This example is the electronic fuel injection (EFI) system in place on the engine of a 1973 450SL. It looks complicated, but makes more sense when you review at the components one by one.

Bob thinks his 1969 and 1972 Mercedes-Benz cars perform well because they have Bosch D-Jetronic fuel injection. John blames the problems with his 1973 on that same system and wishes he’d never bought the car. Why the difference?

Bob never has any trouble starting his 1969 Mercedes-Benz 280SE 4.5 or his 1972 350SL. A quick bump of the key and the engine fires up without a hiccup, then settles down to an 800-rpm idle. He attributes their performance to the Bosch D-Jetronic fuel injection system with which they are equipped.

Meanwhile, when John tries to start his 1973 450SE, even after pressing the accelerator several times, it fires up hesitantly and struggles to idle at 600 rpm. He is certain that the problems originate with the D-Jet system and regrets buying the car.

These three cars have the same engine, the same electronic fuel injection (EFI) system, and the same degree of love and attention. Why is it that Bob is happy and John isn’t? Perhaps it’s simply because the system in John’s car needs to be adjusted or repaired properly and he hasn’t yet found a mechanic who knows how to diagnose and fix its problems.

What Is D-Jetronic?

Bosch D-Jetronic electronic fuel injection (D-Jet, or EFI, for short) is a computerized fuel injection system used on Mercedes-Benz M116 (3.5 liter) and M117 (4.5 liter) V-8 engines manufactured for U.S. distribution between September 1969 and August 1975. The M114 (2.5 liter) and M110 (2.8 liter) 6-cylinder cars sold in Europe from 1969 to 1976 had the same system, but we will focus on the U.S. cars for this article.

The components of this system are basic and simple. Above is a layout of the EFI engine on a 1973 450SL.

The system consists of eight very important components. These are, in order of most to least complex: the fuel injection control unit, the manifold absolute pressure sensor, the injection trigger points, the injection throttle switch, the fuel pressure regulator and damper, the fuel injectors (eight of them), the fuel pump and filter, and the fuel pump relay. All of these units must be in good order for the system to work properly.

• Fuel injection control unit. This device, also known as the injection control unit or EFI control unit, has a very large plug connector that feeds the EFI harness. It takes input from the injection trigger points, the manifold pressure sensor, the engine temperature sensor, and the injection throttle switch. It sends a signal to the fuel pump and relay, the injectors, and a cold-start injector to regulate fuel delivery to the system and, in turn, injection of fuel into each cylinder in the proper amount and at the proper time in the ignition cycle.

Failure of the control unit is rare, but the large plug should always be checked first, as well as the power supply from the positive battery terminal. Often, water can contaminate these units or corrode the plug connections, making them less reliable. Otherwise, they give decades of excellent service.

These have a mixture adjustment knob on them. If the knob is turned clockwise, then the fuel mixture becomes richer (and a little smoother at idle).
In W111 (280SE 3.5), 108, and 109 cars (280SE and 300SEL models with V-8 motors), the controller is in the front right fender under a rubber boot, just in front of the battery.
In W116 cars (350SE and 450SE), the unit is in the inside of the right fender, accessible from the passenger-side footwell where the right leg would rest.
In W107 (SL, SLC) cars, it is underneath the dashboard on the passenger side, just about where the feet would rest, then upward.
 

                                        MAP sensor on 300SEL

• Manifold absolute pressure sensor (MAP). This device reads the amount of vacuum in the intake manifold (or lack thereof) and sends a signal to the EFI control unit. This signal is critical for determining the quantity of fuel injected, which must vary with temperature, acceleration, altitude, humidity, and so forth. Fuel mixture can be adjusted with a 6mm Allen screw in it.

                                            MAP Sensor on 450SL

The MAP rarely fails, but leakage in the vacuum hose from the manifold to the MAP will cause rich running and sometimes a no-start condition. Also, at times, inexperienced mechanics adjust the mixture at the MAP, hoping to improve idle, when the problems are elsewhere. The consequence of poor adjustment is diminished fuel economy. If you have a car with otherwise healthy injectors, and it is running very rich, producing less than 11 mpg, the MAP sensor is usually at fault. Do not adjust it if it has never been touched. Otherwise, it should be adjusted with the engine warm, at idle, until the engine idle speed peaks.
On W111, 108, and 109 cars, it is found on the firewall just behind the engine.
On W116 and W107 cars, it is found in the left fender on the wheel arch.

