1999 GMC Jimmy Fuel Pump Replacement: Your Complete Step-by-Step Guide for Success

Replacing a failed fuel pump in your 1999 GMC Jimmy is a demanding but achievable repair. This comprehensive guide provides detailed, step-by-step instructions based on extensive mechanical experience, covering diagnostics, safety procedures, part selection, and the complete replacement process to get your Jimmy running reliably again. Success requires preparation, patience, and strict adherence to safety protocols when working with gasoline and electricity.

Recognizing the Symptoms of Fuel Pump Failure

Understanding the warning signs helps confirm a pump issue before starting this significant job. The most common symptoms of a failing fuel pump in your 1999 GMC Jimmy include:

1. Engine Cranking But Not Starting: This is the classic sign. The engine turns over strongly but never catches and runs. Listen carefully near the rear of the vehicle while someone turns the key to the "ON" position (without cranking). You should hear the fuel pump prime for about 2 seconds – a distinct electric whirring sound. If silent, suspect the pump, its fuse, relay, or wiring.
2. Engine Sputtering or Loss of Power Under Load: A weak fuel pump cannot maintain sufficient pressure when the engine demands more fuel, such as during acceleration, climbing hills, or towing. The engine may stumble, hesitate, surge, or lose power unexpectedly.
3. Vehicle Stalling After Starting or During Operation: The engine may start briefly then die, or stall while driving, especially at higher speeds or loads. It might restart after sitting for a while.
4. Unusual Noises from the Fuel Tank: A failing pump often produces excessive whining, buzzing, or humming sounds, significantly louder than the normal priming noise. Whining or grinding indicates internal wear or impending failure.
5. Hard Starting After Sitting: If the Jimmy is difficult to start after sitting for several hours (especially overnight), it could point to a pump unable to maintain residual pressure. Fuel pressure bleeds down, forcing the weak pump to work harder to build pressure again on startup.
6. Reduced Fuel Efficiency: While less specific, a continuously running weak pump may not deliver fuel efficiently, leading to noticeable drops in gas mileage.

Crucial Pre-Repair Diagnosis: Confirming the Issue

Never replace the fuel pump based on symptoms alone. Due to the cost and effort involved, perform these essential checks first:

1. Check the Fuel Pump Fuse: Locate the fuse box under the hood. Consult your owner's manual or the fuse box cover diagram to identify the Fuel Pump fuse (often labeled "FP" or "Fuel Pump"). Remove it and inspect the metal strip inside – is it broken? Replace a blown fuse only after investigating why it blew. Simply replacing it may result in another blown fuse if there's a wiring short or pump drawing excessive amperage.
2. Test the Fuel Pump Relay: Locate the Fuel Pump relay (typically in the under-hood fuse box, possibly in the Instrument Panel fuse box). Swap it with an identical relay known to be working from another circuit (like the horn or A/C relay). If the pump now operates normally, replace the faulty relay.
3. Verify Fuel Pressure: This is the definitive test requiring a fuel pressure gauge kit. Locate the Schrader valve fuel pressure test port on the fuel rail near the engine intake manifold. It resembles a tire valve stem cap. Depressurize the system first! Connect the gauge securely. Turn the ignition key to "ON" without starting the engine. Observe the gauge. A healthy pump on the 1999 Jimmy should build and hold pressure within specification (typically 60-66 PSI for the Vortec engines) within 2 seconds and hold it steadily. If pressure builds very slowly, doesn't reach spec, or bleeds down rapidly (more than 5 PSI within 5 minutes), the pump assembly is likely faulty. If there's zero pressure, suspect the pump, relay, fuse, or wiring.
4. Listen for the Pump: With the key turned to "ON" (not crank), have a helper listen at the gas tank filler neck or under the vehicle near the tank. You should clearly hear a humming sound for about 2 seconds. Silence points to a lack of power or a dead pump. A weak or noisy sound points to a failing pump. Use a mechanic's stethoscope or a long screwdriver placed on the tank and held to your ear for better acoustics.
5. Check Inertia Switch: The 1999 Jimmy has a fuel pump inertia safety switch. In a collision, it shuts off power to the pump. Check if it was accidentally triggered. The location varies (consult manual), but often it's under the passenger-side dashboard or kick panel. Press the reset button firmly if triggered.
6. Inspect Wiring: Visually inspect wiring harnesses near the fuel tank for damage, chafing, or corrosion. Look at the connections at the top of the pump module itself once accessed – corrosion or looseness can cause failure.

