1970 Chevy 350 Fuel Pump: Diagnosis, Replacement, and Keeping Your Classic Running Strong

A failing or faulty mechanical fuel pump is a common culprit behind poor performance, hard starting, or a complete no-start condition on your classic 1970 Chevy 350 engine. Understanding its function, recognizing the signs of failure, and knowing how to properly replace it are essential skills for any owner of this iconic V8. Proper maintenance and timely replacement of this critical component ensure reliable fuel delivery from the gas tank to the carburetor, keeping your vintage Chevy running smoothly for miles to come.

Understanding the Heart of Fuel Delivery: The Mechanical Fuel Pump

The 1970 Chevy 350 relies exclusively on a mechanical fuel pump, mounted directly to the engine block. Unlike modern electric pumps submerged in the fuel tank, this pump operates using engine motion. A lever arm (often called a pushrod arm) protrudes from the back of the pump. This arm is actuated by a dedicated lobe on the engine's camshaft. As the camshaft rotates, the lobe pushes the lever arm up and down. This reciprocating motion works a diaphragm inside the pump housing.

Here’s a simplified breakdown of its operation:

1. Suction Stroke: As the camshaft lobe moves away, a spring pulls the pump lever arm down. This pulls the diaphragm down, creating low pressure (suction) within the pump chamber. This suction opens an inlet valve, drawing fuel from the tank through the fuel line into the pump chamber.
2. Pressure Stroke: The rotating camshaft lobe then pushes the pump lever arm up. This pushes the diaphragm up. The upward movement increases pressure inside the pump chamber, forcing the inlet valve closed and opening an outlet valve. The pressurized fuel is then pushed out through the outlet port towards the carburetor.

This simple, robust design provided reliable fuel delivery for decades. It requires no external electricity (beyond the engine running to turn the camshaft) and is generally self-priming once the engine cranks.

Why Does a 1970 Chevy 350 Fuel Pump Fail?

Mechanical fuel pumps are durable, but several factors can lead to their demise, especially in a 50+ year old vehicle:

1. Diaphragm Failure: This is the single most common cause of failure. The flexible rubber or synthetic diaphragm inside the pump is constantly flexing. Over time, this material can harden, become brittle, crack, or develop pinhole leaks. Even a tiny hole compromises the pump's ability to generate sufficient suction and pressure. Diaphragm degradation can be accelerated by exposure to modern fuel blends containing ethanol (which older rubber formulations weren't designed for) and simply due to age.
2. Valve Failure: The inlet and outlet valves inside the pump (small check valves) can become worn, stuck open, or stuck closed due to debris in the fuel, corrosion, or general wear. Stuck valves prevent fuel from flowing correctly in the required direction.
3. Lever Arm Wear: The point where the lever arm contacts the camshaft lobe or the pivot point inside the pump itself can wear down. Excessive wear reduces the amount of movement the lever arm imparts to the diaphragm, severely limiting the pump's stroke and output volume. This wear often leads to low fuel pressure issues under load.
4. Shaft Seal Leak: A seal prevents oil from the engine block from leaking out where the pump's lever arm enters the housing. Conversely, it also prevents fuel that might leak past the diaphragm (in case of failure) from entering the engine oil. If this seal fails, you might see external oil leaks around the pump body or, more critically, fuel diluting your engine oil (a serious hazard).
5. Physical Damage: Impact from road debris or improper handling during other engine work can crack the metal pump housing or damage the lever arm.
6. Contaminated Fuel: Dirt, rust, or debris entering the pump through the fuel lines can accelerate wear on the valves and diaphragm, or cause valves to stick.
7. Improper Installation: Installing a pump without ensuring the lever arm correctly engages the camshaft lobe can lead to immediate failure or poor performance. Using the wrong pump (one designed for a different camshaft lobe profile) can also cause issues.

