, depending on your vehicle's set up. This will involve looking into your alternator or generator's performance, as well as the voltage and amperage it should be supplying. An alternator that is faulty or has a bad external regulator could be overcharging the battery, causing the electrolytic fluid inside to boil; which in turn creates a sulfur smell. Let your engine idle for a while and inspect your battery. If there is fluid escaping the caps on the top of the battery, your charging system probably has a fault that needs to be diagnosed and repaired immediately. Keep in mind that an over charged battery poses the risk of a potentially deadly explosion, making it a good idea to wear safety glasses and fire-proof clothing when inspecting the battery and charging system.

About the Author

Robert Moore started writing professionally in 2002. His career started has head writer and Web designer for VFW post 1224 in Hamburg, Michigan. He has prepared business plans, proposals and grant requests. Moore is a state of Michigan-certified mechanic and is pursuing an Associate of Arts in automotive technology from Lansing Community College.