Gathering Tools and Parts: Essential Preparation

Being fully prepared is vital for efficiency and safety. Rushing this job leads to mistakes. Here’s what you need:

Personal Protective Equipment (PPE) & Safety Gear:

• Safety Glasses: Non-negotiable. Protect your eyes from dirt, rust, and fuel splash.
• Gloves: Heavy-duty work gloves for handling metal and sharp edges. Nitrile gloves underneath offer extra fuel protection.
• Fire Extinguisher: A fully charged ABC-rated fire extinguisher must be within immediate reach throughout the job. Do not proceed without it.
• Well-Ventilated Area: Perform the work outdoors or in a garage with doors wide open to disperse gasoline fumes. Avoid basements or enclosed spaces.
• No Smoking or Open Flames: Obvious, but critical. Extinguish any pilot lights (like water heaters) nearby. Work far from possible ignition sources.

Essential Tools:

• Vehicle Jack and Jack Stands (Minimum 3-Ton Rating): Never rely solely on a hydraulic jack. Use jack stands placed on solid, level ground rated for your vehicle's weight. Wheel chocks on the front wheels.
• Socket Set (Metric) & Ratchets: Deep sockets (1/2" drive recommended), extensions (6" & 12"+), universal joints, and a breaker bar are crucial. Common sizes include 15mm, 13mm, 10mm.
• Wrenches: Combination wrenches (metric), flare-nut wrenches (for fuel lines – essential to prevent rounding).
• Screwdrivers: Assorted flat and Phillips head sizes.
• Pry Bars: Small pry bar for separating stubborn components without damage.
• Fuel Line Disconnect Tools: Specific tools designed for GM "quick-connect" fuel line fittings. Size is typically 3/8" and 5/16". Metal tools are far superior to plastic for tight/discolored fittings.
• Torque Wrench: For correctly tightening tank strap bolts and pump module retaining ring/nuts.
• Fuel Pressure Gauge: For pre-testing and verifying repair success.
• Drain Pan: Large-capacity pan capable of holding 20+ gallons under the tank opening.
• Drip Pans: Place under fuel lines and fittings during disconnection.
• Shop Rags or Absorbent Pads: Plentiful supply for wiping spills and drips immediately.
• Wire Brush & Sandpaper: For cleaning tank flange mating surfaces.
• Penetrating Oil: Like PB Blaster for frozen bolts/straps (apply hours before starting).
• Drain Syphon Kit (Optional but Recommended): Allows lowering tank with less fuel weight.

Parts:

• Fuel Pump Module Assembly: *CRITICAL TO GET THE CORRECT ONE.* Order the pump module specific to your 1999 Jimmy's engine size (4.3L V6) and tank size. Using an incorrect pump (wrong pressure, flow, or sending unit resistance) will cause problems. Research your VIN if unsure. Choose between OE (ACDelco) or reputable aftermarket (Bosch, Delphi, Carter) – avoid the cheapest options. Confirm it includes the strainer (sock filter).
• Replacement Lock Ring: Often severely corroded, making removal/reuse impossible. Get a new locking ring/gasket kit.
• Optional but Highly Recommended: New Fuel Tank Strap Bolts (often corroded/stretched) and new GM Fuel Line O-Rings/Gaskets included with the module or sold separately (ACDelco kit). Reusing old O-rings risks leaks.

Step-by-Step Replacement Procedure

*Caution:* Ensure all safety precautions are in place before beginning (fire extinguisher ready, ventilation, ignition OFF, battery disconnected, PPE worn).

1. Depressurize the Fuel System:

   • Remove the fuel pump relay (located under the hood, consult fuse box diagram).
   • Start the engine. It may run briefly or sputter and die as residual pressure is used. Attempt to restart it 2-3 times to bleed off any remaining pressure.
   • Disconnect the negative (-) battery terminal and secure it away from the post.

2. Drain the Fuel Tank (SAFEST OPTION):

   • Position your large drain pan directly under the fuel tank drain plug.
   • Using the correct socket, carefully remove the drain plug. Fuel will pour out. Ensure no sparks are possible.
   • Allow the tank to empty completely. Replace the drain plug once empty. This step significantly reduces weight and fuel spill risk.
   • Alternative: If no drain plug or inaccessible, you can attempt to lower the tank nearly full, but this is extremely heavy and hazardous. A syphon kit to remove fuel through the filler neck is safer than lowering a full tank. A near-empty tank is vastly safer and easier.

