The Novak Guide to

Jeep Conversion Engine Emissions

ls2Some engine swappers' fear of emissions regulations may often be overblown. It simply is not the challenge that many perceive. The emissions and complexity of engine swaps are actually less of a headache than a few years ago, especially when swapping post-OBDI (1996- ) GM engines into your Jeep. The PCM does now what a myriad of pumps, hoses, EGR, etc. used to do. Instead of taking horsepower away from an engine to meet emissions, the new engines burn cleaner and more powerfully.

And, not just for the purpose of obeying the law. Think of your passengers and the Jeeps following you on the trail. They'll have a more pleasant 4wd experience if they don't have to suck up dirty exhaust.

Your Local Legal Requirements

This guide is not meant to explain, interpret or notify the installer of local or national laws concerning vehicle changes, engine conversions and the emissions systems surrounding them. It is the installer’s responsibility to know and understand their local legal requirements and regulations and to make their installation conform to demands of the jurisdictions in which they live. Novak does not recommend nor condone the disabling or modification of the vehicle or any system within it that could render it out of conformance with any laws in which the vehicle may be licensed or operate. Neither Novak Conversions nor its directors, principles or employees are responsible for any changes made to your vehicle.

We recommend you consult and know your own city, county, state or countries laws. If your vehicle requires referee inspection, you may wish to establish a rapport with a referee and discuss your plans and seek advice and approval from them.

Predominant Standards

Many principalities have emissions standards, but hardly any of them bar proper engine conversions. As a rule of thumb, if you can meet the California laws, you can usually meet all of them. Many jurisdictions have significantly simpler requirements.

Like the weather, trends flow eastward from California. Not just for Californians anymore, vehicle emissions considerations play a big role for most swaps. As a benchmark, we will focus on the way the California Air Resources Board does it.

Good or bad, most states have followed California’s lead when mandating their own emissions standards. Currently, it is legal to perform engine swaps in California if certain criteria are met. Most importantly, the new engine you’re swapping in must be from a GM vehicle of the same year or newer than the vehicle you’re swapping it into. The new engine must retain all of its original, functioning emissions equipment, and any aftermarket performance equipment that has been installed must have a California E.O. number assigned to it to keep your swap legal. You also can’t swap a heavy-duty truck V-8 (trucks over 6,000 pounds GVW) into your S-truck because HD truck engines had less stringent emissions standards than light-duty trucks, but passenger car V-8s are OK.

In order to get your engine swap approved, you need to go to a Referee Station and pay about $30 to have your vehicle inspected. The referee will inspect your installation to see if you used the same or newer year engine. He’ll check operation of all the emissions equipment, and if it passes inspection he’ll slap an “Engine Identification” sticker on your doorjamb, meaning a legal record of your approved engine swap is always on the vehicle. Contact your local DMV office and ask for the number of a Referee Station near you. The person answering the phone may ask why you need to go to a referee, and your answer should be “engine change.” If you say “engine swap” or “ V-8 conversion,” you’ll confuse the employee on the other end of the line who has the power to cause you hours of endless grief, so remember to only speak in language they’re familiar with.

-Mike Petralia,

We’ve read that California law does exempt 1975 and earlier vehicles from emissions regulations. This exemption year has been pulled forward occasionally, so do your research at

Engine conversions, according to California regulations, are to meet the following standards:

“Engine changes are legal as long as the following requirements are met to ensure that the change does not increase pollution from the vehicle:

  • The engine must be the same year or newer than the vehicle
  • The conversion engine must meet or exceed the emission standards of motor being replaced, except California where the conversion motor must meet or exceed the emissions standards of the donor vehicle (which as according to the above rule, is the same year or newer)
  • The engine must be from the same type or family of vehicle (passenger car, light-duty truck, heavy-duty truck, etc.) based on gross vehicle weight. This is why a Jeep can use a truck/SUV donor engine, as well as a car (but not a medium or heavy-duty truck) engine
  • If the vehicle is a California certified vehicle then the engine must also be a California certified engine
  • All emissions control equipment must remain on the installed engine, fuel system and exhaust system
  • Catalytic Converters are not required to be installed on vehicles that did not come with them installed from the factory. However, OBDII donor vehicles have “post-cat” O2 sensors. These cannot be omitted. Without the catalytic converter the PCM will throw a code, thus creating an issue with the conversion. So, although the CATs themselves are not required, if the donor vehicle is OBDII, then by proxy, they are required
  • The evaporative cannister (charcoal cannister) and its purge valve must be operational. The fuel tank pressure sender (a newer feature, since 2003) must also be used on the vehicle if it originally came with it
  • Close-coupled catalytic converters (the ones closest to the engine) must be installed no closer than the factory exhaust (of the donor vehicle) converters were installed and no further than 6" away. Far-coupled converters must be within 12" of factory (again, of the donor vehicle) were
  • Oxygen (O2) sensors must be installed within 1" of their donor exhaust location and be oriented at the same angle
  • Readiness monitors in the PCM must read as passing. This may require several drive cycles and adjustments
  • DLC connector (the OBD port) must be accessible and highly visible
  • MIL (Malfunction Inidicator Lamp) must be clearly visible and labeled

In California and other states, vehicles must first be inspected by a state referee station after an engine change. The vehicle will be inspected to ensure that all the equipment required is in place, and the vehicle will be emissions tested subject to the specifications of the installed engine or the vehicle into which the conversion engine is installed.

We've had reports from California customers in-the-know that claim that pre-2005 Jeeps and GM powertrains are easier to pass, as a general rule, with the emissions referees.

Note that there are two emission systems to consider: Exhaust and Evaporative. The former consists of burning (and reburning) the fuel and air to the cleanest state possible, and then reburning yet again through the catalytic converter in the exhaust circuit. Evaporative emissions consist of how the unburned fuel is stored and transferred in the vehicle. The principal piece of hardware used here is a charcoal canister that absorbs fuel fumes as they slowly evaporate from the tank and lines. Upon starting the engine, the cannister is purged of these fumes through engine vacuum and the temporary opening of a purge valve. Usually, one should retain the Jeep's existing canister as it already fits, and evaporative emissions management are no different for the engines involved.

It has been reported to us that some jurisdictions doing programming checks on the PCM to determine if its been flashed to anything outside factory specs. This may eliminate the possibility of doing an engine swap from a donor vehicle in California, and could limit engine choices to new GM Powertrain engines only. The CARB website does not discuss this, which may indicate that different referees or DMV locations are applying arbitrary rules. We'll try to keep learning more.

Engine Choice and Emissions

Your Jeep is considered to be a “Light Truck” by most jurisdictions. As such, you can usually source your engine from a GM truck or SUV without failing your emissions certification. However, this again is according to local laws and your research is encouraged.


  • The Novak archives and installation projects
  • Novak customers and their project input
  • California Air Resources Board

We welcome any contributions or clarifications to this article, including information about your own local laws. Contact us here.