11 April 2017

How to make your classic car’s fuel delivery system dependable

A few weeks ago, I introduced the idea that, when a vintage car dies while being driven, most of the time the cause isn’t some random lightning bolt from the blue, but is instead one of The Big Six (ignition, fuel delivery, cooling, charging, belts, ball joints). This week we’ll drill down into the fuel delivery system.

On a vintage carbureted car, the fuel delivery system is simple, consisting of just a fuel tank, rubber and metal fuel send lines, a low-pressure fuel pump, a fuel filter, and one or more carburetors. Mechanically fuel-injected cars from the ‘50s, ‘60s, and ‘70s add an injection pump or a fuel distributor, injectors, a high-pressure fuel pump, a pressure regulator, and a fuel return line. Electronically fuel-injected cars kick the level of complexity up significantly, incorporating an electronic control unit (ECU) and a variety of sensors.

But no matter which configuration you have, it all starts with clean gas being pumped at the right pressure through leak-free lines.

Check for Leaks!

You can have a little coolant or oil dripping, but not fuel. Because gas is so flammable, there needs to be a zero-tolerance policy. If the rubber fuel lines and clamps in your vintage car are original or haven’t been changed in, well, you don’t know how long, they should probably be replaced. In addition to the rubber lines themselves ballooning or cracking, the bite of the rubber against whatever metal tube it’s attached to, and the bite of the hose clamp into the rubber, weaken with age, allowing leaks to form.

Further, on a fuel-injected car, the fuel pump pressure is high enough that it won’t just leak a little—it’ll spray fuel. Conversely, on carbureted cars, old braided cloth fuel lines at the gas tank can sometimes be porous enough that, during a springtime start when the float bowls are dry, the low-pressure mechanical fuel pump will try to pull gas out of the tank and instead will suck air through the lines, preventing the float bowls from filling up. So, replace those old fuel lines!

On fuel-injected cars, there are also rubber seals to check. I don’t mean the rubber o-rings at the injectors; these typically don’t cause fuel to leak out. On mechanically-injected cars such as my 1972 BMW 2002tii, old rubber seals at the injection pump can be a major source of fuel leaks, spraying fuel everywhere when they deteriorate. Changing these when you change the rubber fuel lines is good insurance.

leaking 2002tii injection pump seals
Leaking o-rings in the injection pump of a BMW 2002tii

Clean Gas: Performing a Full Back-To-Front Sort-Out

If a car has been sitting for years, the fuel system will almost always require attention. A primary reason is that gasoline itself is problematic. Gas that sits can do one of two things. It can slowly evaporate, leaving a gummy varnish. Or, if it’s oxygenated fuel with ethanol in it, it can absorb water. Gas tanks in vintage cars are usually steel, so water will cause the tank to rust. If a car is run with a rusty tank, the fuel pump will send the rust toward the engine. At a minimum, it can clog the fuel filter and any small mesh screens. If rust gets past these, it can clog up the jets and passageways in a carburetor, or the fuel injectors in an injected car.

rusty injector
Rust-contaminated injector in a BMW 2002tii

A back-to-front fuel system sort-out is flat-out necessary in a long-dormant car, but is also great prophylactic maintenance in a driver before a long trip. First, open up the gas tank, and smell, then look. If it smells like varnish, you need to drain the tank and clean the varnish out with solvent. Be sure to check the pick-up tube that sends the gas to the engine, as this may also be clogged with varnish.

If, on the other hand, the gas smells like, well, gas, look through it with a flashlight for rust and sediment in the tank. If fuel with ethanol in it has absorbed water, though, you can’t really see that. For this reason, you should drain whatever old gas is in the tank of a car that’s been sitting, even if you don’t see or smell anything amiss. And you’ll often be surprised at the amount of rust and sediment that comes out.

Next, remove every rubber fuel line, then use a compressor and an air nozzle to blow through the metal line that runs to the engine. Wrap a clean rag around the end of the line so you can see what’s coming out. If it’s rusty, continue to blow until it’s not. Replace with new rubber fuel lines.

Remove the fuel filter, tap the inlet onto a paper towel to look for rust contamination, and replace it with a new filter. If the car has any fuel screens, be certain to check and clean them.

If the fuel pump is old, or if there’s any evidence of rust in the fuel tank or filter, just replace it. I’ve worked on cars where the tank was cleaned but the fuel pump wasn’t replaced, and the pump was full of rust and continued to spit it forward.

fuel pump rust mound #2
Rust in a BMW 2002 electric fuel pump.

If the car is carbureted and you found rust in the gas tank, pull the top off the carb and check for rust in the float bowl.

Lastly, connect a pressure gauge with a tee to the fuel line where it enters the engine, start the car, check for leaks, and verify the correct fuel pressure for your model.

Sudden Death

If your car is driving down the road one moment and dead in the breakdown lane the next, and it has spark, make no mistake about it, the prime suspect is the fuel pump. Mechanical fuel pumps in carbureted cars use a pushrod, usually run off a lobe from the camshaft, to push a lever that moves a diaphragm. The diaphragm can tear or lose its elasticity, or the little lever can wear down. In an electric fuel pump, the fuel itself actually cools the windings of the electric motor. This makes electric fuel pumps susceptible to contamination in the fuel tank. If an electric fuel pump dies, sometimes it’s bound up with rust or sediment, and rapping it with a ratchet wrench handle (or, if it’s an in-tank fuel pump, smacking the bottom of the fuel tank) will dislodge the rust and get it going again for a while. Replace it as soon as you can.

An electric fuel pump is usually controlled by a relay, and relays do sometimes die. Look up where your relay is, and make a little jumper wire so that, if the fuel pump dies, you can pull out the relay, jumper pin 30 (power) to 87 (fuel pump) to bypass the relay, and see if that gets you going.

Slow Death

There’s an old saying: If a car feels like it is running out of gas, it probably is. If you’re certain it has gas in the tank and an even spark, the problem could be that the fuel filter or fuel screens are clogged. A textbook symptom of this is when a car sputters and dies, then starts and runs five minutes later and repeats the process. The fuel pump is probably pushing rust up against the filters and screens, and when shut off, some amount of it is falling off. At a minimum, change the filters, clean the screens, and, as soon as possible, check for contamination in the tank.

Clean gas and screens. New lines and filters. New fuel pump. Get on down the road!

(Next week: The cooling system.)

Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel Magazine for 30 years. He is the author of Memoirs of a Hack Mechanic and The Hack MechanicGuide to European Automotive Electrical Systems. Both are available from Bentley Publishers and Amazon. Or you can order personally inscribed copies through Rob’s website: www.robsiegel.com.

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