July 2025

Fixing Your 1991 K5 Blazer Fuel Pump: A Complete Diagnosis and Replacement Guide

If your 1991 Chevrolet K5 Blazer is cranking but not starting, sputtering under load, or struggling to maintain power, a failing fuel pump is the most likely culprit. Specifically designed for the unique requirements of the 1991 model year, featuring throttle body injection (TBI), replacing the fuel pump assembly often resolves these frustrating issues. This comprehensive guide covers everything you need to know to diagnose, select the right replacement part, and safely perform the repair yourself, restoring your Blazer's reliability.

The fuel pump is the heart of your Blazer's fuel system. Its job is simple but critical: pull gasoline from the fuel tank and deliver it under consistent pressure to the Throttle Body Injection unit on the engine. For the 1991 K5 Blazer, this pump operates at a system pressure between 9 and 13 PSI. When this pump fails or weakens, the engine doesn't get the fuel it needs, leading to a wide range of drivability problems. Recognizing the signs early and performing the repair correctly is essential.

Recognizing Symptoms of a Failing Fuel Pump in Your '91 K5 Blazer

Ignoring initial symptoms usually leads to complete failure. Watch for these clear warning signs:

Engine Cranks But Won't Start: This is the most obvious signal. If your engine turns over strongly but refuses to fire, especially if it happens suddenly or after sitting, suspect the fuel pump immediately. Listen for the pump's whine when you first turn the key to the "Run" position (before cranking). Absence of this 2-3 second sound strongly points to pump failure or loss of power to it.
Engine Sputtering Under Load/Loss of Power: A weakening pump may struggle to provide sufficient fuel volume when demand is high, like accelerating hard, climbing hills, or towing. This causes the engine to stumble, hesitate, jerk, or simply lack power. Performance returns temporarily once load decreases.
Difficulty Starting When Hot (Heat Soak): Heat causes electrical components inside the fuel pump to expand and potentially fail. If your Blazer starts fine cold but stalls after driving and refuses to restart until it cools down significantly (often 30+ minutes), heat-related pump failure is common.
Engine Surges at Highway Speeds: An inconsistent fuel supply from a faltering pump can cause the engine to surge or lose power momentarily during steady cruising.
Decreased Fuel Efficiency: A struggling pump may not deliver fuel efficiently, forcing the engine to compensate by altering air-fuel ratios, potentially leading to increased fuel consumption.
Stalling at Low Speeds/Idle: Random stalling when coming to a stop or idling, especially combined with hard restarts, suggests insufficient fuel pressure for low-demand conditions.
Whining Noise From the Fuel Tank: While a faint hum is normal, a noticeably loud, high-pitched whining or grinding noise originating near the rear of the truck often signifies a failing pump on its last legs.

Confirming the Diagnosis: Beyond Guesswork

Don't condemn the fuel pump based on symptoms alone. Misdiagnosis wastes time and money. Use these steps:

Listen for the Initial Prime: With the key in the "Run" position (engine off), have an assistant listen near the fuel filler neck or under the rear while you turn the key. You should hear a distinct whirring/whining for 2-3 seconds. No sound? Check power to the pump before assuming the pump itself is dead.
Check the Fuel Pump Fuse and Relay: Locate your underhood fuse/relay center. Consult your owner's manual or fuse box cover diagram. Find the fuse labeled "Fuel Pump" and inspect it visually or with a multimeter for continuity. Swap the fuel pump relay with an identical relay from another circuit (like the horn). If the pump now primes or the vehicle starts, replace the relay.
Check for Voltage at the Tank: Locate the electrical connector going to the fuel tank. This is usually near the top of the tank or along the frame rail leading to it. Carefully disconnect it. Use a digital multimeter set to DC Volts. Probe the terminals (typically the gray wire is hot during prime/crank, black/white is ground) while an assistant turns the key to "Run". You should see 12V+ for those few seconds. If voltage is present but the pump didn't run, the pump is faulty. If no voltage, trace the circuit backward (relay, fuse, wiring harness).
Test Fuel Pressure (Critical Step): This is the definitive test for pump health and system integrity. You need a fuel pressure test gauge compatible with low-pressure TBI systems. Connect the gauge to the Schrader valve on the fuel line near the TBI unit (usually on the driver's side valve cover area). Turn the key to "Run" and note the pressure reading. It should be between 9-13 PSI within a few seconds and hold that pressure steadily for several minutes after the pump shuts off. Low pressure, slow pressure buildup, or pressure that drops quickly after key-off confirms a weak pump or leak, respectively.

