This guide, originally penned by the late Dave Dubois, a renowned expert in MG car mechanics, provides a comprehensive approach to troubleshooting SU mechanical fuel pumps commonly found in classic cars, especially MG models. While written for MGs, the principles apply to any vehicle equipped with these pumps.
Identifying the Problem: Symptoms and Initial Checks
A sudden engine stumble or complete stall, coupled with the absence of the characteristic “tick, tick, tick” of the fuel pump, often signals a fuel delivery issue. Before delving deeper, Dubois recommends a few preliminary checks:
- Gentle Tap Test: Gently tapping the fuel pump (located under the bonnet or near the right rear tire depending on the model) with a screwdriver handle might temporarily restore function if the pump is simply stuck.
- Check for Tank Venting: A clogged fuel tank vent can prevent fuel flow. Remove the gas cap and listen for a whoosh of air. If present, the vent was likely the culprit. Replace the cap if necessary or clear the vent line if your car utilizes a charcoal filter system.
- Fuel Line Test: Disconnect the fuel line from the rear carburetor and direct it towards the ground. Turn the key on. Fuel flow indicates a carburetor problem (e.g., stuck float). No fuel flow points to a problem further upstream.
Diagnosing the Fuel Pump: Electrical and Ground Connections
(Replace this placeholder with an appropriate image from the original article or find a suitable one online showing a multimeter testing a fuel pump)
Once back in the garage, more thorough diagnostics are required:
- Voltage Check: Use a multimeter to verify 12V power at the pump’s terminal with the key on. Absence of voltage necessitates tracing the wiring back to the source using a wiring diagram.
- Ground Integrity: Confirm a solid ground connection using a multimeter or by temporarily connecting a known good ground to the pump. A faulty ground requires tracing and repairing the ground wire (often located under a license plate mounting bolt in the trunk on MGBs). Dave Dubois emphasized the importance of proper ground point preparation in a separate article.
Isolating the Fault: Fuel Lines and Internal Pump Issues
If power and ground are good, further investigation is needed:
- Clogged Fuel Lines: With fuel lines disconnected from the pump, turn the key on. If the pump doesn’t run, refer to a professional for repair or restoration. If it runs, reconnect the tank-to-pump line and retest. Continued operation indicates a clog in the carburetor line. If the pump stalls, the clog lies in the tank-to-pump line or within the tank itself.
- Air Leaks: A continuously running SU pump (not aftermarket types like Facet) with no fuel delivery suggests an air leak on the inlet side or an empty tank. Check the fuel level and inspect the inlet line for leaks, particularly flexible sections. If no external leaks are found, try drawing fuel from a separate container. Success points to a possible hole in the tank’s pickup tube. Continued running without drawing fuel indicates an internal pump leak requiring professional repair.
Considering Repair or Replacement
While SU pumps are durable, they eventually wear out. Before discarding a suspected faulty pump, consider professional repair or restoration. Several specialists listed in Dubois’ original article offer these services. Remember, aftermarket pumps also have a finite lifespan, often comparable to SU pumps.
Conclusion: Dave Dubois’ Lasting Legacy in Tuning MG Car Power
Dave Dubois’ troubleshooting guide offers invaluable insights into diagnosing and resolving SU fuel pump issues, a crucial aspect of tuning MG car power and ensuring reliable performance. His methodical approach empowers classic car owners to tackle these problems systematically, minimizing frustration and maximizing the enjoyment of their cherished vehicles. Remember to consult a qualified mechanic if you are uncomfortable performing any of these procedures.
