Evo X Fuel Pump Relay: The Critical Component That Controls Your Engine's Lifeline

The fuel pump relay in your Mitsubishi Lancer Evolution X (Evo X) is a relatively small but absolutely critical electrical component responsible for switching power on and off to your high-pressure fuel pump. A failure of this relay will prevent your fuel pump from operating, leaving your powerful turbocharged engine completely unable to start or causing it to stall unexpectedly while driving. Ensuring its proper function is essential for your Evo X's performance, reliability, and very ability to run.

Understanding how your car works is key to maintaining it and diagnosing problems when they arise. For drivers and enthusiasts of the iconic Mitsubishi Lancer Evolution X (Evo X, model years roughly 2008-2015 in most markets), one component that can cause significant headaches if it fails is the Evo X fuel pump relay. While seemingly simple, its role is fundamental: it acts as the switch that controls electrical power to your fuel pump. Without a functioning relay, the pump doesn't turn on, and your engine starves for fuel. Let's break down everything you need to know about this vital part.

What Exactly Does the Evo X Fuel Pump Relay Do?

Think of a relay as an electrically operated switch. Its primary purpose is to allow a low-power circuit (controlled by the Engine Control Unit, or ECU) to turn on or off a high-power circuit (the fuel pump motor in this case). Driving the high current required by the fuel pump directly through the ECU would be impractical and risky; the relay handles this heavy lifting.

Here’s the specific process for the Evo X fuel pump relay:

  1. Ignition On: When you turn your ignition key to the "ON" position (before cranking), the ECU provides a brief ground signal to the fuel pump relay.
  2. Relay Activation: This ground signal energizes the relay's internal electromagnet. This pulls a set of internal switch contacts closed.
  3. Power to Pump: Once those contacts close, they complete the circuit from the main power source (the car battery, usually via a dedicated large fuse) directly to the fuel pump motor.
  4. Pump Priming: The fuel pump runs for a few seconds to build up pressure in the fuel rail. This is the "priming" sound you hear when you first turn the key.
  5. Engine Cranking & Running: Once you start cranking the engine (or if the engine is already running), sensors confirm engine rotation. The ECU then provides a continuous ground signal to keep the relay energized and the pump running constantly as long as the engine is operating.
  6. Engine Off: When you turn off the ignition, the ECU removes the ground signal, the relay deactivates, the contacts open, and power to the fuel pump is cut off.

In essence, the Evo X fuel pump relay is the gatekeeper. It listens to the ECU's command and physically connects or disconnects the high-current power needed to run the fuel pump. If this relay fails, the gate stays shut, and no fuel gets delivered, regardless of how the ECU signals.

Where is the Evo X Fuel Pump Relay Located?

The Evo X fuel pump relay is typically located within the main under-hood fuse and relay box. This box is a large, usually black, plastic container situated in the engine compartment. In the Evo X, the primary location is often on the driver's side (left-hand side for US models), near the windshield washer fluid reservoir or the battery.

To locate it specifically:

  1. Open the Hood: Ensure the vehicle is parked safely with the parking brake engaged.
  2. Identify the Main Fuse Box: Locate the large plastic box in the engine bay.
  3. Remove the Cover: The cover usually has clips or simple fasteners. Press or unclip these and lift the cover off. There might be a diagram printed on the underside of the cover showing fuse and relay positions.
  4. Identify the Relay: The Evo X fuel pump relay is often (but not always) positioned near other critical system relays like the Engine Control Unit (ECU) relay or the radiator fan relays. It should be identical in shape and size to other standard Mini ISO or Micro ISO automotive relays within the same box. Look for labels printed next to the socket or within the diagram on the cover. Common labels include "Fuel Pump," "FP," "Fuel," "F/Pump," or sometimes the manufacturer's part number designation (e.g., MR***). If the cover diagram is unclear or missing, you might need to consult the owner's manual or a reliable Evo X service manual for the exact position.

It's important to note that some forums or confusion with other Lancers might mention a relay behind the glovebox. In the Evo X, the fuel pump controller (a more sophisticated module that manages pump speed for certain models and modifications) might be located there, but the primary power-switching Evo X fuel pump relay is almost exclusively found in the under-hood fuse/relay box.