Symptoms of a Failing 1970 Chevy 350 Fuel Pump

Recognizing the warning signs early can prevent being stranded or causing further issues:

1. Engine Sputtering Under Load: A classic sign of fuel starvation. The engine might idle fine but sputter, stumble, or lose power drastically when accelerating or climbing hills. This indicates the pump cannot deliver enough fuel volume as demand increases.
2. Hard Starting or Extended Cranking: If the pump isn't delivering adequate pressure, it takes longer cranking to get fuel up to the carburetor. You might need to pump the gas pedal excessively to prime the system if the carburetor bowls have run dry.
3. Engine Stalling: The engine may start and idle, then suddenly die, especially after running for a short period or when coming to a stop. This can happen as fuel demand at idle might still be met briefly after priming, but the failing pump can't sustain delivery.
4. Loss of Power at High Speeds/RPM: Similar to sputtering under load, the engine runs out of breath at higher speeds because fuel delivery can't keep up with the engine's consumption rate.
5. Engine Won't Start (No Fuel): A complete pump failure means no fuel reaches the carburetor. You won't smell fuel at the carburetor air inlet when manually operating the throttle linkage while cranking.
6. Visible Fuel Leak: Cracked housing or a severely leaking diaphragm/seal can lead to fuel dripping from the pump body itself. This is a FIRE HAZARD and requires immediate attention.
7. Oil Dilution/Overfull Oil Level: If the internal diaphragm ruptures, fuel can leak past the shaft seal into the engine crankcase. This dilutes the engine oil, drastically reducing its lubricating properties. Check your dipstick – if the oil level is higher than normal, smells strongly of gasoline, or appears unusually thin, suspect a ruptured fuel pump diaphragm immediately. Driving with fuel-contaminated oil can lead to catastrophic engine failure. This often accompanies hard starting and poor running.

Diagnosing a Suspect Fuel Pump (Simple Checks)

Before rushing to replace the pump, perform these basic checks:

1. Visual Inspection:
   • Look closely at the pump body for any obvious cracks, dents, or signs of fuel leakage. Look for wetness or fuel staining around the pump body and mounting surface.
   • Check the rubber fuel lines connected to the inlet and outlet ports for cracks, brittleness, or leaks. Check hose clamps for tightness. Old, cracked fuel lines are a common source of air leaks causing starting/running issues.
2. Check Fuel Delivery at Carburetor:
   • SAFETY FIRST: Ensure no sparks or open flames nearby! Work in a well-ventilated area. Have a fire extinguisher accessible.
   • Locate the fuel line where it connects to the carburetor inlet.
   • Carefully disconnect the fuel line at the carburetor. Position the end of the line into a clean container (like a clear soda bottle) to catch fuel.
   • Have an assistant crank the engine for about 15 seconds (do NOT start it). WARNING: Do not crank excessively without allowing the starter motor to cool.
   • Observe the fuel stream: A good pump should deliver a strong, steady pulse of fuel with each stroke. Weak spurts, dripping, or no fuel at all indicates a problem (pump, line blockage, or empty tank!).
3. Check Fuel Pump Pressure & Volume (More Accurate):
   • Pressure Test: This requires a simple fuel pressure gauge designed for carbureted systems (typically expecting 4-8 PSI). Disconnect the fuel line at the carburetor and connect the gauge inline using appropriate adapters if needed. Crank the engine (or preferably start and run it at idle). Compare the pressure reading to the specification for your pump (usually listed on the pump or its box, generally 5.5-6.5 PSI is typical). Low pressure indicates pump wear or diaphragm issues.
   • Volume Test: Often more telling than pressure for diagnosing volume starvation. Disconnect the fuel line at the carburetor and run it into a graduated container. Crank the engine for exactly 15 seconds. Multiply the captured amount by 4 to get pints per minute (PPM). A healthy pump on a 350 typically delivers 1 pint (or more) in 15 seconds (4+ PPM). Less than 1/2 pint in 15 seconds (<2 PPM) strongly suggests a failing pump.

Choosing the Right Replacement Fuel Pump

Not all mechanical fuel pumps are created equal. Selecting the correct one is crucial:

