Solving Slant 6 Fuel Pump Problems: A Complete Diagnosis, Repair, and Prevention Guide

Replacing a faulty Slant 6 fuel pump is often necessary to restore proper engine performance, fuel delivery, and reliability to this iconic Chrysler inline-six engine found in classic Mopar vehicles. Diagnosing pump failure correctly, selecting the right replacement part, and performing the installation with careful attention to detail are crucial steps in getting your Slant 6 running smoothly again. Problems like engine stalling, hard starting, lack of power, or overheating directly stem from inadequate fuel pressure and volume caused by a failing mechanical fuel pump. Addressing this component promptly prevents secondary issues and ensures the longevity of your classic car's engine.

Understanding the Unique Challenges of the Slant 6 Fuel System
The Chrysler Slant 6, nicknamed the "Leaning Tower of Power" for its distinctive 30-degree cylinder bank angle, presents specific fuel pump considerations compared to conventional vertical engines. Its orientation means the mechanical fuel pump, mounted low on the engine block, has a different stroke and potential vulnerability to heat soak from the exhaust manifold positioned nearby. Early (pre-1970) pumps often used a distinct design compared to later models. Recognizing these engine-specific factors is vital before troubleshooting or repair. All Slant 6 engines utilize a mechanical fuel pump driven directly by an eccentric cam on the engine camshaft. Its sole purpose is to draw fuel from the gas tank using suction and deliver it under pressure, typically 4-7 PSI, to the carburetor bowl. Consistent, uninterrupted fuel flow at the correct pressure is essential for the carburetor to meter fuel properly for combustion.

Recognizing the Unmistakable Signs of Slant 6 Fuel Pump Failure
Ignoring early warning signs can lead to more severe problems. Key symptoms indicating a potential Slant 6 fuel pump failure include:

• Engine Sputtering or Stalling, Especially Under Load: Climbing hills, accelerating hard, or maintaining highway speeds requires maximum fuel volume. A failing pump can't keep up, causing the engine to stumble, misfire, or die completely. Often, the engine might restart after cooling down slightly if vapor lock is part of the issue.
• Difficult Starting or Extended Cranking: The fuel bowl might drain back overnight if the pump check valves are weak. This requires extended cranking to pull fuel from the tank to fill the bowl before the engine fires. You may also need excessive pumping of the gas pedal to prime the system if the accelerator pump circuit is the only source of initial fuel.
• Noticeable Loss of Power and Performance: The engine feels sluggish, lacks its usual low-end torque, or struggles to reach higher RPMs, particularly noticeable on manual transmission vehicles or when carrying passengers or cargo. Throttle response becomes dull.
• Engine Overheating on Hard Drives: This occurs primarily during sustained high-speed runs. As the pump fails to deliver sufficient fuel volume at high RPM, the engine runs lean. A lean air-fuel mixture burns hotter, raising combustion chamber temperatures significantly and potentially causing detonation or knocking.
• Whining or Clicking Sounds from the Pump Location: While mechanical pumps are usually quieter than electric ones, failing internal components like worn diaphragms or stiff valves can cause unusual audible clues originating from the engine block near the fuel pump mounting point.
• Visible Fuel Leaks Around the Pump Body: Cracks in the pump casting, porosity in cheap castings, or severely degraded seals/gaskets can allow fuel to weep or drip. This creates a major fire hazard and demands immediate replacement.
• The Vapor Lock Phenomenon (Especially Common on Slant 6 Engines): Heat radiating from the exhaust manifold and the engine block can cause volatile gasoline to vaporize prematurely within the pump body or fuel lines close to the engine. As vapor bubbles form, they block the flow of liquid fuel, causing sudden loss of power or stalling, often resolving once the engine cools slightly. This is a frequent issue exacerbated by modern gasoline blends and insufficient heat shielding.

