Don’t throw away that old vacuum gauge

Most of us remember that we have an old carburetor fuel pressure gauge sitting somewhere in our tool box. Remember? Only reading up to 15 psi and also measuring vacuum? This tool can be an invaluable aid in helping to diagnose problems with the newer fuel systems we see today. One procedure involves suspected intake air leaks in V-type engines with the intake valley sealed to the crankcase and heads. Please, DO NOT add propane into the crankcase. This method has been known to cause explosions within the crankcase from unexpected combustion, sending valve covers into orbit. Instead, disable any crankcase breathing or ventilation systems, then block all of the breather ports leading to the crankcase. Next, install your vacuum/pressure gauge onto the oil dipstick tube. After starting the engine, observe the gauge. Your gauge should never indicate that vacuum is present. If your gauge does, then you have found your intake leak, which is in the valley area. Even a tight engine has some blowby, and since you have closed the crankcase, the gas has nowhere to go. This creates some pressure that should be indicated by the gauge.

Another use for your old gauge is to check fuel system operation. Many vehicles use dual fuel pump systems. Others use three pumps as well as more than one fuel tank. Most of the dual pump systems use a lift or supply pump in the fuel tank together with an external high-pressure pump. When the pressure at the fuel rail is low, here is a way to determine which pump is not working properly. Using a tee fitting, install the gauge between the high and low pressure pumps. If the low-pressure pump is O.K., you will read low pressure. If the low-pressure pump is not working correctly, or if the fuel inlet is restricted, you will read vacuum. This is because the high-pressure pump is doing all the work.

Jeff Auerbach, Domestic Specialist