1. OEM Spec vs. Performance: Most rebuilds will be fine with a standard OEM-replacement pump. These match the original flow rate and pressure. For modified engines (larger carb, mild cam, etc.), a high-performance pump with slightly higher flow capacity might be beneficial, but rarely necessary for stock setups.
2. Inlet/Outlet Orientation: Pay close attention to the direction the inlet (from tank) and outlet (to carb) nipples point. They MUST align correctly with your existing steel and rubber fuel lines without kinking or excessive strain. Pumps are available with different port configurations (straight in/out, 90-degree bends). Compare visually to your old pump.
3. Pump Arm Profile: While generally standardized for the small-block Chevy, ensure the new pump's lever arm length and profile look identical to your old one. A mismatch could affect engagement with the camshaft lobe.
4. AC Delco vs. Aftermarket Brands: AC Delco is the traditional GM/OEM supplier. Brands like Carter, Airtex, Mr. Gasket, and Edelbrock also produce quality pumps. Reputable parts stores will cross-reference based on your engine/application.
5. Material: Cast iron housings are traditional. Some aftermarket pumps use aluminum housings, which are lighter but can be more susceptible to cracking if overtightened or impacted.
6. Fuel Compatibility: If you regularly use ethanol-blended fuel (E10), ensure the replacement pump specifically states compatibility with ethanol. This usually relates to diaphragm material.

Tools and Parts Needed for Replacement

Gather these before starting:

• New Fuel Pump: Ensure it's correct for a 1970 Chevy 350.
• New Fuel Pump Gasket(s): Usually included with the pump. Crucial for a proper seal. Some pumps use one gasket against the block; others might use a thin paper gasket and a thicker insulating spacer block, requiring gaskets on both sides. Know your setup.
• Wrenches and Sockets: Typically 1/2" drive sockets and wrenches in sizes like 1/2", 9/16", and 5/8". A small socket set and combination wrenches are essential. A long extension may be needed depending on access.
• Screwdriver: Flathead and/or Phillips for hose clamps (if applicable).
• Drain Pan: To catch spilled fuel and coolant (if removing bypass hose).
• Shop Towels/Rags: For cleanup.
• Silicone Grease or Motor Oil: To lubricate the pump lever arm before installation.
• Small Container & Funnel: To manage fuel spillage.
• Thread Sealant (Optional but Recommended): For the bolts securing the pump, especially if they thread into the water jacket. Check the pump bolt holes – if coolant comes out when you remove the bolts, they do thread into the water jacket. Use a non-hardening sealant like Permatex High Tack or Aviation Form-A-Gasket on these bolt threads. If the holes are dry, sealant isn't strictly necessary for the bolts, but the gasket must seal properly.
• Safety Glasses and Gloves: Essential.
• New Rubber Fuel Hose (Optional but Recommended): Consider replacing the short sections of rubber hose connecting the pump to the hard lines if they look old or cracked. Use SAE J30R9-rated fuel hose.
• New Hose Clamps (Optional but Recommended): Constant-tension (spring) clamps or fuel-injection style screw clamps are better than standard worm-drive clamps which can cut into the hose.

Step-by-Step Guide to Replacing the 1970 Chevy 350 Fuel Pump

SAFETY: Disconnect the NEGATIVE (-) battery cable before starting any work. Relieve fuel system pressure by cranking the engine briefly after disconnecting the coil wire (to prevent starting). Be prepared for some fuel spillage. Ensure ignition is OFF.

