After spending five weeks working the bugs out of a 1985 Mercedes 500SEC, I decided to drive it to soccer one Sunday morning, a mere 70 miles from my house. Following the game, I cleaned up and got in the car to fire it up. Unfortunately, after five seconds of cranking, nothing happened. I tried several more times, with no results. This SEC had an old battery, and I knew that each crank meant I was closer and closer to needing a jump.
As luck would have it, I had a pair of needle-nose pliers. This allowed me to run the fuel pump constantly to overcome either a bad fuel pump relay or fuel vapor lock. I removed the fuel pump relay, bridged pins 30 and 87 in the relay plug and let the pump run for about ten seconds. After that I tried to start the engine, and it fired up after a few seconds, stumbling and running rough but alive once again.
What happened? Did I purchase a batch of bad gas? Did I have leaking injectors? How about a bad fuel pump relay?
Thankfully, the solution is more straightforward.
What is a fuel accumulator?
The issue was a bad fuel accumulator. The accumulator is a metal reservoir that stores pressurized fuel. It’s located by the main fuel pump, under the rear of the car. It serves two functions: To help stabilize fuel line pressures when the engine is shut off, and to provide a reserve of pressurized fuel under heavy throttle, just in case the pump(s) cannot keep up.
On one side of the accumulator, a pipe connects to the main fuel line. On the other, a low-pressure return line allows excess fuel to return to the tank. When the accumulator starts dumping excessive fuel volume out of the return side, line pressure falls below 3 bar.
When I was cranking the 500SEC, everything that needed to work was working. The fuel pump relay was delivering fuel, and the pump was pumping. However, about 50 percent of the pressurized fuel was dumped back to the low-pressure side of the accumulator. For this reason, the engine could never build up the pressure needed to force fuel vapors out of the fuel distributor and spray a healthy amount of fuel through the injectors.
Diagnosing a bad accumulator
Although an accumulator was used on all K and KE Jetronic systems, it went through three design phases. The 3-line accumulator (used from 1976 to 1978) and the similar two-line unit (used from 1979 to 1985) display similar failure patterns like the one described above. A quick and dirty test involves removing the air filter housing and pushing on the air flow meter plate. If the plate has no pressure, the accumulator is likely your culprit.
On KE–Jetronic systems (W201 models from 1984, all others from 1986), accumulator failure is harder to detect. The most reliable test involves running your main fuel supply line into a fuel pressure gauge with no outlet. Pressures should stabilize at about 5 bar, and then drop to 3 bar. The pressure drop should stop at 3 bar. If it continues, look for a decline to around 1.8 bar. At this point, you will have hot starting issues, so replacement is a must.
What about cold starts and accumulators?
Many technicians are stumped by the presence of normal cold starting and poor hot starting with a bad accumulator. On both iterations of K-Jet, a cold start injector supplies instant raw fuel from the fuel pumps straight into the intake when the engine is cold. This allows the engine to run and purge the fuel distributor. The lack of heat helps too. Fuel will boil when it heats up, unless it is pressurized. Post-shutdown, most gasoline engines heat up to about 100 C or more. This causes non-pressurized fuel to vaporize, making atomization and injection of the fuel vapors hard to accomplish. When Mercedes developed D-Jetronic, they believed 2 bar (29.5 psi) was enough to prevent fuel boiling and vapor lock. This proved barely adequate, as seen by the number of D-Jetronic Mercedes that had partial failures of fuel pressure regulators and developed hot starting issues.
Finally, Bosch figured out that 3 bar was ideal, where the fuel would stay liquid at temperatures well above 100 C. This principle was applied with all K-Jet systems.
What if it isn’t the accumulator?
On K-Jetronic systems, the O-ring on the fuel pressure regulator can fail. The O-ring separates the control pressure side from the return circuit. This regulator is in the fuel distributor itself and can easily be accessed and replaced. The O-ring plays a part in hot starting issues about 209 percent of the time. Physical inspection is the best means of diagnosis.
On KE-Jetronic systems, a electrohydraulic actuator can cause these issues if it is set too lean. This part and its adjustment were covered in one of our online workshops, which can be found on our “MBCA events and member resources” Youtube page.
Last, but not least, don’t forget about that 3mm Allen screw that adjusts the mixture. If it is improperly adjusted, you may need a new battery after cranking the car for too long.