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Can I Convert My Diesel to CNG?

SkyCNG gets this question all the time. Diesel fuel prices have been getting hit just as hard (if not harder) than gasoline prices in the USA. "I have a 2008 Silverado 2500 diesel, can I convert to CNG?". Light diesel truck applications for CNG are very difficult to find... let me re-phrase that: a RELIABLE diesel truck conversion is hard to find.

The diesel conversions you read about in the news for big commercial fleets are usually DECICATED CNG engines on a big tractor trailers. The diesel engine is being completely overhauled to run only on CNG (no more running on diesel) for about $20-30k each. The cheaper kind you can find online are called DDF (diesel dual fuel) conversions that give you about 50/50 mix of fuel as it is sprayed into the engine... and they're very unreliable (see below).

The biggest drawback to a DDF conversion is PAYBACK. Even if you got it to work perfectly, it would take about 75k miles per year to justify the initial cost of the kit and tank (easily over $4,000 for the install even with a cheap eBay kit). Oh, and don't believe the gas mileage hype... your truck display may say 40 or 50 mile per gallon, but that's because it is not factoring in the CNG that is being sprayed inside the engine. Your cost per gallon is only a little bit better than normal diesel.

SkyCNG has not found anyone that can answer all our questions about CNG conversion kits for diesel engines. There are some videos on YouTube and a few more articles on the internet where people have successfully performed a DDF conversion, but our attempts to contact those companies are a dead end. Most of the websites are either closed down or have no mention of CNG conversions. The truth is that the technology DOES exist, but it can be expensive compared to gasoline CNG conversions, it's not consistent, and definitely not reproducable on most vehicles. They may have got it to work on a couple trucks in a certain year, but odds are that the kit won't "hook up" on most diesels. SkyCNG has found that after about 2010 you'll get a DPF (diesel particulate filter) error code and check engine light when trying to run on CNG. The EPA has a sensor on the DPF that triggers when your truck is running dirty AS WELL AS when it is running TOO CLEAN... yeah, can you believe that? When you mix the CNG with your diesel engine it will give you a cleaner exhaust, and the DPF won't like it. The truck will give an error code until you fix it.

The most common type of CNG application for light truck and car diesel engines is usually referred to as a DDF "fumigation" kit. This is a cheap ($200-500 without tank) type of CNG conversion kit that sprays CNG together at the same time as the diesel fuel... usually 50% CNG and 50% diesel. Like gasoline CNG conversion kits, the diesel kit does have ECU versions (computer controlled and tuned) and mechanical kits that don't have a computer to control them. We assume this technology has not been perfected or is not financially responsible since we can't find anyone to speak to us or sell us one. The good news is that some people have supposedly gotten it to work, and the results are pretty cool. The driver sees significantly increased horsepower (some claim 100 more HP)... however when you increase HP you also burn more fuel a lose efficiency. But like we said above, we have not found a way to make the solution work well enough to offer our customers. If you can get it to work at all, it usually requires a certain year vehicle (older than about 2009) and will likely require repeated tinkering and shop visits.

You may find a few websites with conversion information, but when you dig a little deeper you will see that the companies are probably only doing diesel/CNG conversion for very specific year trucks. Also, they may not even return a phone call.


There are some systems that will convert a diesel engine to CNG by adding all sorts of components and even spark plugs, but the engine will not be a fumigation type. It will be bi-fuel. It is very expensive and very difficult to find reliable suppliers of information and parts.

If you have anything to add about diesel CNG systems please email us and let us know!