Choosing the Correct Replacement Pump Assembly for Your 1991 K5

The 1991 K5 Blazer with TBI uses a specific fuel pump assembly. Buying the wrong part leads to leaks, poor fitment, or incorrect pressure. Key considerations:

Complete Fuel Pump Assembly: Always replace the entire assembly unless you have proven expertise repairing the assembly itself. The assembly includes the pump motor, strainer (sock), sending unit for the fuel gauge, float, lock ring, and sealing gasket. Failure to replace wear components like the strainer can cause immediate repeat issues. Common brand choices with strong OE fitment include Delphi (often the original supplier), AC Delco (GM Genuine replacement), Airtex, Spectra Premium, and Carter. Delphi FG1565 or AC Delco MU1731 are typical examples for a 1991 K5 TBI. Always verify compatibility using your VIN or seller's application chart.
Avoid Cheap, Low-Quality Pumps: Cheapest isn't best with fuel pumps. Low-cost imports often have inadequate pressure, lower flow rates, unreliable pump motors, or flimsy components. Paying slightly more for a Delphi, AC Delco, or reputable aftermarket brand like Spectra Premium FG103B (again, verify application!) significantly increases longevity and reliability.
Consider a Pre-Assembled Sending Unit: Many brands offer the pump already mounted on a new sending unit. This is highly recommended as it replaces worn wiper contacts on the unit that cause erratic fuel gauge readings – a common separate issue in older GM trucks. This simplifies installation.
Strainer (Sock) and Gasket: If buying components separately (less common now), ensure the strainer filter matches the hose diameter and is compatible with pump flow. A fresh sealing gasket (usually neoprene or cork composite) is mandatory to prevent leaks.
Replacement Fuel Pump Relay: Given the low cost and critical function, replacing the fuel pump relay with the pump is sensible preventative maintenance, especially if the original relay is several years old. Use only OE-spec relays.

Essential Tools and Supplies for the Job

Gather these tools before starting:

Quality Jack and Heavy-Duty Jack Stands: Safety is paramount. The tank must be securely supported and you need clearance underneath.
Wheel Chocks: Secure the front wheels.
Basic Socket Set (SAE and Metric): Including extensions and a breaker bar for stubborn bolts.
Wrenches (Standard and Adjustable): Specifically for fuel line fittings (often 5/8" and 11/16" line wrenches help prevent rounding).
Torx Bits (T15, T20 common): For trim/pump ring screws.
Flathead and Phillips Screwdrivers
Pliers (Regular and Hose-Clamp pliers)
Floor or Scissor Jack (Optional but helpful for gently lowering the tank)
Block of Wood or Tank Support Strap Tool (to rest the tank on while lowering/lifting)
Drain Pan (Large enough to catch spilled fuel)
Fuel Line Disconnect Tools (Proper size for GM quick connects – often 3/8" and 5/16", especially the release style for nylon fuel lines if yours are plastic)
New Fuel Pump Assembly (With strainer and filter)
New Fuel Pump Strainer/Filter (if not included with the assembly)
New O-Ring Sealing Gasket (Crucial for leak-free installation)
New Locking Ring (Often included in pump kit, check)
Replacement Fuel Pump Relay (Recommended)
Safety Glasses and Chemical-Resistant Gloves
Fire Extinguisher (ABC Rated)
Plenty of Rags

Critical Safety Precautions Before You Start

Work in a Well-Ventilated Area: Gasoline vapors are explosive. Absolutely no smoking or sources of ignition nearby (open flame, pilot lights, sparks).
Depressurize the Fuel System: Before disconnecting any lines, locate the fuse for the fuel pump. Run the engine until it stalls. Crank the engine for a few seconds more. Reinstall the fuse only after work is complete. Alternatively, with the key OFF, carefully wrap a rag around the Schrader valve on the TBI unit and depress the core slightly to release pressure. Catch escaping fuel in a small container.
Disconnect the Battery: Prevent accidental sparks. Remove the negative battery cable completely.
Have a Fire Extinguisher Handy: Keep it within arm's reach. Know how to use it.
Wear Safety Glasses: Protect your eyes from dust, debris, and fuel splash.
Wear Chemical-Resistant Gloves: Gasoline is a skin irritant and carcinogen.
Don't Work Alone: Especially when lowering a heavy fuel tank, have a helper nearby.
Properly Disposal of Fuel: Capture spilled fuel and drain old gasoline from the tank responsibly. Contact your local hazardous waste disposal center for guidelines. Never pour it down drains or on the ground.
Beware of Rusty Components: Sending units and lock rings can become stuck, leading to frustration. Penetrating oil (applied well before starting) and careful application of force are necessary.