3. Raise and Support the Vehicle:

   • Park on level, solid ground. Engage parking brake firmly. Chock front wheels securely front and back.
   • Jack up the rear of the vehicle at the manufacturer's specified jacking points near the rear axle or frame. Do not jack on the fuel tank.
   • Place jack stands under secure points on the frame or designated lift points. Lower the vehicle slowly onto the stands. The stands must bear the full weight. Give the vehicle a solid shake to ensure stability before crawling underneath. Keep the jack slightly engaged as an additional safety measure.

4. Disconnect Fuel and Vapor Lines:

   • Locate the top of the fuel pump module access point under the vehicle, near the center of the fuel tank. You'll see wiring harness plugs, a fuel supply line (pump to engine), a fuel return line (engine to tank), and an EVAP vapor line.
   • Use the appropriate fuel line disconnect tools. Push the tool firmly into the fitting around the fuel line nipple until it clicks/seats fully.
   • While holding the tool in place, push the plastic connector toward the fitting and simultaneously pull the entire line off the fitting. Some wiggling may be needed. Expect a small amount of fuel spillage – have rags/drip pans ready. Be patient; forcing it breaks plastic parts.
   • Disconnect all lines: Fuel Feed, Fuel Return (if equipped), EVAP Line. Note their positions or take pictures.
   • Disconnect the main electrical wiring harness connector. Depress the locking tab firmly and pull it apart.

5. Lowering the Fuel Tank:

   • Locate the two large steel straps running under the tank, one near the front and one near the rear. Each strap is secured by a bolt passing through a bracket on the frame/body.
   • Support the tank securely from below. A transmission jack is ideal. A large block of wood on a floor jack works cautiously. Do not allow the tank to fall.
   • Using appropriate sockets and extensions, carefully loosen and remove the bolts securing the front and rear tank straps. These bolts are often very tight and corroded. Penetrating oil applied beforehand helps. Use a breaker bar if needed.
   • Slowly lower the support jack/block several inches, just enough to create access space above the tank. Ensure the straps are free and won't obstruct. The tank only needs to drop a few inches for access; complete removal isn't always necessary unless replacing straps or tank itself. Ensure it is stably supported.

6. Removing the Old Fuel Pump Module:

   • Clean the area around the large circular lock ring at the top of the tank. Brush away rust and debris. Use compressed air if available.
   • Position your drain pan under the access opening.
   • Using a brass punch and hammer (NEVER steel near fuel tank vapors, sparks cause explosion), strike the lock ring tangs firmly in the counter-clockwise direction. A few solid blows should break it free. If severely stuck, penetrating oil and persistence are key. Special lock ring removal tools exist but often aren't necessary. Protect your eyes.
   • Once loose, unscrew the locking ring completely by hand or with the punch/hammer carefully. Remove it and the large O-ring seal beneath it. Discard the old ring and seal.
   • Carefully lift the fuel pump module assembly straight up out of the tank. Avoid damaging the float arm (fuel sender). Some tilting/rotating might be needed to clear the internal baffles. Be prepared for fuel remaining in the tank – it will pour out at this stage. Place the old module into your drain pan.

7. Installing the New Fuel Pump Module:

   • Crucial: Compare the new module assembly carefully to the old one. Ensure the strainer (sock filter), electrical connector, and fuel line nipples match. Verify the float arm moves freely. Replace the small O-rings on the fuel lines fittings on the top of the module if included or recommended (they often are).
   • Crucial: Clean the tank's sealing surface meticulously. Remove all traces of the old gasket, dirt, and rust using a wire brush and clean rags. A clean surface ensures a proper fuel-tight seal. Wipe the inside lip where the O-ring sits.
   • Lubricate the large NEW O-ring seal (included with the pump or the lock ring kit) lightly with clean engine oil or fresh gasoline. Do not use silicone grease which can contaminate fuel.
   • Position the new O-ring perfectly into its groove on the tank's opening.
   • Align the new pump module correctly (usually a notch or tab aligns with a point on the tank flange). Carefully lower the new module straight down into the tank. Ensure the float arm doesn't catch.
   • Place the new locking ring onto the flange and turn it clockwise by hand as far as possible. It should sit flat and turn easily initially.
   • Using the brass punch and hammer, tap the lock ring tangs firmly in the clockwise direction to fully seat and tighten it. Ensure it is seated evenly all around. Do not force it unevenly. It should be tight and secure.

8. Reinstalling the Fuel Tank:

   • Carefully raise the tank back into its original position using your jack/support. Ensure the straps rest correctly in their channels on the tank.
   • Maneuver the tank so it aligns properly under the straps.
   • Install the NEW tank strap bolts (highly recommended if old ones were corroded or stretched). Tighten them securely by hand initially.
   • Gradually tighten the strap bolts diagonally (front a bit, then rear a bit) to distribute tension. Consult a manual for the specific torque specification (typically around 35-40 ft-lbs) if available, but "very snug" is generally appropriate – the tank should not move or sag. Do not overtighten.