• Injection trigger points. Injection trigger points, located in the bottom of the distributor, are triggered with engine rotation. This means that any adjustment to ignition timing also affects injection timing. Genius!

Each trigger point fires two injectors at a time, which means that each injector has a pilot trigger, one before the intake valve opens and one when the intake valve opens. The EFI control unit makes sense of these signals.
When these fail, the car likely won’t run. If it does run, it will consistently miss on several cylinders. However, idle speed and low-rpm running become erratic as they wear, often leading to a very poor low-speed range on EFI motors.

• Injection throttle switch. This is another unit that never really fails but is often misadjusted. This unit sends a signal to the EFI control unit to open all of the injectors when the throttle is depressed. It also sends a signal to the EFI control unit, based on throttle position, to tell it how long to keep the injectors open. The more you push, the greater the quantity of fuel sprayed. The easy way to test this unit is to turn on the ignition, depress the throttle, and listen for a squealing sound from the injection rail. This is the sound of all the injectors opening at once. There is an adjustment on this switch, but it rarely needs to be touched.

• Fuel pressure regulator and damper. The fuel system has a pressure regulator to ensure that pressures don’t go too high or too low. It also has a damper to keep the fuel flow smooth and free of bubbles and vapor. The pressure regulator keeps the fuel system at a mean of 30 PSI, the ideal pressure. As age and time take their toll, we find that sometimes pressures up to 38 PSI are needed.

This device, which rarely fails, can be checked by plumbing a fuel pressure gauge into the fuel distribution rail (illustrated in photograph on page 70). The regulator can be adjusted with a 10mm wrench. Outward adjustment of this screw lowers the pressures, and inward adjustment raises them.
 

Fuel injectors are blue in this picture, each attached to the common fuel rail.

• Fuel injectors. The eight fuel injectors are responsible for distributing fuel to each individual cylinder. They are either yellow (M116, 3.5-liter engines) or blue (M117, 4.5-liter engines). They fire according to a final signal from the EFI control unit and can open for varying amounts of time, based on throttle position, and fire individually or all at once.

Failure usually involves leakage from the tip, the body, or the supply hose. The solenoid inside the injector rarely fails. They are all tied in together with a fuel supply rail. The hoses in this rail and from the rail to the injectors should be checked frequently for leakage or cracking. Many EFI cars have burned to the ground from injector hose leakage.
 

                                   Fuel Pump in the fender well of a 450SL

• Fuel pump system. Early 3.5-liter cars had a pump with a supply, output, and return line port. The picture below  shows the fuel pump from a 4.5-liter car. On the later pumps, the return port is eliminated. Although these pumps are reliable for years, over time they can leak, fail to deliver the right amount of fuel, or simply fail to spin. They are located under the back of the car, by the tank. On W111 models, the filter is in the front left fender, under the hood, behind the headlight.

The fuel pump relay triggers the pump. It has a purge function, which causes it to run for a second while the ignition is turned on. Otherwise, it should trigger the pump when the engine is cranked or running. It receives a signal from the EFI control unit.

For a while, the location of this relay posed a quandary for me. Where was it on W111, 108, and 109 cars? If you look below the fuel injection control unit, there is a group of relays. The fuel pump relay is the most obscured one, the one in the lowest position. If you miss it, then look twice – it’s that hard to find. On W116 cars, it is in the fuse box and is one of the ice-cube relays. It should have four prongs instead of five, however. The later versions of these were dark red.

On W107 cars, the relay sits above the fuse panel, behind the cardboard cover that the fuse lid fits into. It is also an ice-cube relay. Inside the panel there are several of these. It should be the one in the middle, in the row closest to the back of the car. This is a five-prong relay. A bad fuel pump relay will keep the pump from working and, when it fails, may also cause erratic running after the engine warms up, lack of power, rough idle, and fluctuating high-rpm running.

The Fun Part:
Troubleshooting the D-Jet EFI System

The D-Jet EFI system has been plagued by a bad reputation from day one. Some of its problems tend to strike the owners of nearly any D-Jetronic-equipped Mercedes-Benz, but others are uncommon and capricious.

D-Jet systems age gracefully if the cars do not overheat or suffer exposure to excessive humidity. These forces of mechanics and nature can wreak havoc on wiring harnesses and injection control units. As these systems age, the cars do not run as smoothly, fuel economy decreases, and light missing may develop at idle. For 108/109 cars with 4.5 engines, harnesses are still available new for good prices. Replacing the harnesses can result in substantial improvements in fuel economy and running.