Step-by-Step Replacement Procedure

Park and Prepare: Park the Blazer on a level, solid surface. Engage the parking brake firmly. Chock the front wheels. Disconnect the negative battery terminal. Depressurize the fuel system. Put on safety glasses and gloves. Position a large drain pan under the fuel tank area.
Access the Fuel Pump: The fuel pump assembly is accessed on top of the fuel tank. Removing the entire tank is necessary. For two-door models, the rear bench seat usually flips forward, revealing an access panel in the floor. Remove the seat bolts and flip the seat up. Remove trim clips/screws holding the access panel. On four-door models, or if no hatch exists, the entire fuel tank must be lowered. Skip to step 3 if accessing via an interior panel.
Remove Fuel Tank Shield (If Applicable): Some trucks have protective shields. Remove any retaining bolts or straps.
Disconnect Fuel and Vent Lines: Carefully trace the fuel lines and vapor return line from the top of the tank towards the front of the truck. Identify the connectors. Use the appropriate fuel line disconnect tool for quick-connect fittings. Slide the tool between the connector body and the plastic line and push/pull the lines apart. For simple hose clamps, compress the clamp and slide the hose off the metal nipple. Cap or plug lines and ports to prevent contamination. Also disconnect the smaller fuel vapor canister hose if present.
Disconnect Electrical Connector: Unplug the electrical connector for the pump assembly. Note its position for reconnection.
Disconnect Fuel Filler Neck: Loosen the hose clamp securing the rubber fuel filler hose to the tank neck. Carefully pull the hose off the neck. Be prepared for a small amount of fuel spillage. Have rags ready.
Support the Tank: Position a large floor jack securely under the center of the tank, preferably with a wide block of wood between the jack pad and tank surface for stability. Raise the jack just enough to take the tank's weight off the straps.
Remove Tank Support Straps: Locate the straps encircling the tank. There are usually two. They are bolted to the frame. Remove the retaining bolts using a socket and breaker bar if needed. Keep the bolts and any associated hardware. Carefully lower the straps away from the tank.
Lower the Fuel Tank: Slowly and steadily lower the jack. Keep one hand steadying the tank. Lower it far enough to gain ample access to the top of the tank. Block the tank securely once it's down low enough.
Remove the Fuel Pump Locking Ring: With the tank lowered (or accessed via interior panel), clean debris from around the pump flange and lock ring. The ring is large and usually requires a special tool (large hammer and brass punch work carefully, or a lock ring tool available at auto parts stores). Note the ring's direction. Tap it counterclockwise until it unscrews. It can be stiff – penetrating oil helps. Lift the ring off. Carefully note the orientation of the ring tabs for reassembly.
Remove the Old Fuel Pump Assembly: Lift the pump assembly vertically out of the tank. You may need to rotate it slightly to navigate the float arm past obstructions inside the tank. Be prepared for gasoline spillage (have your drain pan positioned).
Drain Old Fuel and Clean: Use a syphon pump to remove any remaining gasoline from the tank into an approved container. While the tank is accessible, inspect the interior as best you can. Look for excessive rust, debris, or sediment. If significant contamination is present, professional tank cleaning or replacement should be considered. Wipe the tank opening flange clean and inspect the surface where the seal sits.
Transfer Critical Parts (If Necessary): If replacing with a basic pump element only, you must transfer the pickup tube, strainer, fuel sending unit/float assembly, mounting bracket, and electrical contacts carefully. This is time-consuming and requires precision to ensure fuel gauge accuracy. Using a complete new assembly is vastly preferred and less error-prone.
Prepare the New Assembly: If not pre-assembled, attach the new fuel strainer/filter (sock) securely to the inlet tube of the pump assembly. Lightly lubricate the new O-ring sealing gasket with clean engine oil or a dab of petroleum jelly (petrolatum) – never gasoline. This prevents pinching and tearing. Place the O-ring into the groove on the tank flange itself, not the pump assembly.
Install New Fuel Pump Assembly: Carefully lower the new pump assembly straight down into the tank, guiding the float arm correctly (it usually points towards the rear of the tank). Ensure the alignment tab on the pump flange matches the slot in the tank opening.
Secure the Locking Ring: Place the locking ring onto the flange, aligning its tabs correctly with the tank's tabs. Ensure the O-ring seal hasn't been dislodged. Use the brass punch or ring tool to tap the ring clockwise until it is fully seated and tight. Ensure it sits evenly all around.
Reconnect Electrical Connector: Plug the electrical connector back onto the top of the pump assembly. Ensure it's fully seated.
Reinstall the Tank: Carefully raise the tank back into position using the jack. Reinstall the support straps, securing them with their bolts. Tighten bolts securely. Remove the jack.
Reconnect Fuel and Vent Lines: Reattach the fuel supply, return, and vapor vent lines securely to the correct ports. Ensure quick connects "click" fully or hose clamps are positioned and tightened behind the nipple flares. Double-check the fuel supply line goes to the correct port (usually the larger one leading towards the engine).
Reconnect Filler Neck Hose: Reattach the large filler hose to the tank neck and tighten the clamp securely.
Reinstall Tank Shield/Access Panel: If applicable, replace the protective shield. If you used an interior access panel, clean the sealing surface, replace the panel, secure its fasteners, and then reinstall the rear seat securely.
Reconnect Battery: Reattach the negative battery cable.
Prime the System: Turn the key to the "Run" position for several cycles. Listen for the fuel pump to run for 2-3 seconds each time. This primes the system.
Check for Leaks: Before starting the engine, visually inspect all connections you disturbed, especially the fuel lines at the tank and the top of the new pump assembly. Also, smell carefully for gasoline fumes. If no leaks are detected, turn the key to "Run" again and recheck.
Start the Engine: Attempt to start the engine. It may crank for a few extra seconds as fuel fills the lines.
Final Pressure Test (Recommended): Once running, ideally perform another fuel pressure check at the TBI Schrader valve to confirm the new pump delivers 9-13 PSI.
Verify Fuel Gauge Operation: Fill the tank and observe the fuel gauge reading to ensure the sending unit portion of the new assembly is working correctly.

