Fuel pump problem or something else?

[todorbaja]

Hi everyone,

I have a 2005 Toyota Yaris 1.0 with the 1SZ-FE engine and I’m having problem with starting the car.

When the car has been sitting for a longer period of time, it usually won’t start on the first attempt. It cranks normally and usually starts after the second, third, or sometimes fourth attempt.

However, if the car has been sitting for a long time and I hit the bottom of the fuel tank before trying to start it, I can then hear the fuel pump running and the car starts immediately on the first attempt.

Because of this, I initially suspected a bad fuel pump. I replaced the fuel pump with a new one, but the exact same problem is still present.

I have also checked the fuel pump relay by swapping it with the identical horn relay, but this made no difference.

Once the engine starts, it runs normally.

Has anyone experienced this?



Thanks!

[AzianCuz]

Striking the fuel tank of a Toyota Yaris 1SZ-FE likely fixes an intermittent issue caused by corroded electrical contacts or a loose connector at the fuel pump lid rather than the pump motor itself. A comprehensive, forum-ready diagnostic procedure focuses on testing for 12V power at the harness plug, cleaning the connector pins, ensuring a secure ground connection to the chassis, and verifying the fuel pressure regulator's O-ring.

Step-by-Step Diagnostic & Fix Guide

Step 1: Access the Fuel Pump Assembly:

Remove the rear seat bench by pulling up sharply on the front edges to release the clips

Locate the access cover on the floorboard directly underneath the seat

Pry off the metal/plastic cover plate (usually held down by black butyl rubber adhesive) using a flathead screwdriver to expose the top of the fuel tank.

Step 2: Inspect and Clean the Wiring Harness Plug:

Disconnect the electrical plug from the top of the fuel tank lid (flange).

Inspect the female pins inside the wiring harness connector. Look for a green/white powdery substance (copper corrosion).

Check pin tension. If the female slots look stretched out or wide, use a sewing needle or dental pick to gently bend the internal metal tabs back inward for a tighter grip.

Clean the connector. Spray both the male pins on the tank and the female pins on the plug heavily with electronic contact cleaner. Scrub gently with a small wire brush or old toothbrush, then let it air dry completely.

Step 3: Perform a Voltage and Ground Test:
"Do this test before plugging the connector back in, and ensure the car has been sitting long enough that it would normally fail to start."

Set your multimeter to DC Volts (20V scale).

Identify the power and ground wires on the harness side (usually the two thickest wires).

Insert the multimeter probes into the corresponding slots of the unplugged harness.

Have an assistant crank the engine.

If you see 12V+ while cranking: The car's computer, fuses, and relays are working perfectly. The issue is definitely a bad physical connection at the plug or inside the tank lid.

If you see 0V or very low voltage: Move the black multimeter probe to a clean, unpainted piece of the car's metal chassis. Crank again. If you now get 12V, your body ground wire is bad/corroded and needs to be traced, cleaned, and re-bolted to bare metal.

Step 4: Test the Lid Flange for Internal Breaks:

Set your multimeter to Continuity Mode (the beep setting).

Touch the probes to the top (outside) male pin and the corresponding bottom (inside the tank) male pin of the plastic lid assembly.

Wiggle the pins gently. If the multimeter stops beeping or the resistance jumps around while wiggling, the metal circuit embedded inside the plastic lid is fractured. You must replace the fuel tank lid/flange assembly.

Step 5: Check the Fuel Pressure Regulator O-Ring:

Pull the fuel pump housing back out of the tank.

Disassemble the housing to reach the internal fuel pressure regulator and the fuel pump tip.

Inspect the small rubber O-rings. Ensure none of them are pinched, torn, twisted, or missing. A misplaced O-ring allows all fuel to drain out of the lines and back into the tank while sitting, requiring 3 to 4 long cranks to refill the lines. Reassemble using a drop of clean engine oil to lubricate the O-rings so they seat perfectly.