1. Access: The fuel pump is located low on the passenger side of the engine block, near the front and slightly below the cylinder head. It may be somewhat obscured by alternator brackets, the distributor, or other components. You might need to remove an air cleaner assembly for better access. Determine if working from above, below, or a combination will be best.
2. Disconnect Fuel Lines:
   • Identify the fuel inlet line (coming from the tank at the rear) and the outlet line (going towards the carburetor at the front) connected to the pump.
   • Place the drain pan under the pump area. Use wrenches to carefully loosen the fittings connecting the fuel lines to the pump nipples. Be prepared for fuel spillage. Sometimes the lines are connected with a flare nut directly to the pump; other times, short pieces of rubber hose connect steel lines to the pump – loosen the hose clamps and pull the hoses off.
   • Once disconnected, gently move the lines aside. Cap or plug the lines from the tank temporarily to minimize leakage if needed (a vacuum cap or golf tee can work in a pinch).
3. Disconnect Bypass Hose (Critical Step!): The 1970 Chevy 350 fuel pump typically incorporates a heat exchanger. You will see a metal pipe (about 5/8" diameter) bolted to the underside of the pump housing with a single small bolt or nut. This pipe connects to the engine's cooling system via a short length of larger diameter heater hose. This system uses engine coolant to warm the fuel pump body slightly, preventing vapor lock and aiding cold starts.
   • Loosen the clamp securing the heater hose to the pipe under the pump. Carefully pull the hose off. Coolant will likely spill out – have your drain pan ready!
   • Unbolt the small bolt holding the metal pipe bracket to the pump housing. The pipe can now be moved aside slightly with the pump, but leave it connected to the heater hose hanging down for now.
4. Remove Mounting Bolts: Locate the two bolts securing the fuel pump body to the engine block. They are usually large (1/2" or 9/16") bolts running horizontally into the block directly above and below the lever arm slot. Slowly loosen and remove these bolts completely. Important: Note if coolant or oil comes out from the bolt holes as they are removed. This indicates they thread into the engine's water jacket (coolant) or oil passages, and you MUST apply thread sealant when reinstalling the bolts later.
5. Remove Old Pump: With bolts out and fuel/coolant lines disconnected, carefully wiggle the pump straight outwards from the block. As you pull it free, the pump lever arm will disengage from the camshaft lobe. Be mindful of the metal bypass pipe still attached – it might be stiff.
6. Remove Pipe from Old Pump: Now, with the pump off the engine, unscrew the large nut connecting the bypass pipe to the bottom of the old pump housing. The pipe should separate from the old pump. The rubber heater hose will still be attached to the other end of the pipe. If replacing the hose is practical due to age or damage, now is the time (requires draining more coolant temporarily). Otherwise, you'll need to transfer this pipe assembly to the new pump. You may also see a small gasket between the pipe and the pump housing – replace this if damaged or if a new one is provided.
7. Clean the Mounting Surface: Using shop towels and a scraper carefully if needed, thoroughly clean the pump mounting surface on the engine block. Remove all traces of the old gasket material and any debris. Ensure the bolt holes are clean. Wipe the surrounding area to prevent dirt from falling into the engine or onto the new gasket.
8. Prepare and Install the Bypass Pipe on the New Pump:
   • Compare the new pump to the old one to ensure it matches. Verify correct port orientation.
   • Obtain the correct replacement gasket (thin paper ring) for the bypass pipe connection, if included or if the old one is damaged.
   • Thread the bypass pipe into the port on the bottom of the new pump housing and tighten securely (use a wrench on the pipe flats). Ensure the gasket is seated correctly.
   • Apply a thin film of silicone grease or clean engine oil to the entire surface of the pump lever arm where it contacts the camshaft lobe.
9. Apply Thread Sealant: If the pump bolts were removed from holes connected to coolant or oil passages (i.e., if fluid came out), apply a thin, non-hardening thread sealant (like Permatex High Tack or Aviation Form-A-Gasket) to the threads of the bolts. Avoid getting sealant on the bolt head or the underside of the head.
10. Position New Gasket and Pump:
    • Place the new pump gasket (and spacer block if your pump uses one) over the bolt holes, aligning perfectly with the block mounting surface and port openings. Do NOT use any additional sealant on this gasket surface unless specified by the pump manufacturer – the dry gasket is designed to seal. Spacer blocks often require gaskets on both sides.
    • Carefully insert the lever arm of the new pump into the slot in the engine block. The camshaft lobe rotates constantly; you need to align the lever arm directly against the rounded low point (base circle) of the lobe for easiest installation. The lever arm must sit flat against the camshaft lobe surface before tightening. This might take slight wiggling or turning the engine manually via the crankshaft bolt using a socket/wrench (ignition OFF!). Forcing the pump can bend the lever arm.
11. Reinstall Mounting Bolts: With the pump lever arm correctly seated flat against the camshshaft lobe (it should sit with slight pressure against the lobe due to its internal spring), start the mounting bolts by hand. Ensure they turn easily – cross-threading can ruin the block threads. Once both bolts are started, tighten them evenly and firmly using a crisscross pattern. Follow the torque specification if provided (usually "snug" – around 15-25 ft-lbs). Avoid overtightening, as it can crush the gasket or crack the pump housing, especially aluminum ones.
12. Reattach Bypass Hose: Push the heater hose back onto the pipe stub attached to the bottom of the new pump. Ensure it goes on completely. Tighten the hose clamp securely in its original position.
13. Reconnect Fuel Lines: Reconnect the outlet (to carb) and inlet (from tank) fuel lines to the corresponding ports on the new pump. Ensure flare fittings are tight or hose clamps are properly positioned and securely tightened. Avoid kinks in any rubber hose sections. Ensure any new rubber hose used is fuel-rated and clamped tightly.
14. Final Checks: Double-check all connections – fuel lines, bypass hose clamp, pump mounting bolts. Visually inspect for leaks. Ensure no tools or rags are left in the engine bay. Reconnect the battery negative cable.
15. Priming and Starting: Before attempting to start, it's helpful to prime the fuel system to reduce cranking time. Pour a small amount of fresh fuel (a few ounces) directly into the carburetor float bowl vent(s) if accessible. This gives an initial charge.
    • Turn the ignition key to the "ON" position for a few seconds (no need to crank yet). This allows the ignition system to energize but won't help mechanical pump priming.
    • Now crank the engine. It may take 5-15 seconds of cranking (in short bursts of 10-15 seconds with rest periods for the starter) for the new pump to draw fuel up from the tank. Crank until you hear the engine attempt to fire, then briefly pause before cranking again if necessary. Be patient. Pumping the gas pedal during cranking can help activate the carburetor accelerator pump.
16. Check for Leaks: Once the engine starts and runs, carefully inspect the fuel pump mounting surface, fuel line connections (inlet and outlet), and the bypass hose connection under the pump. Look closely for any signs of fuel or coolant leaking. Pay close attention! Fuel leaks are a major fire risk. If you see any leaks, shut off the engine IMMEDIATELY, correct the connection, and retest.
17. Test Drive: After ensuring no leaks and stable idle, take the vehicle for a short, gentle test drive. Listen for any unusual sounds and check that the engine responds normally to acceleration without hesitation or stumbling. Ensure it doesn't stall when coming to a stop.