Symptoms of a Failing Evo X Fuel Pump Relay

When the Evo X fuel pump relay begins to fail or fails completely, the symptoms are directly related to the fuel pump not receiving power:

  1. Engine Cranks But Won't Start (No Fuel): This is the most common symptom. You turn the key, the starter motor cranks the engine normally, but the engine doesn't fire up or attempt to start. You won't hear the characteristic brief "whirring" sound from the fuel pump priming when you first turn the ignition to "ON".
  2. Intermittent Starting Problems: The engine might start fine sometimes, but other times it cranks without starting. This is common with relays that have internal contacts that are worn, corroded, or sometimes sticking intermittently. It might work when cold but fail when hot, or vice-versa.
  3. Engine Stalling While Driving: This is a critical and dangerous symptom. If the relay loses its signal internally or the contacts open while driving, the fuel pump shuts off instantly. This cuts fuel delivery abruptly, causing the engine to stall. It may restart immediately if the relay re-engages, or it might not. This can happen unexpectedly at any speed.
  4. Loss of Power / Hesitation Under Load: While less common for a complete failure, a relay with arcing or high-resistance contacts might cause voltage drops to the fuel pump. This reduces fuel pressure, leading to noticeable hesitation, power loss, jerking, or misfires, especially when accelerating or under higher engine loads where fuel demand is greatest.
  5. No Fuel Pump Priming Sound: As mentioned, a key indicator is the absence of the fuel pump's brief humming sound (~2-3 seconds) immediately after turning the ignition to "ON" (before cranking). This is the first thing to check.

IMPORTANT CAUTION: Symptoms like no-start or stalling can also be caused by a bad fuel pump itself, a blown fuse, wiring problems, faulty fuel pump controller, fuel filter clog, or ECU issues. Never assume it's only the relay without proper diagnosis. However, checking the relay is generally straightforward and a sensible first step given the symptoms.

How to Test Your Evo X Fuel Pump Relay

Testing a relay is a manageable task with a basic multimeter. Always prioritize safety: disconnect the negative battery terminal before working on electrical components to prevent accidental shorts. If unsure, consult a professional.

Method 1: Listening & Feeling (Basic Check)

  1. Locate Relay: Find the Evo X fuel pump relay in the under-hood fuse box.
  2. Have Helper Turn Key: Get an assistant to turn the ignition key to the "ON" position (not start).
  3. Listen and Feel: Place your fingers firmly on the relay body. You should hear a distinct, audible "click" sound and feel a physical click/vibration within the relay immediately as the key is turned to "ON". This click should happen only once at key-on. Lack of a click strongly suggests the relay isn't activating.

Method 2: Swap Test (Simple Functional Check)

  1. Identify a Known Good Relay: Find another relay in the fuse box that is identical in size and markings to the suspected Evo X fuel pump relay. A common candidate is the radiator fan relay (ensure it's the same type). Avoid critical relays like ECU main relay unless necessary.
  2. Swap Them: Carefully remove the fuel pump relay and the identical relay you've chosen (you might need small pliers or just fingers; wiggle gently). Install the known good relay into the fuel pump relay socket. Install the suspected bad relay into the socket where the known good relay came from.
  3. Test Operation: Turn the ignition to "ON". Listen for the fuel pump priming sound (if it wasn't happening before). Try to start the car. If the car starts normally and you hear the priming sound, the original fuel pump relay is likely faulty. Conversely, if the known good relay now causes the symptom (e.g., fan doesn't run) when placed in another circuit, that also incriminates the original relay you swapped out.

Method 3: Multimeter Testing (More Precise)
You need a multimeter set to measure DC Volts and Resistance (Ohms). Consult the relay schematic often printed on its side or a service manual to identify the terminals:

  • Terminals 85 & 86: These are the control circuit coil terminals. Applying 12V across these energizes the relay.

  • Terminals 30 & 87: These are the switch contacts. Terminal 30 is usually connected to constant battery power via a fuse. Terminal 87 is the output that goes to the fuel pump (via the controller in the Evo X). When energized, 30 connects to 87. (Terminal 87a, if present, is Normally Closed and opens when energized, but most auto relays are Single-Pole Single-Throw - SPST - meaning 30 and 87 only connect when coil is powered).

