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A Pressure Problem

Q. I am going to change the fuel filter. In reading the manual, I read that it is important to relieve pressure in the fuel system before removing the old filter. I found the "valve stem" pressure relief, but when I pushed it, there was no pressure. The truck had been sitting for several hours, but I thought the system stayed pressurized. Where might the pressure leak out, and how might I find it? How do you tell if the injectors are operating properly? Is this an indication that something is wrong, or is it not a cause for worry?

  • 1996 Mazda B2300 Extended Cab
  • 2300cc (same as Ford Ranger)
  • Manual transmission
  • 54000
  • Fuel Injection
  • Rear ABS
  • A/C, P/S, no cruise
  • Rack and pinion steering


A. The manual is quite correct, it is very important to relieve the fuel pressure before doing any work on the fuel system. High pressure fuel sprayed into your eyes is not fun, I know that from personal experience. Another thing I learned is not to use carb cleaner to clean the gas out of your eyes. That's not to mention the fire hazard involved. The best way to relieve the fuel pressure is to remove the fuel pump fuse and crank the engine for a minute or two until the pressure is gone.

There are three main reasons for fuel pressure to drop in an engine, one is a leak in one of the lines. A very dangerous condition. Two is a bad fuel pump check valve and three is a leaky injector(s).

To check for a fuel leak, run the engine and visually inspect all the lines from the fuel tank to the engine. Replace any leaking hose with Hi-Pressure fuel line specifically for fuel injected engines.

To test the fuel pump check valve you'll need a fuel pressure tester. Connect it to the test port in the injector ring line and start the car. Let it fun for 2 or 3 minutes and then shut it off. Watch the fuel pressure. If it drops fairly quickly, then chances are it's a bad check valve. If it's a slow drop off, then it's probably an injector(s) leaking down. This is not a definitive test, just something to give you and idea. To definitely isolate it, you need to pinch off the line going from the fuel filter to the fuel tank. That will isolate the check valve and if you lose pressure, it's definitely an injector(s). If it remains high, it is the check valve.

In some pumps it's possible to replace the check valve without replacing the whole pump. Some you do have to replace the whole fuel pump assembly. I would be concerned about the possible fuel leak. That is the most dangerous of the three, The other two will cause hard starting until the pump can build up the needed fuel pressure.

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