The Critical Role of Accurate Diagnosis Before Replacement
Do not immediately assume the fuel pump is faulty based on symptoms alone. Misdiagnosis wastes time and money. Before tackling the potentially awkward pump replacement, perform these crucial diagnostic steps:

• Confirm Fuel Flow at the Carburetor: Disconnect the fuel line at the carburetor inlet. Point the open end into a suitable container. Have an assistant crank the engine (ensure ignition is disabled to prevent starting). Look for strong, rhythmic spurts of fuel. A weak or non-existent flow points strongly to the pump or its supply line. Remember that flow must be observed over many seconds of cranking to fully assess volume delivery.
• Evaluate Fuel Volume Over Time: Don't just check for presence. Crank the engine for at least 15-30 seconds while capturing the fuel output into a container. A healthy pump should deliver a substantial volume – typically at least a quarter to a third of a pint or more during that time. Significantly less indicates insufficient performance.
• Check Fuel Pump Pressure (If Possible): While less common for home mechanics on older cars, renting or borrowing a simple fuel pressure gauge with a low pressure range (0-15 PSI) provides definitive confirmation. Disconnect the fuel line at the carburetor and install the gauge inline or directly to the pump outlet using appropriate fittings. Start the engine and observe pressure at idle and raise RPMs. Pressure should consistently measure within the factory specification range (usually 4-7 PSI). Pressure that's too low or rapidly drops when the engine stops confirms pump failure.
• Inspect the Entire Fuel Delivery Path: A fault could lie before the pump. Inspect the fuel lines from the tank to the pump for severe kinks, rust holes, or damage. Ensure the fuel filter (often located inline before the pump) is clean and unobstructed. Verify the gas tank cap vent is not clogged, creating a vacuum lock preventing fuel flow. Listen for a distinct sucking sound at the tank fill neck when loosening the gas cap after running, indicating a blocked vent. Check inside the fuel tank for significant sediment or rust blocking the pickup tube sock. Blow compressed air backward from the pump inlet line towards the tank to check for obstructions (remove tank filler cap first).
• Rule Out Carburetor Issues: A stuck float needle valve, debris blocking fuel passages, or a failed accelerator pump diaphragm inside the carburetor can mimic fuel pump symptoms. Carefully inspect the carburetor float bowl and jets. Clean jets with appropriate spray carb cleaner and soft wires if necessary. Rule out severe ignition problems (like intermittent coil failure) that can feel similar to fuel starvation.

Essential Preparation for a Slant 6 Fuel Pump Replacement Project
Gathering the right tools and parts before beginning prevents frustration and ensures a successful repair. Necessary items include:

• The Correct Replacement Fuel Pump: Purchase a new, high-quality mechanical fuel pump specifically listed for your Slant 6 engine's year, model, and displacement (170, 225, or 198 cubic inches). Verify it matches the old pump visually regarding inlet/outlet size, location, and mounting bolt pattern. Consider an OEM-spec brand like Carter for reliability. Avoid extremely cheap, low-budget pumps with inferior materials. Check if the new pump includes the necessary mounting gasket.
• A Quality Fuel Pump Mounting Gasket: Obtain a new gasket specifically designed for Slant 6 fuel pumps. A fiber-composite or rubber-coated metal gasket is best. Never reuse the old gasket as it will leak. Purchase extra if the new pump doesn't include one. Check if the pump uses sealant instead of a gasket (less common). Have the appropriate anaerobic sealant ready if required.
• Basic Hand Tools: Wrenches and sockets in appropriate sizes (typically standard/SAE sizes like 1/2", 9/16", 5/8" depending on year). A universal joint socket and extensions are invaluable for accessing the lower bolts. A sturdy flat-blade screwdriver may be needed for the inlet/outlet fittings. Pliers (channel locks or slip-joint). A small pry bar or large screwdriver can help gently persuade the pump arm past the cam eccentric during installation. An inspection mirror and good lighting are crucial.
• Safety Glasses and Gloves: Imperative for eye protection against dripping fuel and debris. Nitrile or rubber gloves protect hands from fuel and grime.
• Shop Towels or Rags: Essential for cleanup and managing drips. Plenty are needed.
• Oil Drain Pan: Place this under the pump location to catch dripping oil or fuel when removing the old pump.
• Thread Sealant: High-temperature rated thread sealant for the bolt threads if the pump bolts go into oil passages (common on older blocks). Use sparingly.
• New Fuel Line Gaskets/Ferrules: Get replacements for the compression fittings at the pump inlet and outlet.
• Penetrating Oil: Like PB Blaster, useful if mounting bolts are rusty. Apply liberally beforehand.