Post-Replacement Considerations and Maintenance

• Monitor: Keep an eye on engine performance and fuel consumption for the first few days. Check occasionally for leaks around the new pump installation.
• Oil Check: If you suspect the old pump's diaphragm was leaking fuel into the oil (overfull level, gas smell), change the engine oil and filter IMMEDIATELY after replacing the pump. Driving with contaminated oil causes severe damage.
• Fuel Filter: Replacing an old mechanical fuel pump is an ideal time to also replace the fuel filter (usually located near the carburetor inlet or inline). Debris from the failing pump or old lines could clog it.
• Gas Tank & Lines: Consider the age of the fuel tank and lines. Sediment, rust, or debris buildup in the tank or old lines can damage the new pump by restricting flow or causing valve wear. If issues persist after pump replacement, inspect or replace filters, lines, and consider tank cleaning.
• Fuel Quality: Using fuel with stabilizers, especially for vehicles driven infrequently, can help prolong the life of pump components. Opt for "Top Tier" gasoline if available. While designed for newer cars, it often has enhanced additives beneficial for older fuel systems.
• Spare Parts: Consider keeping a spare pump and gasket set specific to your 1970 Chevy 350 in your garage. They are relatively inexpensive and offer peace of mind on longer drives or classic car tours.

When to Seek Professional Help

While replacing the mechanical fuel pump on a 1970 Chevy 350 is a manageable task for many home mechanics, consider professional assistance if:

• You are uncomfortable working with fuel lines due to the fire hazard.
• Access is exceptionally difficult on your particular vehicle setup.
• Diagnosing the problem seems inconclusive even after testing.
• Bleeding the fuel system or getting the engine started afterward proves impossible.
• Significant coolant loss occurred during the bypass hose disconnect and refilling/bleeding the system seems complex.
• You suspect oil dilution from a ruptured diaphragm necessitates a thorough oil change process.

Conclusion

The mechanical fuel pump is a vital, straightforward component on your 1970 Chevy 350. Recognizing the symptoms of failure – particularly sputtering under load, hard starting, oil dilution, and visible leaks – allows you to diagnose issues quickly. Replacing it is a fundamental repair requiring careful preparation, the correct parts, attention to the critical bypass hose, and proper installation to ensure the lever arm engages correctly. By performing this maintenance proactively or correctly addressing a failure, you maintain the reliable fuel delivery essential for the smooth performance and longevity of your classic Chevy engine. Regular checks of related components like fuel lines and filters contribute to the overall health of your fuel system for many more enjoyable miles.