So, you walked out to your classic car turned the key and while the car turns over, it just will not start; and maybe in need of replacing a fuel pump. For a combustion engine to run, it is in need of four things. These include the following:
- Air
- Fuel
- Compression
- Spark
If any of these items are missing the car will not run.
These elements are all part of the Otto Cycle. If this is the case, then something is missing from the equation. The first person to build a working four-stroke engine, a stationary engine using a coal gas-air mixture for fuel (a gas engine), was German engineer Nicolaus Otto. This is why the four-stroke principle today is commonly known as the Otto cycle and four-stroke engines using spark plugs often are called Otto engines.
In a future blog we will address the four cycles of the modern combustion engine, but right now we will discuss the second element in the above list and work at getting you classic car back on the road.
How does a mechanical Fuel Pump Work?
A mechanical fuel pump is driven by the camshaft, or by special shaft driven by the camshaft. As the shaft turns, a cam passes under a pivoted lever and forces it up at one end.
The other end of the lever is linked loosely to a rubber diaphragm forming the floor of a chamber in the pump, goes down and pulls the diaphragm with it.
When the lever pulls the diaphragm down, it creates a suction that draws fuel along the fuel line into the pump through a one-way valve.
As the revolving cam turns further, so that it no longer presses on the lever, the lever is moved back by a return spring, relaxing its pull on the diaphragm.
The loosely linked lever does not push the diaphragm up, but there is a return spring that pushes against it.
The diaphragm can move up only by pushing the fuel from the chamber. Since the fuel cannot exit through the one-way valve in entered through above, it exits through another one-way valve and it pushed up to the carburetor.
How to Change a Mechanical Fuel Pump
Replacing a fuel pump can be a messy job, so be sure you are dressed appropriately. Do not be nervous though it is relatively easy you just want to stay organized and make sure you have all the necessary tools ready for the job. You will need the following common tools:
- Ratchet wrench set with universal joint.
- Tubing or open-end wrenches
- Hose clamp pliers or screwdriver
- Bolt or wooden dowel
- Shop rags
- Gasket sealer
- Fire extinguisher rated for gasoline fires (This is just a precaution and good to have around any shop or garage. It is always best to be prepared.)
Remember, you will be working around fuel and fuel vapors, so be sure that your workspace is well ventilated. Do not smoke, use an open flame, or do anything that may cause sparks or otherwise present a safety hazard.
Replacing A Fuel Pump
Once you have gathered your tools, turned off your vehicle, and made sure you are working in a safe area, you can begin working. First, you will need to remove the old fuel pump in this order:
- Disconnect the negative battery cable.
- Disconnect the fuel tank hose at the fuel pump and plug the hose with a bolt or wooden dowel to keep any fuel from flowing out. Also, disconnect the vapor-return hose if the vehicle is equipped with one. Be sure to wipe up any gas that spills.
The fuel pump is generally located on the front passenger side of the engine. However, in the photo below you will see it is located on the front driver’s side, as the fuel pump that was being replaced was on a1964 Pontiac Lemans with a 326 Cu.In. V8.
- Carefully examine the old fuel hose; if it is frayed or cracked, replace it with new fuel line hose.
- Disconnect the outlet line to the carburetor. Use a wrench on the fuel pump fitting and another on the line nut.
- Remove the two attaching bolts and extract the old fuel pump. Clean off any old gasket material from the mounting surface of the engine using a shop rag.
Once the old fuel pump has been removed, it is a good idea to compare the old unit with the new just to make sure they are compatible and all the holes line up.
So, they are a match! It’s time to prep and install the new unit in this order:
- Apply a coat of gasket sealer on both sides of the new gasket. Put the attaching bolts through the new pump and slip the gasket over the bolts.
- Install the new pump on the engine. Make sure that push rod is correctly installed in both the engine and the fuel pump. If the push rod slides out, you can pack it with some heavy grease to hold it in place while you install the pump.
- Attach the fuel outlet line that runs to the carburetor. If it’s difficult to connect, remove the other end of the line from the carburetor. Connect the line to the fuel pump, and then reattach the other end to the carburetor. Use a wrench to hold the fuel pump fitting and tighten the line nut with another wrench.
- Attach the fuel inlet hose from the gas tank and the vapor-return hose. Tighten all clamps.
- Reconnect the battery ground cable, start the vehicle, and check for leaks.
Once you have inspected your work and made sure it is free of leaks, your vehicle is good to go. Unless of course it still does not start, in which case it is time to look at some of the elements like compression or more likely spark.
Jamie Pappas
