How To Test The Fuel Pump (1990-1995 5.0L V8 Ford Mustang)

The fuel pump on your 5.0L V8 Ford Mustang can be accurately tested to find out if it's good, failing, or has failed completely.

So whether your Mustang has an engine performance problem you suspect is caused by the fuel pump, or it cranks but doesn't start and you think the pump is bad, a fuel pressure test will help you get to the bottom of it.

This test isn't difficult since Ford included a fuel pressure test port (a Schrader valve) on the fuel rail of your Mustang's V8 engine and I'll show you how step by step.

APPLIES TO: This tutorial applies to the following vehicles:

• 5.0L V8 Ford Mustang: 1990, 1991, 1992, 1993, 1994, 1995.

FUEL PUMP CIRCUIT WIRING DIAGRAMS:

• Fuel Pump Circuit Wiring Diagram (1990 5.0L V8 Ford Mustang).
• Fuel Pump Circuit Wiring Diagram (1991 5.0L V8 Ford Mustang).
• Fuel Pump Circuit Wiring Diagram (1992-1993 5.0L V8 Ford Mustang).

Symptoms Of A Bad Fuel Pump

Having diagnosed a ton of engine performance issues and no-start conditions, I can tell you fuel pumps usually fail in one of two ways:

• Engine cranks but won't fire up: A completely dead fuel pump produces zero fuel pressure. The starter will spin the engine, but it simply won't catch and run.
• Engine starts but runs poorly: A weak fuel pump can still build just enough pressure to get the engine started, but not enough to keep it supplied with the fuel it needs once it's under load (like when you're driving).

These are the most common symptoms you'll run into when a fuel pump has failed or is in the process of going bad:

• Engine no-start: The starter motor cranks the engine but it won't start.
• Stalling after start: The engine starts but shuts off after a few seconds of run time.
• Stalling on acceleration: The engine starts but as soon as you step on the accelerator pedal, the engine stalls.
• No power under load: Lack of power while driving the vehicle.
• Backfiring: Explosions (loud popping sounds) can be heard coming from the intake manifold when you step on the accelerator while the engine is under load.

NOTE: I want to mention that other parts can fail and create the same symptoms as a bad fuel pump. A common one is the fuel pump inertia switch. When it trips, it cuts power to the pump and makes it look like the pump has gone bad. For more details, check out this section: Checking The Fuel Pump Inertia Switch.

Where To Buy A Fuel Pressure Test Gauge And Save

If you don't already have a fuel pressure test gauge to check the fuel pump on your Ford Mustang, below are some recommendations that'll get the job done without costing an arm and a leg:

Fuel Pressure Specifications

These are the fuel pressure specifications for the 1990-1995 5.0L V8 Ford Mustang

Year	KOEO	KOER
1990-1993	35-40 PSI	30-45 PSI
1994-1995	35-45 PSI	35-45 PSI

• KOER: Key On Engine Running.
• KOEO: Key On Engine Off.

TEST 1: Using A Fuel Pressure Gauge To Check Fuel Pressure

The Schrader valve (fuel pressure test port) on your V8 Ford Mustang is located on the fuel rail that feeds the injectors for cylinders 1, 2, 3, and 4 (this side of the engine is known as Bank 1).

To be more specific: the Schrader valve is at the back of the Bank 1 fuel rail, near the firewall.

Once you find the Schrader valve, connect your fuel pressure test gauge to it and check the fuel pressure.

• If the engine doesn't start: The fuel pressure spec is 30 to 45 PSI.
• If the engine starts and runs: The fuel pressure spec is 30 to 35 PSI.

IMPORTANT: Before starting your fuel pressure test —especially if your Mustang won't start— check the fuel pump inertia switch and make sure it hasn't tripped. If it has, the fuel pump won't get power and this will throw off your test results. See this section for more info: Checking The Fuel Pump Inertia Switch.

OK, let's get started with this test:

1. 1

   Locate the Schrader valve on the fuel injector rail.

2. 2

   Remove the dust cap from the Schrader valve and place a shop towel under the general area of the Schrader valve.
   
   NOTE: The shop towel's job is to absorb and fuel that may leak when performing step 3.

3. 3

   Connect the fuel pressure gauge to the Schrader valve on the fuel injector rail.

4. 4

   When ready, ask your helper to cycle the key on and off but don't crank the engine while you observe the fuel pressure tester's gauge.
   
   Check the connection at the Schrader valve for fuel leaks and if any tighten the fuel pressure a bit more (by hand only) to eliminate them.

KOER: Engine starts and runs:

1. 5

   Start the engine and let it idle.

