For more information about classic fuel injection systems, check out our 1984 VW Rabbit GTI project car. As part of the restoration process, we had to repair the GTI's old-school Bosch CIS system.
By Staff Writer
Sep 28, 2024
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Tech Tips,
fuel injection
| Posted in
Drivetrain
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Restoration & Renovation
| From the May 2007 issue
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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.
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:
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.
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.
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.
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.
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.
Minor additions to this great write-up:
For future reliability, change all fuel lines to braided stainless Teflon lined hose. It is impervious to virtually all fuels including ethanol. Most stainless hose is rubber lined, so be careful. Get rid of that rubber if you are going to used pump gas.
Don't be scared to tune mechanical injection with a wide band set up. If possible, check every cylinder separately. You can change injector size to match flow rates to individual cylinders. The aircraft industry has been doing this for years.
Thanks again MM for the great article!
i thought this thread was gonna be about USA HILBORN fuel injection, came out around 1949-50, before Mercedes 300sl, but wiki says some german engines used it in the early 1930S, or earlier!
are we talking port injection or direct chamber injection?
more info about fuel injection, little known fact is Electronic fuel injection was pioneered and patented by a USA company,BENDIX corp. i actually drove a 1957 AMC Rambler REBEL with efi in 1957, it was fairly good(not perfect), but Transistor technology was in its infancy!
and BOSCH thought well of it and bought license and patent rights from Bendix.
thats just KOOL.
Very important for early FI cars is to use non-ethanol gas too. Pure-gas.org.
In reply to CFreeborn :
I agree with you. This should be the first thing on the list.
An add-on to a good article. If you got disgusted with your factory EFI and retrofitted Webers or a Holley, do NOT expect the EFI fuel pump to work as a plug-in. At 50 psi regulated down to 5 psi, the EFI pump will thermally overload, shut off and you'll spend quite a bit of time alongside the highway waiting for it to cool down & reset
At least on a SPICA. Other brands may just pop a fuse so its tow-truck time for you.
A friend owned a Maserati 3500 GT with injection and Mercedes used it on the 300 SL back in the early 1950s.
Triumph even used it on the TR6 in the home market but North America had to make do with carbs and instead of 150 bhp they got 104 bhp.
Relieve pressure in line first; D/C battery, pull fuel pump fuse, start engine and run till it dies. Now disconnect fuel filter hoses and replace filter. Otherwise, you will get gas spray all over if you disconnect with pressure in the line.
After just going through a Porsche CIS system, I would add to the above the importance of getting a smoke machine and looking for leaks. And to keep going, after fixing leaks, to smoke test again until all sealed. My box was fine. But I had some rubber things that needed replacing or tightening. And while going through that, I never noticed the smoke coming out of the throttle shaft until the others were fixed.
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