Injectors are a more serious issue. While a damaged harness or computer can cause a running problem, a damaged or leaking injector or fuel hose can cause an engine fire. The injector bodies – color-coded blue (4.5 cars) or yellow (3.5 cars) – have small hoses connecting them to the fuel rail. Over time, heat can and will take its toll on injector hoses and bodies, and a leaking injector hose is dangerous. If you ever buy one of these cars, inspect the hoses and bodies during the car’s running cycle, preferably just as the engine starts, and then after it’s warmed up. Hose kits are available for those who wish to repair the system.

Even as the cars age, fuel economy shouldn’t fall too far off. EFI cars were all good for 10 to 15 mpg. If you are getting 8 mpg, something is wrong and should be corrected. If you are getting 11 mpg, you may be able to do better. Somewhere around 13 to 14 mpg is excellent for these cars. If fuel economy is poor, injectors should be checked for leakage, both into the engine and on the outside, and the MAP sensor should be examined. Although you can read resistance values on these, I find it easier to swap in a working spare, as a MAP can have good resistance values but still deliver poor fuel economy and idle.

Following are case studies of several common problems, illustrating diagnosis of the problem and how to solve it.

In all situations, before making corrections to the injection system, be sure that correct plugs (Bosch W7DCO, W9DCO, or non-resistor, non-platinum equivalent) are installed, and the engine’s valves, lubrication, ignition, and cooling system are in order.

Case No. 1: The story of the misfiring 1973 450SL.

As the injection trigger points age on these cars, they can contribute to a lumpy idle and poor low-speed running, characterized by loping engine speeds and slight missing below 2,000 rpm, with these symptoms eventually followed by the car not running at all.

On a 1973 450SL, the owner complained of a miss every two to four minutes at high-speed driving, and an erratic, jerky idle when the car had warmed up. After making sure the ignition was well-sorted, we checked the injection system. On this car we had already replaced a MAP sensor, bringing economy up to 14 mpg (from 8-10 mpg), but the miss was still there, making the car less enjoyable. We replaced the trigger points with a set we knew were okay, and voila! The miss was gone. The car also idled much better, and the engine was smoother at low speeds.

Case No. 2: The 280SE 3.5 “barn find” that started and ran sporadically.

The new owner of a 1971 280SE cabriolet, which looked as if it had been found in a barn, told us that he had taken it to four different shops trying to solve the car’s problems of starting and running sporadically. All of them told him that the car needed a new harness and that it would not run properly until the harness was replaced. The harness was also blamed for the car’s worst symptom: the tendency to die without warning and fail to restart for days. After confirming that the battery connector for the EFI system was tight and the harness looked good, I began to do some searching. When the car ran, the fuel pump had power. When it did not run, the fuel pump had no power and the injector throttle switch would not activate the injectors. (Tests were run with the ignition on or the engine cranking.) I took apart the plug on the EFI control unit and found excessive corrosion on it. While I cleaned it up, I swapped in a different control unit that I knew worked and tested the system, then put the cleaned-up original control unit back in. One 250-mile trip later, I felt the problem had been solved.

Case No. 3: The asthmatic 1972 280SE 4.5 that ran poorly after running for a long time.

A mechanically sound 1972 280SE 4.5 was delivered to our garage in November 2007. This car was the victim of years of neglect. It would run well for approximately 45 minutes, then had no power and would miss or occasionally die at low speeds. To test fuel delivery problems from the pump, we installed a pressure gauge into the fuel system with enough hose to watch the gauge as we drove. After driving for approximately 40 minutes, pressures on our gauge would fall below 28 PSI on acceleration. After another 10 minutes, the pump was struggling to supply any fuel at all. The car started up okay after a five-minute cooldown and seemed to behave pretty well after that for another 20 minutes or so. But the missing, jerking, and lack of power all came back.

The pump ran fine for more than an hour on a bench with diesel fuel in circulation, so we concluded that the problem wasn’t the pump, but some obstruction in the fuel supply. After undoing the 22mm Allen head holding the strainer in the base of the tank, we removed more than two pounds of debris. After several hours of driving, the debris would plug the delivery slits in the base of the tank, leading to poor fuel delivery. When I cleaned the tank, a shower of mud and rust rained out of the drain hole.

