Diagnosing Your Fuel System

Description and Diagnosis

If you are having fuel pressure-related problems, use these tests to narrow down the problem.  You will need a fuel pressure gauge.  You can either make one, using the instructions on the Make A Fuel Pressure Gauge page, or you can buy one for between $20 and $40.  They are available at most larger auto parts stores.  The better ones have brass fittings, bleed valves (to relieve the pressure), and metric adapters.  Here you can see a picture of mine.

  1. Check fuel pressure using a fuel pressure gauge.  Connect it to the fuel rail service valve on the fuel rail or by installing a "T" fitting between the fuel supply line and the fuel rail, as described on the Make A Fuel Pressure Gauge page.
  2. Turn the ignition key to "run" (engine off).  The pressure should rise to about 34 psi to 38 psi for 1984-1985 N/A engines, 13.5 psi to 15.5 psi for 1986 and up N/A engines, or 53 psi to 57 psi for turbo engines.  Try cycling the key once or twice if it is low.  If you still can't get the pressure up this way, don't worry about it for now.  Your battery or wiring may not be strong enough to power the pump without the engine running.
  3. Watch the gauge.  The pressure should hold.  If the pressure drops quickly, there may be a leaking fuel injector, a bad fuel pressure regulator, or a bad check valve on the fuel pump.  If the pressure is high, the fuel pressure regulator is bad or the fuel return line is somehow blocked.  Don't worry about low pressure at this point.
  4. Start the engine up.  The fuel pressure regulator is designed to maintain a constant pressure on the injectors, relative to the intake pressure (or vacuum).  There will be a vacuum at idle and the pressure will drop.  If you have a vacuum gauge, you can verify this.  For each inHg of vacuum at the manifold, you should see a 0.49 psi drop in fuel pressure.  So at 15 inHg of vacuum, you should see about 47.6 psi of fuel pressure.
  5. Remove the vacuum line from the regulator.  The pressure should rise back to its atmospheric level (about 36psi for 1984-1985 N/A, 15psi for 1986 and up N/A or 55 psi for turbo).  If the pressure is low, there may be a fuel flow problem due to a dirty fuel filter or a worn fuel pumpBe sure to connect the vacuum line back up!
  6. If the pressure measures OK, try revving the engine with the vacuum line hooked back up.  The pressure should rise to match the changing manifold pressure (a vacuum gauge or boost gauge for turbos is helpful), keeping a constant pressure difference between the two.
  7. If the pressure is OK while parked, you will need to try driving around with the gauge attached.  With a long enough hose, you can run the hose through the back of the hood and clip the gauge to the windshield with the windshield wiper.  You will also need a vacuum/boost gauge to compare the readings.
  8. If the fuel pressure drops off at higher engine speed or while driving, there may be a fuel flow problem or a bad regulator.

For information on upgrades, see the Upgrading The Fuel System page.

Fuel Flow Problems

If you are having fuel flow problems, try replacing the fuel filter.  It is fairly cheap and is mounted next to the gas tank in front of the rear, right wheel well.  There are also tiny screens in each of the injectors.  Most injectors can be disassembled and the screens can then be cleaned.  Also, look for any pinched or kinked fuel lines.  If this is not the problem, then the fuel pump may be getting weak.

Fuel Pump Failures

Some pumps will only fail after warming up, so they appear to work fine after they have "rested" for a while.  This is the sign of a dying fuel pump motor.  Some pumps have a defect where a hose clamp is missing on the short fuel hose between the pump and pump bracket, inside the tank.  The hose will pop off and will suddenly produce very low pressures.  Other pumps will still run strong, but the gear pump itself is worn and cannot produce enough flow at high pressure (usually accompanied by a loud whining sound from the tank).  You can verify a weak pump (at idle) by pinching the fuel return line on the regulator.  If the pressure does not rise, then the pump is the likely culprit.

Fuel Regulator Failures

Fuel regulators don't often fail, but if they do, it may result in low pressure.  You can verify this by pinching off the fuel return line to see if the pressure rises.  More often the diaphragm will develop a leak and while the fuel pressure is not effected, fuel finds its way into the vacuum lines causing any number of problems (usually MAP sensor related).

Fuel Injector Failures

To test for leaking fuel injectors, remove the fuel rail and reconnect all the fuel lines.  Repressurize the fuel system by cycling the ignition key to "run" once or twice.  No fuel should flow or drip from the injectors.  If the injectors are leaking from the tips, you can try to disassemble and clean them.  If the are leaking from the connection to the rail, remove the injector and inspect the o-rings.  Replace any bad o-rings.  Injectors sometimes crack along the back or sides and must be replaced.  Always replace the fuel injector in sets to maintain equal fuel flow (in a pinch you can try replacing it with a used injector).

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Updated 11/05/2003.

Copyright © 1996-2003 Russ W. Knize.