    • Testing Coil Resistance (Relay Removed):

    1. Set multimeter to Ohms (Ω).

    2. Measure resistance between terminals 85 and 86. You should get a reading, typically between 50 and 150 ohms (check relay specs or compare to a known identical relay). An "O.L." (overload) or infinite resistance indicates an open coil – the relay is bad. A reading near 0 ohms indicates a shorted coil – also bad.

    • Testing Switch Contacts (Relay Removed):

    1. Set multimeter to Ohms (Ω) or Continuity (which usually beeps).

    2. With relay unpowered, measure between terminals 30 and 87. You should have "O.L." / no continuity (open circuit).

    3. Optional: Apply 12V to terminals 85 (+) and 86 (-) using fused jumper wires connected to a battery or a 12V power source. Caution: Ensure polarity (if coil has Diode, + on 85). Do not apply 12V to coil unless you are certain the relay rating and your power source. Use proper fused jumpers.

    4. While 12V is applied to 85/86, you should hear/feel the relay click. Measure between terminals 30 and 87 again. You should now get a reading near 0 ohms or hear the continuity beep (closed circuit). If it doesn't close the circuit when energized, or the resistance isn't very low, the contacts are faulty.

    • Testing Voltage in-Circuit (Relay Installed): Diagnose control signal and power supply.
    1. Check Voltage at Terminal 30: Set multimeter to DC Volts. With the relay installed and ignition OFF, place the red probe on Terminal 30 (check diagram, may require back-probing wire carefully or checking socket). Place black probe on chassis ground. You should see battery voltage (~12V). If not, there's a blown fuse or wiring issue upstream of the relay.
    2. Check Control Signal at Terminal 85 or 86:
    • Have assistant turn ignition to "ON".
    • Place black probe on solid ground. Place red probe on either terminal 85 or 86 (you need the diagram).
    • You should see voltage: either ~12V on one terminal and ground switching on the other, OR a voltage drop indicating the ECU is completing the ground path. Most often, one terminal should go to near ground (0V) when ignition is turned to "ON". Crucially: Voltage or ground must change at one terminal specifically when the ignition is turned to "ON". Lack of signal change indicates an ECU problem, wiring issue to the ECU, or a faulty input on the ECU side. A constant ground or voltage suggests a wiring short.
    1. Check Output at Terminal 87 (Use Caution): This tests if power is leaving the relay when commanded.
    • With ignition turned to "ON", carefully probe terminal 87 (output) with red probe, black probe on ground.
    • You should see battery voltage for the few seconds that the fuel pump primes (if it's priming). If you hear the pump prime but see no voltage at the terminal the relay socket feeds, suspect wiring between the relay and pump controller/pump. If you see voltage during priming here, the relay's output circuit is functioning at that moment.

Replacing a Faulty Evo X Fuel Pump Relay

Once you've confirmed the Evo X fuel pump relay is the culprit, replacement is simple:

  1. Source the Correct Replacement: The Evo X typically uses a standard automotive Mini ISO or Micro ISO relay. The exact specifications (amperage rating - usually 20A to 30A or higher, coil resistance) must match. The physical pin layout must also match (Mini ISO has terminals arranged in a specific pattern). Crucially, use the correct relay type specified for that socket. The part number is almost always printed on the relay itself (e.g., Mitsubishi part numbers like MR* are common, or manufacturers like Panasonic, Omron, Bosch). You can:
    • Get the exact OEM part number from a dealer or parts lookup.
    • Buy from an auto parts store (bring the old relay to ensure match).
    • Source a high-quality aftermarket equivalent (Denso, Bosch, Omron, etc.). Cheap generic relays are notorious for early failure.
  2. Disconnect Battery: Always disconnect the negative battery terminal first for safety.
  3. Locate and Remove Old Relay: Open the fuse box cover. Identify the fuel pump relay socket. Carefully pull the old relay straight out. It shouldn't require excessive force; wiggle gently if stuck.
  4. Inspect the Socket: While the relay is out, visually inspect the socket for any signs of burning, melting, corrosion, or bent pins.
  5. Install New Relay: Align the new relay correctly with the socket pins. Push it firmly down until it seats completely. Ensure all pins go into their corresponding holes.
  6. Reconnect Battery: Reconnect the negative battery terminal.
  7. Test: Turn the ignition to "ON" and listen for the fuel pump priming sound. Attempt to start the engine. Verify normal operation. It's also wise to monitor for any immediate issues or stalling on a short test drive in a safe location. Clear any ECU trouble codes if they were set.

