5 tips to keep a classic fuel injection system running well

By Staff Writer
Sep 28, 2024 | Tech Tips, fuel injection

Vintage car buffs may scoff at modern technology like fuel injection, but the truth of the matter is that fuel injection has been around nearly as long as the venerable carburetor. Experiments in fuel injection began early in the 1900s, and the technology became available on commercial diesel engines by the 1920s. From there it saw use in airplanes during WWII, where fuel injection yielded better power and reliability in the heat of battle.
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Bosch pioneered the use of mechanical fuel injection in gasoline-powered cars with the introduction of the Mercedes Benz 300 SL in 1955, while Porsche started using Bosch’s MFI in 1969 for their 911E and 911S. BMW and Alfa entered into the realm of mechanical fuel injection with their own versions, Kugelfischer and SPICA, respectively, which were aimed at delivering top performance with an eye toward ever-tightening emissions regulations.

The mechanical pump, driven by a cogged belt off of the crankshaft, acts as a metering device to precisely deliver the appropriate amount of fuel to each cylinder during the firing order. Linkages from the throttle give an indication of load, while the speed of the pump varies with the rpm of the engine itself. The MFI pump is fed by a high-pressure fuel pump further upstream, while injectors on each cylinder deliver the fuel to the combustion chamber in an atomized spray.

Mechanical fuel injection gave way to the modern era of electronic fuel injection, where sensors and small computers determine the proper amount and timing of the fuel delivery. The computer signals injectors with miniature electronically operated valves to open and close, which then squirts fuel sent from a high-pressure fuel pump. The 1967 Volkswagen 1600TL, with its Bosch D-Jetronic, featured one of the first uses of true electronic fuel injection; from there, it and other similar systems were adopted industrywide throughout the subsequent decade.

Here are some basic tips to keep your fuel injected classic running in tiptop shape:

1. Insepct Fuel Lines

Fuel injection operates at a much higher pressure than a carburetor, on the order of 60 or more psi versus less than 7 psi. This higher pressure can cause small leaks to become major problems as the highly atomized fuel sprays out. This can turn a classic into a fireball in seconds.

Inspect all of the fuel injection lines and fittings for leaks on a regular basis. Pay particular attention to flexible rubber lines. If in doubt, replace rubber lines with new ones that are rated for fuel injection. Do not use low-pressure fuel lines on an injected car. Always use top-quality hose clamps that do not cut into the rubber as they are tightened.

2. Check for Leaks

MFI, Kugelfischer and SPICA pumps can wear out and leak over time. This leaking will often lead to a crankcase full of gasoline instead of oil. In addition to the explosion hazard, the diluted oil won’t properly lubricate your engine, leading to scored and seized bearings in short order.

Fortunately, there are specialists that can rebuild MFI, Kugelfischer and SPICA pumps to like-new status for about what it costs to rebuild a pair of Webers. Some rebuilders will even allow you to specify what fuel curves you would like the pump to deliver, based on the modifications to the rest of the engine. Unless you are extremely familiar with the workings of your fuel injection system, resist the urge to tamper with the rebuilder’s pump settings, and follow the factory-specified techniques for adjusting the air and fuel mixture of the engine. Unlike a carbureted engine, once the mixture is correctly adjusted, the fuel injection system shouldn’t need too much more attention.

3. Replace Fuel Filters Often

Keep your fuel clean by replacing fuel filters on a regular basis. It sounds elementary, but the high pressure of the fuel injection system can quickly clog a filter if there is any crud in the gas tank. If your tank has any rust on the inside, get it flushed and treated before you drive the car. A contaminated tank will clog everything that lies downstream.

4. Professionally Have Injectors Cleaned and Blueprinted

Get your injectors cleaned and blueprinted by a professional. Like a clean set of properly sized carburetor jets, injectors are crucial to performance. Dirt and varnish can clog an injector, making it impossible to get a proper spray pattern. Have your injectors cleaned by a shop like Marren Fuel Injection or RC Engineering.

5. Look for Vacuum Leaks

Fuel-injected cars often use vacuum sensors to sense load on the engine. This means that vacuum leaks can play havoc with proper performance. Regularly inspect all of the intake hoses and vacuum lines. A quick shot of carburetor cleaner on the outside of a suspect hose will help you locate any leaks: a quick rise in rpm after a blast of the cleaner will indicate a leak.