North American Turbocoupe Organization

Fuel System Diagnostics for Turbo Coupes
andrewjs18 Offline
Fuel System Diagnostics for Turbo Coupes
By Pete Dunham


You suspect you have a fuel system problem but not sure where to start with diagnosing the system. The following article will lead you through the steps to take to determine if it’s a fuel problem and what part of the system is faulty. If the problem is determined to be the fuel pump you may want to see the article on Fuel Pump Diagnostics by Jeff Korn, also in the Technical Articles

Tools needed

You will need a diagnostic fuel pressure gauge capable of being threaded on the Schrader valve, located on the fuel rail, behind the upper intake. Fuel pressure gauges are available at many parts stores, from $30 to $50. Just be sure it is capable of being threaded on the Schrader valve, which is exactly like a tire valve. A cheap volt/ohm meter is needed. You may need the special tool used to disconnect fuel and air conditioning lines that use a garter spring.

Step 1.  Check Fuel Pressure

Thread the fuel pressure gauge on the Schrader valve. The valve is on the fuel rail, between the upper intake and the firewall. It may have a removable plastic cap on it. With the engine and key OFF, remove the cap and thread on the gauge. Protect face and eyes as there can be pressurized fuel in the fuel rail, which will be released as the gauge is threaded on.


Find the EEC test connector near the front end of the brake master cylinder. See above diagram. When found install one end of a test lead to the indicated terminal. Turn the key to RUN and ground the other end of the test lead. Pump should run. Get the pressure reading from gauge. Pressure should be 35 to 45 psi. Record pressure and disconnect lead from ground. You will be using this step to pressurized the system in some of the other steps.

If pressure is acceptable go to Step 5. If not go to Step 2.

Step 2.  Fluid circuit check

If pressure was low but greater than 3 psi then the pump did run. Go to Step 3.

If pressure was less than 3psi go to Step 4

Step 3.  Fluid circuit check

Since the pump ran but the pressure is low, there are some things to check for the cause.

Check each item, one by one and retest for proper pressure after each check, as in Step 1.
  1. Plugged fuel line filter. Replace with new and  retest. The filter is just in front of the fuel tank on the passenger side
  2. Check for kinked/restricted fuel lines
  3. Disconnect fuel return line and note if fuel is returning. The return line is the smaller of the two lines connected near the back of the fuel rail. If fuel is being returned then adjust/replace the fuel pressure regulator. (It may be easier to disconnect the return line at the bottom back of the engine compartment, driver’s side than at the fuel rail) Run line to a container and then run pump as in Step 1. Be sure to reconnect the return line at the end of this check.
  4. Check for low voltage at the pump. Voltage should be within 0.5 volts of battery voltage.
  5. (See Jeff Korn’s article Fuel Pump Diagnosis for further details)
  6. Another possibility for low pressure is  the hose connecting the pump (in the tank) to the outlet port on  top of the tank has split/come loose. Also there is the possibility that the filter sock in the tank has become plugged. The tank will have to be dropped to investigate.

If after checking/repairing the above, pressure was restored to acceptable limits (35-45 psi) per Step 1, then go to Step 5. If none of the above restored proper pressure, than replace the pump and go to Step 1.

Step 4.  Electrical circuit check.

Refer to Jeff’s article “Fuel Pump Diagnosis” for this step. Repair /replace per the instructions there. Then repeat Step 1.

Step 5.  Check valve test - rate of pressure drop

Do as per step 1 to get a pressure reading then disconnect the test lead from ground to turn off the pump. Observe pressure for 3 minutes. Pressure should not drop more than 2 psi in 3 minutes. (In reality pressure can drop a little faster than this and not be a problem but I can’t say how much is acceptable. A larger capacity pump can stand a faster drop rate than the stock pump, which is pretty limited. The guys running adjustable fuel pressure regulators may see faster drops.) This test is to check the functioning of the check valve in the pump, the fuel pressure regulator, and how the overall system hold pressure. The check valve is to prevent the backflow of fuel out of the supply line back into the tank. If the system does not hold pressure per the above spec, it does not automatically mean the check valve in the pump is defective, go to Step 6 for further testing.

If the system passes this test, go to Step 7. If it doesn’t pass, go to Step 6.

Step 6.  Checking for leaks

If you are at this step its because the system failed the check valve test in Step5. In this step, you will check for leaking injectors and the fuel pressure regulator will be tested. Pressurize the system as in Step 1 and disconnect the test lead from ground. Observe each fuel injector for sign of leaking fuel. Use both eyes and nose.

Now you will have to disconnect the fuel return line at the fuel rail. It’s the smaller of the two lines at the back of the fuel rail. It requires a special tool. When it removed, plug the end of the fuel rail and pressurize the system again per step one. Repeat the pressure drop test in Step 5. If the pressure drop is now within the spec, then the fuel pressure regulator is bad and must be replaced. Then repeat step 5 and if OK, go back and repeat Step 1. Be sure to reconnect the fuel return line at the fuel rail.

If none of the injectors leaked and if the pressure drop results were still not within spec, then the pump check valve is bad. Replace the pump and repeat Step 1 and then Step 5.

Step 7.  Engine running test

You only got to this step if the system was OK in Step 1 and then Step 5.

Disconnect the vacuum line to the fuel pressure regulator (FPR) at the FPR and plug the end of the hose to the intake manifold. Start the engine and let it idle. Fuel pressure should be 35 to 45 psi. Record this pressure for later reference. If pressure is OK, go to Step 9. If pressure is not OK go to Step 8, be sure to reconnect the vacuum line to the FPR.

Step 8.

You are at this step because the fuel pressure was not in the 35 to 45 psi range specified in Step 7.

If the fuel pressure was too high, then the fuel pressure regulator is defective. Replace it and return to Step 1, then Step 7

If fuel pressure was too low, check the following items.

Restricted fuel filter

Improper fuel regulator adjustment (factory regulator is non-adjustable, if out of spec, replace it)

Fuel line restricted

Improper voltage to the fuel pump or leak in the hose between the fuel pump and the tank outlet  or clogged filter sock inside the tank. See Jeff Korn’s article “Fuel Pump Diagnosis”

If any of the above are a problem, repair as needed and repeat Step 1, then 5, then 7

If none of the above are a problem, then replace the pump and repeat Steps 1,5 and 7.

Step 9.  High speed test

This is the last test in this routine. Your going to have to figure a way to route the fuel pressure gauge so it can be seen through the windshield as you drive the car. This isn’t hard for the 87-88s but might be a challenge on the earlier TCs. You need to be able to get into the boost and see what fuel pressure is doing. What ever the fuel pressure reading was in Step 7, that is your base figure and you should see 1 psi added to that figure on the gauge for each psi increase of boost. Example. If you got 39 psi in Step 7 and you run 10 psi of boost then at 10 psi of boost the fuel pressure gauge should read 49 psi. If you get less than this 1 for 1 increase, replace the fuel pump and retest.

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