Preventing Evo X Fuel Pump Relay Failure: Tips

While relays can fail spontaneously, some practices can extend their life:

  1. Use Quality Relays: Invest in OEM or known high-quality brands (Bosch, Denso, Omron, Panasonic). Avoid the cheapest generic options.
  2. Address Electrical Issues Promptly: If you experience other electrical gremlins (flickering lights, voltage fluctuations), have them diagnosed. Low system voltage or spikes can stress relays.
  3. Minimize Aftermarket Electrical Loads: Adding high-power accessories like stereos, lighting, or fuel system components (like a bigger pump requiring a higher-amp relay circuit) increases electrical system load. Ensure any upgrades are wired appropriately with proper relays and fuses, ideally separate from the factory fuel pump circuit.
  4. Keep Connections Clean: During routine maintenance (battery checks, air filter changes), visually inspect the main fuse box for moisture ingress or corrosion. Keeping the under-hood environment clean helps.
  5. Consider Preventative Replacement (For High-Mileage/Critical Use): If your Evo X is heavily modified, tracked, or has very high mileage, proactively replacing the Evo X fuel pump relay (and main ECU relay) with high-quality new units is cheap insurance against a potentially dangerous failure.

Modifications and the Fuel Pump Relay: A Note

Many Evo X owners upgrade the fuel pump to support increased power (Walbro 255 or 450lph pumps are common). While a standard Walbro 255 might work with the stock wiring and relay on a stock turbo setup, larger pumps or sustained high-load situations (track use) draw more current.

Here’s where the Evo X fuel pump relay comes into play:

  • Stock Relay Limits: The stock relay is typically rated for 20A-30A. A significant upgrade like a Walbro 450 can draw significantly more current, especially under boost when pressure is highest. This can exceed the stock relay's rating.
  • Relay Failure Risk: Overloading the relay causes overheating. This melts plastic, welds internal contacts (relay stays stuck "on", draining the battery), or causes contact arcing leading to failure and potential stalling or fire risk.
  • Solution - Relay Upgrade Kits: Serious fuel system upgrades require upgrading the power feed. This typically involves:
    • A High-Current Relay: A dedicated 30-40A or higher relay kit. These are readily available from aftermarket suppliers. This relay handles the pump's high current draw.
    • Heavy-Gauge Wiring: Running a new, thick (e.g., 10-12 AWG) wire directly from the battery (through a new large fuse) to the new relay's power input (Terminal 30).
    • Existing Relay Control: The existing stock Evo X fuel pump relay socket output is usually repurposed. Instead of driving the pump directly, its output terminal (87) is now used only to trigger the coil of the new, high-current relay. The original relay's small signal wiring can easily handle this low-current task.
    • Pump Power: The output of the new high-current relay (Terminal 87) connects via the new heavy-gauge wire directly to the upgraded fuel pump.
  • Why Upgrade: This protects the critical stock ECU and fuse box wiring from high current, significantly reduces voltage drop to the pump (meaning more consistent fuel pressure), prevents stock relay burnout, and provides reliable power delivery even under extreme conditions.

If you're upgrading your fuel pump significantly (beyond a basic Walbro 255), incorporating an upgraded fuel pump wiring harness with a high-capacity relay is highly recommended and considered essential best practice for reliability and safety. This essentially makes the stock Evo X fuel pump relay a trigger for a much more robust power switch.

Conclusion: Don't Underestimate This Small Part

The Evo X fuel pump relay is a prime example of a small, inexpensive component playing an absolutely critical role in your vehicle's operation. Its failure can leave you stranded or, worse, cause dangerous stalling while driving. Understanding its function, knowing its location, recognizing the symptoms of failure, and knowing how to test or replace it are essential pieces of knowledge for any Evo X owner or technician. While a failing fuel pump itself is often the first suspect for fuel delivery issues, a quick and easy check of the relay can save significant time, money, and frustration. If you experience no-start or stalling symptoms, especially if the fuel pump doesn't prime, inspecting the Evo X fuel pump relay should be one of your first diagnostic steps. Keep a high-quality spare relay in your glove box – it's an inexpensive safeguard against a very inconvenient problem. Ensuring this small electrical switch is in perfect working order keeps the lifeblood of your Evo X – the fuel – flowing reliably.

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