9. Reconnecting Electrical and Fuel Lines:

   • Reconnect the main electrical wiring harness plug. Ensure the locking tab clicks firmly into place. Listen for the click.
   • Install NEW O-rings on the fuel line fittings on the top of the module pump if you haven't already. Lubricate them lightly with fresh gasoline.
   • Align the fuel feed line and firmly push the quick-connect fitting onto its nipple until it audibly and distinctly clicks. Tug on the line to ensure it's fully locked. Repeat for the fuel return line (if equipped) and EVAP vapor line. Double-check all connections are secure and leak-free visually.

10. Final Assembly and Verification:

    • Carefully lower the vehicle off the jack stands.
    • Reconnect the negative (-) battery terminal.
    • Install the Fuel Pump Relay back into its slot.
    • Before starting: Turn the ignition key to the "ON" position (do not crank) for 2-3 seconds, then back to "OFF". Repeat this 2-3 times. This primes the system and builds pressure, allowing you to listen for leaks without the engine running.
    • *VITAL LEAK CHECK:* Get under the vehicle again with good lighting. Inspect EVERY fuel connection point – the quick-disconnects on the tank, the pump module top itself, and any fittings you touched. Look for any sign of dripping or wetness. *If there is ANY leak, DO NOT START THE ENGINE.* Shut off the ignition, disconnect the battery, and fix the leak immediately. Even a small leak is dangerous.
    • If no leaks are found, you can proceed.
    • (Optional, Recommended): Reconnect your fuel pressure gauge at the engine's test port. Turn key to "ON" and check pressure builds to spec (60-66 PSI) and holds.

11. Start Up and Final Testing:

    • With the leak check confirmed and potentially fuel pressure verified, start the engine. It may crank slightly longer than usual as air is purged from the lines.
    • Once running, monitor the engine idle. Listen for smooth operation.
    • While the engine is running, do one final visual leak check underneath. Be thorough. Any dripping requires immediate engine shut off and repair.
    • Test drive the vehicle cautiously first in a safe area. Check for smooth acceleration, consistent power delivery under load, and no stalling. Fill the tank and check around the pump access gasket for leaks (rare if properly installed). Reset your trip odometer to monitor fuel economy over the next few tanks.

Critical Tips for Success and Avoiding Common Mistakes

• Patience & Time: Allocate at least 4-6 hours for an inexperienced DIYer. Rushing causes errors or skipped safety steps. Work methodically.
• Strap Bolts: Replacing the old, corroded tank strap bolts eliminates a major headache during reassembly. Do it.
• Lock Ring: Trying to reuse the old rusty lock ring is nearly always a mistake. Buy the new one. Cheap insurance.
• Quick-Connect Fittings: These are the #1 frustration point. Using the right disconnect tool size is crucial. Push the tool in firmly and fully before trying to separate the line. Lubricate new O-rings slightly with gasoline. Pushing the connector toward the fitting while pulling the line off helps immensely. Replace plastic connectors if they break (solder splice kits available). Flare nut wrenches prevent rounding fittings.
• Tank Mating Surface: The single biggest cause of leaks after installation is debris on the tank flange preventing the big O-ring from sealing. Clean, clean, clean. Wipe with brake cleaner after wire brushing.
• Component Matching: Ensure the pump module part number matches your exact vehicle configuration. Engine size and tank size matter. Double-check before leaving the parts store. An online VIN decoder is helpful.
• Wiring Harness: Handle the wiring connector carefully. Inspect pins for corrosion. Ensure the plug clicks firmly. Resistance or connection issues here cause intermittent problems hard to diagnose.
• Sock Filter Orientation: Position the strainer correctly per the old unit. It often points towards the front corner of the tank. Bending the pickup tube is possible if needed but do it gently.
• Post-Install Issues: If the vehicle won't start after install: Double-check relays, fuses, inertia switch, wiring harness connection, and ground points. Verify power at the pump connector during key-on.

Replacing the fuel pump in your 1999 GMC Jimmy is intensive but represents significant cost savings compared to a shop. Meticulous safety practices, thorough preparation, careful execution following these detailed steps, and attention to common pitfalls will lead to a successful repair restoring reliable performance to your Jimmy. Take it step-by-step, prioritize safety above all else, and don't hesitate to seek help if needed during the process.