2. 6

   Your fuel pressure gauge should register the specified KOER fuel pressure.
   
   See the KOER fuel pressure specifications here: Fuel Pressure Specifications.

KOEO: Engine does not start:

1. 5

   Crank the engine and let go of the key for it to remain in the RUN position (do not switch the key to its OFF position).

2. 6

   Your fuel pressure gauge should register the specified KOEO fuel pressure.
   
   See the KOEO fuel pressure specifications here: Fuel Pressure Specifications.

Let's take a look at what your results mean:

CASE 1: The fuel pressure gauge shows 0 PSI. This points directly to a lack of fuel as the reason your Mustang won't start.

Before I call the pump bad, I like to double-check that it's actually getting 12 Volts while cranking. This step makes sure the fuel pump relay and fuse are doing their job.

CASE 2: The fuel pressure gauge shows a reading lower than the specified range. This is a clear sign the pump is weak and on its way out.

Any pressure below spec will cause drivability issues. At this point, replace the fuel pump and the fuel filter together.

CASE 3: The fuel pressure gauge shows pressure within the specified range. This result tells you the pump is healthy and supplying enough fuel to the injectors.

If your Mustang still cranks but won't start, or runs rough, then the cause of the problem isn't the fuel pump.

Checking The Fuel Pump Inertia Switch

Your Ford Mustang is equipped with a fuel pump inertia switch. Its purpose is to shut off power (or Ground) to the fuel pump whenever there's an impact to the vehicle.

If the inertia switch trips, the engine won't start —or if it was already running, it'll shut off— because the fuel supply gets cut.

While it's meant to activate during a serious collision, it can sometimes get triggered by smaller bumps or jolts. That's why it's always smart to check and reset the inertia switch anytime you're diagnosing the fuel pump.

Resetting it is easy: push the button on top of the switch straight down until it clicks and bottoms out.

TEST 2: Using Starting Fluid To Confirm A Lack Of Fuel

The starting fluid test has been one of my go-to diagnostic tests whenever I have to troubleshoot an engine no-start problem.

After checking the fuel pump inertia switch and making sure the spark plug wires are sparking (with a spark tester), I grab my can of starting fluid, spray it into the throttle body, and crank the engine.

If the engine starts —even for a few seconds— I know the problem is fuel delivery related. My next step is to check fuel pressure with a gauge, just to be sure.

IMPORTANT: This is a very fast and easy test but you do have to take one very important safety precaution and this is to reconnect the air intake duct after spraying starting fluid down the throttle bore (although you don't have to fasten it). This will prevent any backfire, that might occur, from scaring the heck out of you when cranking the engine.

CHECK FOR SPARK: Be sure and check for spark at all eight spark plug wires first before doing the starting fluid test. Otherwise, if there isn't any spark, it's going to throw off your starting fluid test result.

This is what you have to do:

1. 1

   Remove the intake air duct from the throttle body. You don't have to completely remove it, since you'll have to reconnect it in one of the next steps.

2. 2

   Open the throttle plate and spray starting fluid down the bore.
   
   As a safety precaution reconnect the air duct after you have sprayed a good squirt of starting fluid (but you don't have to tighten the air duct's hose clamp).

3. 3

   Crank the engine once the air duct is back on and you're clear of the engine compartment.

4. 4

   You'll get one of two results with this test:
   
   1.) The engine will start momentarily and after a few seconds will die or.
   
   2.) The engine will only crank but not start at all.

OK, let's find out what your results mean:

CASE 1: If the engine started and ran for a few seconds. This test result tells you that the reason your Mustang is cranking but NOT starting is due to a lack of fuel.

CASE 2: The engine did not start, not even momentarily. This usually means that a lack of fuel IS NOT the reason your car is not starting.

Now, remember what I said about this test not being very accurate? Well, I suggest you do one more test and this is to check the fuel pressure with a fuel pressure test gauge.

More 5.0L V8 Ford Mustang Diagnostic Tutorials

You can find a complete list of wiring diagrams and diagnostic tutorials for the 5.0L V8 Ford Mustang in this index:

• 5.0L V8 Ford Mustang Index Of Articles.

Here's a sample of the diagnostic tutorials you'll find in the index:

• How To Test The Throttle Position Sensor (1994-1995 5.0L Ford Mustang).
• How To Test The MAF Sensor (1991-1995 5.0L V8 Ford Mustang).
• How To Test For A Blown Head Gasket (1990-1995 5.0L V8 Ford Mustang).
• How To Do And Interpret An Engine Compression Test (1990-1995 5.0L V8 Ford Mustang).