Step-by-Step Guide to Removing the Old Slant 6 Fuel Pump
Performing this job requires patience due to the pump's location near the exhaust manifold and steering linkage. The engine must be cold.

• Relieve Fuel System Pressure: Start with a cold engine. Do not run it immediately before starting work. Disconnect the battery negative terminal as a safety precaution against accidental sparks. Disconnect the fuel inlet and outlet lines from the pump using wrenches. Be prepared for residual fuel spillage – have rags ready and use an oil drain pan underneath. Plug the end of the fuel supply line from the tank to prevent excessive draining (a golf tee or pencil can work temporarily). Cover nearby exhaust components to prevent fuel drips.
• Access and Remove Mounting Bolts: Carefully locate the two bolts securing the pump body to the engine block. On most Slant 6 engines, these bolts are accessible from underneath the vehicle, though often obstructed by steering linkage, engine mounts, or the oil pan. A universal joint socket on an extension, used with a ratchet or breaker bar, is often required. Removing the alternator or its lower bracket (if equipped) might provide slightly better access on some models. Apply penetrating oil to the bolts if they appear corroded before attempting removal.
• Extracting the Old Pump: Once the mounting bolts are completely removed, gently work the old pump out of its cavity. The pump arm is engaged with the actuating lever on the camshaft eccentric inside the engine. Wiggling while pulling firmly should free it. Expect it to feel stiff initially as you overcome the spring tension holding the arm against the eccentric. Be mindful of the old gasket stuck to the block – scrape it off carefully after removing the pump.
• Preparation of the Mounting Surface: Use a gasket scraper or carefully wielded razor blade to meticulously remove all remnants of the old gasket and any sealant from the engine block mating surface. Any leftover material will cause an oil and fuel leak. Clean the surface thoroughly with brake cleaner and a lint-free cloth until shiny metal is revealed. Ensure no debris falls into the open pump cavity – cover it temporarily with tape or a clean rag if necessary during cleaning.

Installing the New Slant 6 Fuel Pump Correctly
Proper installation ensures immediate function and prevents leaks.

• Positioning the New Gasket: Ensure the new mounting gasket is positioned correctly on the engine block surface. Verify it aligns perfectly with the bolt holes and the pump cavity opening. Apply a very thin, uniform coat of engine oil or gasket sealer appropriate for fuel/oil contact (check gasket instructions) to one side of the gasket only to help it adhere temporarily during installation. Avoid excessive sealer that can squeeze inward and block passages.
• Engaging the Pump Arm: This is the trickiest step. The pump arm must engage correctly behind the camshaft eccentric inside the block cavity. Position the new pump so its arm is roughly parallel to the eccentric's lobe profile. Apply light engine oil to the pump arm tip to reduce friction. Carefully maneuver the pump body and arm into the hole. You need to compress the pump arm inward against its spring tension while simultaneously fitting the arm past the eccentric. Often, the arm rests against the outer diameter of the eccentric lobe. Rotating the engine manually (using a wrench on the crankshaft pulley bolt) to move the eccentric to its lowest point (lobe pointing directly away from the pump cavity) can sometimes provide more space. A small pry bar used with extreme care or a large screwdriver handle pushing on the pump body can help push the arm past the high point of the eccentric. Avoid forcing anything excessively.
• Bolt Tightening Sequence: Once the pump body is flush against the gasket and block (meaning the arm is seated correctly behind the eccentric), start the mounting bolts by hand to avoid cross-threading. Double-check pump alignment. Tighten the bolts gradually and evenly in a criss-cross pattern. Follow the manufacturer's torque specifications if available. Typically, these bolts require moderate torque, far less than head bolts – over-tightening risks stripping threads in the block. Snug is sufficient, usually in the range of 10-20 ft-lbs, but specifications vary.
• Reconnecting Fuel Lines: Install new sealing washers or ferrules onto the inlet and outlet line fittings. Apply a drop of oil to the threads to facilitate assembly. Hand-tighten the fittings first, ensuring they thread on smoothly. Hold the fitting at the pump with a backup wrench while tightening the fuel line nut with another wrench to avoid twisting and stressing the pump. Ensure the lines are not kinked or contacting hot exhaust components when routing. Double-check tightness, but avoid over-tightening compression fittings.

