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Ram Jet 350 MEFI 4 Controller Upgrade

Jeep Front Three Quarter Crwaling
Trent Riddle | Writer
Posted July 1, 2011

Brain Transplant

When it was introduced in 1999 the Ram Jet 350, from GM Performance Parts, was hailed as the best crate engine to hit the market. It was the first production crate engine to be offered with plug-and-play fuel injection and 350 hp and 400 lb-ft of torque right out of the box. It’s no surprise that the Ram Jet 350 quickly became the engine of choice for Chevy V-8 conversions in Jeeps, Land Cruisers, and other rigs.

The only downside to the early Ram Jet was the open-loop-only engine controller, the MEFI 3 (multipoint electronic fuel injection, version 3). It seems that the brain of the system had some limitations.

GM Performance Parts addressed these issues around 2004. GMPP dropped the MEFI 3 controller, and now all new Ram Jet engines come with a MEFI 4. The unit is a true open-loop system. Just like your fuel-injected truck or car, the unit starts up in open loop and, after warm-up, switches to closed loop. With a closed-loop EFI system the computer constantly takes in input from several sensors, including the O2 sensor. The result is an engine that produces more power, gets better mileage, and doesn’t make your eyes water from a rich condition, or knock and over heat from being lean.

For owners of Ram Jet engines with the old MEFI 3 controller, GMPP offers an upgrade kit. This kit consists of a MEFI 4 controller, a wire harness, an intake air temp sensor, and a heated oxygen sensor with mounting bung. This bung must be welded into the exhaust pipe, and the air temp sensor goes into the intake tube or manifold plenum.

The MEFI 4 only uses a single O2 sensor, much the same as the early OBD-I, throttle-body injection systems. Placement is best done in the first few feet of the exhaust pipe after the manifold. Jeff Abbott of Painless Performance points out that the Ram Jet MEFI 4 uses a narrow-band oxygen sensor and adjusts all injectors the same regardless of which bank the sensor is placed in.

You can’t simply take a MEFI 4 controller and plug it into your MEFI 3 harness. Most of the wires are in different places, and the MEFI 4 has a host of new ones. The inverse is true too: You can’t use a MEFI 3 controller with your new MEFI 4 harness. This means that your old harness and controller is not a backup option. It’s best to sell it to one of those boat owners who needs the MEFI 3 for an existing application. (MEFI 3 controllers are no longer made, so availability is limited.)

Installation of the MEFI 4 harness is actually quite simple. Most of the connections are the same as your original ones. There are several new connections that need to be made, but these are simple. You should be able to make the conversion in a weekend: one day to remove the old harness, weld in the oxygen sensor bung, and locate the air intake sensor mounting point, and one more day to install the new harness and MEFI 4 controller. One thing is for sure. If you liked your MEFI 3, you’ll love the new MEFI 4.

The Ram Jet 350 is essentially a ’97 Chevy Vortec motor with a custom cam and port injection EFI intake system. The engine produces 350 hp and 400 lb-ft or torque out of the box. It’s a perfect swap for Jeeps and other trail rigs. This is partially due to the smooth operation of the EFI in off-camber situations and at any altitude.

The Ram Jet 350 has been a popular engine for Jeep, Land Cruiser, and other motor conversion projects since its introduction in 1999. This small-block Chevy was the first production EFI crate motor with a plug-and-play harness and computer included. The MEFI 4 controller makes the Ram Jet even more versatile than the original engines with MEFI 3 controllers.

One of the key components of the MEFI 4 controller upgrade is the inclusion of an oxygen (O2) sensor. This sensor allows the controller to sense the fuel mixture and adjust it on the fly. This prevents the engine from running too rich or lean, improving performance, increasing fuel economy, and extending engine life.

The Ram Jet 350 MEFI 4 upgrade includes an IAT/MAT sensor. This sensor measures the intake air or manifold air temperature depending on where it’s mounted. On the Ram Jet 350 the sensor is mounted into the intake manifold air plenum. Also be aware that the parts bag says “IAT Sensor” and the harness plug tag says “MAT Sensor.” They are the same thing. (Note that the Ram Jet kit sensor has a 1⁄4-inch pipe thread and must be installed in a grounded location.)

The two MEFI controllers look the same. The only way to tell the difference is by the part number on the sticker on the back. A MEFI 3 (right) for the 350 says PN 12489488. The MEFI 4 (left) will either say PN 12499120 or 88962717 depending on whether it’s an older or later version.


GM Performance Parts
Detroit, MI 48232
GM Tech Books
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