Can't Get Enough
Join Date: Sep 1999
Location: Cotopaxi, Colorado
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Re: 4.0 SWAP Fuel Rails????????
The '91-'95 engines use the older SBEC computer, with the 60 pin connector. These all use a 35 to 42 lb. system with a return line and a vacuum controlled fuel pressure regulator. The vacuum control on the regulator is used to maintain a constant differential across the injector. If you had 16 inches of vacuum pulling fuel from the injector, with 35 lbs. of pressure behind it, it would deliver the same amount of fuel as no vacuum in the engine, and 42 lbs. of pressure behind it, with the same injector opening time.
The newer,'96 and up, systems use a much faster JTEC computer. The JTEC controllers use three 42 pin connectors. This computer has the ability to sense manifold vacuum from the MAP sensor, and make it's own corrections. These systems use a fixed 50 lb. fuel rail, with no return line. The biggest dis-advantage of swapping the JTEC system is the "check engine" light driver. There is none in the JTEC systems, it is all controlled by two wires on the CCD bus. In fact, every guage in the instrument cluster is controlled by these two wires. The oil pressure, vehicle speed, tach, coolant temp, odometer, even the fuel level. There are advantages and dis-advantages to this system. If you were to hook up the new style dash, it would be a breeze to wire, and you wouldn't have to fool around with a mechanical speedo cable. The drawback is the amount of wires, and the mess in the relay center.
The fuel rails between the two will easily swap over, I think 3 screws. If you have plans on using a newer engine with an older SBEC controller, you should swap to the old style rail and regulator. Another downside is the different style plugs on the sensors. You would have to change act, iac, tps, map, etc or the wire connectors. I believe the values are all the same, but the newer systems use a 5 volt vref instead of 8 volts.