A fuel pressure gauge is designed to indicate the exact pressure the fuel is exerting on the carburetor needle and seat--or in the case of fuel injection, the pressure on the injectors. Repairing a fuel pressure gauge is limited to repairing leaks or replacing the stabilizing oil.
The fuel pressure on a carbureted engine should not exceed five pounds. The needle and seat in the float bowl is the same as the float in a toilet. When the water rises in a toilet the float rises and at the preset level shuts the valve off. The same applies to a carburetor, however the float only applies a small amount of pressure on the needle to stop the flow. Too much pressure will de-seat the needle and allow fuel into the bowl and cause flooding. Pressure is not the issue with a carburetor, just volume.
If nitrous oxide is being used, a high volume electric fuel pump is necessary. There are two ways to manage this system safely: with a single electric pump two separate fuel pressure regulators and two separate fuel gauges or one engine driven and one electric pump with one fuel gauge and regulator. The latter is the easiest and cheapest way to go and is safe. The engine-operated pump needs no pressure regulation. The nitrous side needs to be regulated so that the minimum pressure when the nitrous button is pushed is eight pounds of pressure. Anything less is instant disaster in most cases. The more pressure, the more fuel flows through the unrestricted nitrous solenoid. Too low a pressure causes a disastrous lean mixture, which sends the cylinder head temperature into orbit. With the engine running and the nitrous bottle turned off, pushing the nitrous button momentarily should show no less than eight pounds of fuel pressure while the button is on.
On a fuel-injected engine, a specific high pressure is necessary continuously to maintain the correct flow from the injectors. The computer regulates the time that the injectors are open but the amount of opening is not adjustable on any engine.
Repairing the fuel pressure gauge is limited to replacing the stabilizing oil in the face of the gauge should it have leaked out or repairing any leaks found. If the gauge does not work or fluctuates wildly with the oil in place, replacement is the only option.
Check the connections at the gauge and at the fuel line connection. Any leak should be corrected by separating the connections and applying Teflon tape to the connectors to prevent further leaks. If the oil is contaminated and causes the gauge to darken to the point that the needle is undistinguishable, or the gauge has a small oil leak, the oil can be removed and replaced with a silicone such as WD40. Pull the small rubber plug out of the back of the gauge and drain the oil. Spray a little carburetor cleaner into the hole to clean the remainder of the contaminated oil out of the gauge. Spray WD40 into the gauge with the nozzle until it is full and replace the rubber plug.
Fuel gauge troubleshooting comes down to checking out a few basic components in the fuel gauge.
Signs of a fuel gauge problem can present themselves without warning. Be prepared to act.