Get your own Free Home PageBefore you start twisting things on your Spica injection pump, let’s be sure the fuel delivery system is correct. Tuning or diagnosing a problem is impossible without adequate delivery pressure f from the fuel supply pump. The Spica injection pump’s plungers draw fuel from a chamber in the pump section that is constantly pressurized with fuel (we hope). The plungers need this ever-flowing reserve of gasoline to properly deliver to the injection valves and for cooling purposes.
First, if the low fuel pressure lamp isn’t working, try grounding the wire that connects to the pressure switch atop the main filter canister. With the key on and that wire grounded, the lamp should be on. If not check for 12 volts on the wire. No juice? Probably no bulb either. Usually the overpriced pressure switch is at fault. Its contact closes when the pressure drops to seven PSI or lower.
Now that that’s working right, try turning on the key and observe the low pressure lamp. It should wink at you for just a moment as pressure builds up to the seven PSI level. It will take longer if you’ve done any work on the system as the air is bled off to the tank. A worn fuel pump will take several seconds to turn off the light, especially if the pump is hot from running for some time. The most common cause of low fuel pressure is a clogged tank filter. This inline filter has a very fine mesh element inside it to trap very small particles and protect the fuel pump. If you need a tank filter, it’s a good idea to take a gander at what style your car uses. Alfa made several different configuration of tube sizes and angles.
On most 1977 and newer Spiders, a recall campaign was initiated to install a small prepump on the bottom of the tank sending unit. This was done to alleviate fuel starvation on hard corners when the tank was full or nearly empty. A small rubber hose connects the tank unit pipe, which exits the fuel tank, to the prepump. When this hose splits, the prepump becomes a little gasoline fountain inside your fuel tank that doesn’t supply much fuel to the main supply pump.
Always be sure the fuel hoses are routed properly at the injection pump! The return hose goes on the fitting with a small obstruction inside it This causes the fuel to dwell inside the pump for better cooling. The supply hoses to the injection pump goes to the fitting with no orifice or obstruction inside. If these were reversed, the low pressure warning device or test gauge would indicate good fuel pressure even though the injection pump itself was doing without. When testing system pressure, it would be optimum to attach a gauge where the low pressure switch screws in. Since you may be without metric hardware to tap into this hole, a tee fitting clamp on the hose that runs from the filter canister to the injection pump is fine. DO NOT just plug the hose into the gauge! This "dead end" arrangement not only gives an optimistic reading, but can also damage things. Besides, with the Tee setup, the car can be driven while the pressure is monitored. A large drop in pressure when the engine is revved could indicate a low output alternator.
Exactly how much fuel pressure you should be reading is a matter of some debate. Alfa doesn’t seem too worried about exact numbers here, but all agree seven PSI (which triggers the low pressure lamp) is too low to run the car or cool the plungers. Although these supply pumps are capable of putting out 35 to 40 PSI, I personally believe 12 to 20 PSI is correct. Anything higher can push gas past the plunger’s seals into the oil system (Yech!)
What if your gauge shows less?? Blindfold the liability lawyers for this one, gang. If your fuel pump is just getting tired but is still reliable, this is a pretty nifty fix: fit a restriction in the hose that is the return line back to the tank. The restriction in the fitting on the side of the injection pump is about 4 mm to 4.5 mm (0.16 in to 0.18 in). I start with an orifice .060" smaller than that to “dial in” the fuel pressure. We used an idle jet from an SU carburetor for our restrictor. Solder it up and drill it out. Note: Do not do this without a gauge hooked up or you’ll have no idea what kind of change you’ve effected. Buon Fortuna!
[Ed. note: The submersible pump also pressurizes the line to the main supply pump eliminating vapor lock and consequent pump cavitation.(fdm)]
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