Because of the exposure of the old fuel pump to all of this crud, I replaced it with a new Bosch unit. However, after driving the car for approximately 40 miles, the symptoms returned, worse than before. Frustrated, I took the car back to my garage. I noted that if I gave it three minutes to cool down, it would run okay for another 30 minutes. I hooked up the pressure gauge to the car again and found that the fuel pressures were abysmal.

Would you believe the new pump was bad? I put the old one back on, and the car ran pretty well after that. The lesson to be learned here is that one should always make sure that the fuel tank is clean. And never assume that a part is good just because it’s new until it has proven its reliability.

Case No. 4: The hot-start problems.

There must be a book full of myths about the EFI problem of hot-starting challenges stemming from vapor lock.

An owner brought  in a 1972 350SL 4.5 to solve its erratic and unpleasant hot-starting problems. It missed. It loped. It bucked. It wouldn’t idle for more than a few seconds, even 20-30 minutes after shutdown. If the car sat for one hour, then everything was golden, but after running at speed for a while, it would die and not restart for at least an hour.

We concluded that either there was so much vapor in the lines that the engine wasn’t getting any fuel or there had to be something seriously wrong with the engine. To diagnose the problem, we decided to conduct a fuel volume delivery test.
The EFI pump must deliver one liter of fuel in 30 seconds. There can be no exceptions to this rule. It must deliver it from the return port on the damper, and it must do it hot as well as cold. The paltry stream of fuel that the 350SL car spit at me did not amount to more than a half-liter in 30 seconds, less than half of what it needed to.

Although EFI pumps for these cars are difficult to find, I was able to source a new one. After replacing it, we had a different car, which started under all conditions.
The lesson here is that if you have hot-start problems, run a fuel volume delivery test first. If the pump can’t deliver sufficient fuel, then you need to address that matter first, before you start to worry about other problems. Also, because fuel flow has a cooling effect, good flow may solve other hot-start problems.

Sadly, the aftermarket Bosch pumps, their Japanese replicas, and many rebuilt ones even fail to meet the requirement. Also, many original pumps have been in service for 30 years or more, which means that often the only pump that will work is a new $600 unit from Mercedes-Benz. You may think this is absurd, but the tolerances to which its pumps are built exceed all others.

So what if a new pump fixes nothing? The 3.5-liter cars also tend to develop hot-start, but a new pump won’t always fix the problems. Our old cabrio had a hot-start glitch as well. For two hours after shutdown, that car would not start at all. The fuel pressure was at 34 PSI, which is high but not excessive. I tried everything from a MAP sensor to a new fuel pressure regulator and drilling a small hole in the output line of the fuel pressure regulator, as suggested by Mercedes technicians “to allow vapors to vent from the fuel distribution rail,” but all in vain.

Finally, frustrated, I decided that another 5 PSI on the fuel pressure regulator wouldn’t cause a major catastrophe, so I went out on a limb and made the adjustment.

That car started and ran like a champ. Apparently, the fuel wanted to vaporize at anything under 36 PSI. Perhaps too much heat was rising from the engine, or perhaps the car had some valve-sealing issues, but at the end of the day, it started well at all times. We did have to tighten some injector hose clamps after the repair, but hopefully your mechanic will watch for leaks.

Is D-Jet Actually Reliable?

When these cars are set up properly, the components seem to last for many years. I rarely see them arrive on tow trucks, and, despite their poor fuel economy, they always seem to have a certain efficiency in their operation that derives from the virtue of being well-engineered in the first place. In my opinion, it is still practical to select a D-Jetronic-equipped Mercedes-Benz as a daily driver. While this article may help answer some of your questions, if you do own one of these cars, seek out as much literature as you can. The old Mercedes-Benz 108/109/111/113 service manual outlines the system very well.

As a final comment, I would avoid the 1975 model-year cars, as their emissions equipment (giant catalytic converters right by the head) was more of an experiment than anything. These cars can experience problems due to the excessive heat in their engine bays. If you do purchase one, plan on fitting the early-style exhaust manifolds from a 1974; at this time, you can legally upgrade the exhaust systems.

These D-Jet-equipped models are great cars whose operation has given me many memories and smiles. My childhood would never have been complete, and my passion for Mercedes-Benz may have never been fulfilled, if it weren’t for that Cabernet Red 280SE 4.5 my father owned when I was a little boy. Happy motoring!