Post-Replacement Checks and Preventative Tips

Test Drive: Carefully drive the vehicle, paying attention to acceleration, hill climbing, and overall drivability. Ensure there are no hesitations or power loss.
Monitor Fuel Economy: Keep an eye on your gas mileage over the next few tanks. A significant improvement indicates the old pump was struggling.
Keep the Tank Above 1/4 Full: Fuel helps cool the submerged fuel pump motor. Allowing the tank to frequently run very low reduces the pump's lifespan as it heats up more easily.
Replace Fuel Filter Regularly: A clogged fuel filter forces the pump to work harder, shortening its life. Follow the factory recommended interval for your Blazer.
Use Quality Fuel: While TBI systems aren't overly sensitive, consistently using known low-quality gas stations with potential water or debris contamination isn't advisable for the pump or injectors.
Address Wiring Issues Promptly: If you discovered damaged wiring to the pump during the repair, fix it properly with solder and heat shrink tubing, not just crimp connectors or electrical tape. Corrosion or damaged wiring causes premature pump failure.

Finding Parts and Service

Local Parts Stores: AutoZone, O'Reilly Auto Parts, Advance Auto Parts, NAPA: Stock replacement pumps (Delphi, AC Delco, Airtex, Spectra Premium) and necessary tools/gaskets. Use store locators online.
GM Dealerships: Provide Genuine GM parts, though often at a higher cost than premium aftermarket options like Delphi.
Online Retailers: RockAuto.com (wide selection, competitive pricing), Summit Racing, Jegs, Amazon, GM Parts Direct. Always triple-check part compatibility and ensure OE fitment is guaranteed. Read reviews carefully.
Repair Shops: If the job seems overwhelming, get quotes from reputable independent mechanics (specializing in GM/Chevy is a plus) or your dealership. Labor time is typically 3-5 hours plus parts cost.

Replacing the fuel pump assembly on a 1991 K5 Blazer is a significant DIY project, but absolutely achievable with the right tools, preparation, safety awareness, and this guide. Investing in a high-quality pump assembly and following each step meticulously ensures your classic SUV regains its reliable starting and smooth power delivery for many miles ahead.

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