Priming and Verifying Successful Replacement
The new pump won't immediately push fuel without priming the system.

• Priming the Pump: Before starting, disconnect the coil wire to prevent ignition. Crank the engine continuously in 15-20 second bursts with short pauses in between to avoid starter damage. This allows the new pump to fill the fuel lines and the carburetor float bowl. Listen for changes in cranking sound or observe if fuel begins filling the filter bowl if transparent. Be patient; it may take considerable cranking after the entire system was drained.
• Final Inspection for Leaks: Before reconnecting the coil wire, meticulously inspect the entire pump assembly and fuel lines. Pay close attention to the mounting gasket perimeter and the inlet/outlet connections. Look for any sign of fuel seepage or weeping. A flashlight helps. Also, check that no tools or rags were left near moving parts. Check the oil dipstick level to ensure no significant drop occurred during the procedure.
• Start-up and Test Drive: Reconnect the coil wire. Start the engine. It may run rough initially as air purges from the system. Let it idle and observe pump operation and any leaks. Listen for unusual noises. After a few minutes, carefully feel the pump body – it should be cool to the touch. If possible, check fuel pressure at idle again. Perform a test drive, paying attention to previously experienced symptoms. Confirm smooth acceleration, consistent power at highway speeds, stable idle, and no hesitation or stalling.

Essential Maintenance and Long-Term Care for Your Slant 6 Fuel Pump
Prevention extends pump life. Key practices include:

• Consistent Fuel Filter Changes: Install an inline fuel filter between the gas tank and the fuel pump inlet. Replace this filter at least annually or more frequently in dusty environments or if you suspect old tank debris. A clogged filter forces the pump to work harder, accelerating diaphragm and valve wear. Ensure the filter orientation respects flow direction.
• Gas Tank Cleanliness: Sediment and rust inside the tank act like abrasive paste. Overhauling or replacing a heavily contaminated tank is a worthwhile investment. Installing an additional, easily accessible filter near the tank before the pump provides an extra layer of protection. Treat rust in old tanks if feasible.
• Heat Management Strategies: Address Slant 6 vapor lock tendencies proactively. Install adhesive-backed insulation or reflective heat barrier material on the fuel pump. Re-wrap deteriorated OEM heat shields over exhaust manifards. Check that existing heat shields are intact. Re-route fuel lines away from the hottest engine components wherever possible using additional clips. Consider using ethanol-free gasoline if available and practical, as it vaporizes at a higher temperature than ethanol-blended fuels.
• Use Fuel Stabilizer: For vehicles that sit unused for extended periods (weeks or months), add a quality fuel stabilizer to a nearly full gas tank. This prevents gasoline varnish formation inside the pump, which can gum up valves and diaphragms. Start and run the engine periodically to circulate stabilized fuel.
• Monitor Oil Level and Viscosity: Leaking mounting gaskets or internal pump seal failures can dilute engine oil with gasoline. Regularly check the dipstick. If the oil level seems abnormally high or the oil smells strongly of gasoline, investigate immediately. Also, running excessively thick oil can overstress the pump actuating arm and linkage components. Use the manufacturer-recommended oil weight.

Conclusion: Reliable Fuel Delivery Restores Slant 6 Performance
A properly functioning mechanical fuel pump is the cornerstone of the Slant 6’s renowned durability. Recognizing the specific signs of failure unique to this engine, accurately diagnosing the issue, and executing a careful replacement with a high-quality pump transforms a potentially frustrating breakdown into a straightforward repair. By understanding the system, using the right parts and tools, and implementing preventative maintenance focused on fuel purity and heat control, you ensure consistent fuel delivery for miles to come. Address fuel pump problems promptly to maximize the performance and longevity of your classic Slant 6-powered vehicle. The effort invested guarantees reliable starts and smooth power delivery, preserving the experience of driving a piece of automotive history.