Tech Session
By Richard Simonds
A Fix for 230/250SL Start-up Misfires – Modify the fuel injection pump
Mercedes-Benz was one of the early pioneers of fuel injection as the means to create the fuel-air mixture in engine cylinders instead of traditional carburetion. During development of the technology, a variety of different mechanisms and approaches were tried, some better than others. The result today is that owners of earlier classic cars must contend with problems inherent in the designs; but with modern technology, some of these problems can be cured. A specific example is the fuel injection pump used on the on the early W113 Pagodas (230SL and 250SL up to engine number 129.982-10/12-001364).
These early W113 Pagodas used a Bosch 6-plunger mechanical injection pump (Bosch designation: PES 6 KL 70/120 R11). This pump can have misfiring problems upon startup due to the design of the pressure valves at the top of the pump that feed the fuel pipes going to the injectors (injection nozzles). This condition worsens as the pump wears, making it appear for the first several seconds after startup as if the sparkplugs are not firing regularly. In fact, it is not likely to be an ignition system problem, although that is the first area to check. The more likely cause is the pressure valves in the injection pump on the 1963-1967 230SL and early 250SL.
These first-generation pressure valves had a cone seal that could cause leak-down of fuel pressure after the engine was shut down. Then when starting the engine, there would not be sufficient pressure in the fuel pipes to reach the injectors, causing the engine to misfire until full pressure had been restored to all of the injectors. Because more than one pressure valve could leak-down – and at different rates – the engine would misfire and gradually smooth out until all six cylinders were receiving fuel and the engine could fire on all cylinders.
But that’s the end to a long detective story that started as I chased a common 230SL problem: my engine misfired on cold start-up. My M127-981 engine generally had been pretty fault-free. I had done a valve job (valve guides, seals, and machining the head) around 180,000 miles, and after that it started and ran well. However, around 210,000 miles it started occasionally misfiring when starting – especially if it had not been run for a week or more.
Working with my mechanic Ken Adams, a technical contributor to The Star magazine, I initially checked ignition timing and replaced the points and condenser in the original distributor with an electronic ignition module. Then I replaced the distributor shaft, believing a worn and slightly out-of-alignment shaft could be causing the problem. In retrospect, that wasn’t the problem because the engine still would occasionally misfire upon startup, but always smoothed out after running for six to 10 seconds.
I recalled attending a workshop at StarTech 2005 where W113 gurus Pete Lesler and Joe Alexander discussed setting up the linkages on the fuel injection system. Getting all of that aligned correctly did make a difference in performance, improved fuel mileage somewhat, and stopped the black smoke out the exhaust above 4,000 rpm when I accelerated enthusiastically; but it did not cure the occasional misfiring upon startup.
The next step was to send the fuel injection pump to one of the world’s best rebuilders – Gus Pfister at Pacific Fuel Injection Service in South San Francisco, California. All six injectors were replaced – and replaced a second time when further testing revealed some poor spray patterns on a few of the first replacements. The engine ran better, but still would occasionally misfire upon startup. Still, after sitting idle for several days, the engine would misfire upon startup but then smooth out after several seconds.
Next, Adams performed a leak-down test on the engine that revealed one cylinder was dropping to 100 pounds pressure per square inch, two were at 130 psi, and three were at 145 pounds. Consequently, Adams pulled the engine and we sent it to Metric Motors Inc. in Canoga Park, California, for a long-block rebuild. That made a huge difference in performance and a notable improvement in fuel economy. But the misfiring upon startup continued.
Lots of money and time had been invested by this time and still the problem persisted. As you might guess, that led to several serious conversations with my mechanic – and my wife – about the future of our relationship with the 230SL. It was not a pretty picture.
At this point, Adams sent the pump back to Pacific Fuel Injection, this time with specific details about the problems that continued to plague my engine. With the additional information, Pfister determined that there was a problem with the old cone seals in the pressure valves and confirmed the problem by performing a leak-down test on the fuel injection pump. To correct the problem, he remachined six ball-seal pressure valves that came with the engine on the later 250SLs (Bosch designation: PES 6 KL 70 A 120 R18z) to fit in my 230SL’s fuel injection pump housing. If you’re wondering why he didn’t simply install a 250SL or 280SL injection pump, the answer is because the pump housing in the 250SL/280SL has a different 3-dimensional cam, fuel lines that enter on the opposite side, and a different housing unit; the original 230SL pump had to be used.
The end result of this small modification: My 230SL now starts well cold, warm or hot. It warms up without stumbling or stopping. It runs well at any speed, from city driving to highway driving. And, a big benefit is that my fuel mileage improved from 12-16 mpg to 17-24 mpg (city-highway). Everything is now all right again with my 230SL, my mechanic – and my wife and me.
Images
Bosch Fuel Injection Pump Testing Machine. Pacific Fuel Injection (PFI) has several of these machines set up to test and calibrate diesel and gasoline mechanical fuel injection pumps manufactured by Bosch. The day I visited, PFI had one setup for a 230SL, another for a 300SL, and a third for a 280SL. However, each machine can be adjusted to test and calibrate any pump that comes in – including fuel injection pumps for Porsche and others from the era of Bosch mechanical fuel injection pumps
Pressure Valve Assembly for 230SL and early 250SL fuel injection pumps. From left: cap at bottom of pressure valve, cone, seal, spring, pressure valve housing.
The weak link in the original setup: a close-up view of the stock cone valve. The cone surface is more susceptible to wear, so eventually stops maintaining fuel pressure.
By comparison, the pressure valve assembly used on fuel injector pumps beginning with the mid-production 250SL fuel injection pumps. From left: ball bearing seat, ball bearing, ball bearing retainer, spring, seal, brass cap at bottom of pressure valve. As can be seen from this disassembled view of the components (Do not try this at home) pressure in the fuel system is maintained while the engine is shut off by mechanical action of the spring pressing against the ball, rather than the earlier cone, and is not as susceptible to wear.
This image illustrates the modification that is necessary to use a pressure valve assembly from later fuel injection pumps in the earlier pumps. Since the cap at the bottom of the pressure valve – earlier and later versions at left and far left – has to fit into the fuel injection pump body, the outer circumference of the later cap has to be machined to the same size as the early cap. This, of course, has to be done for each of the pressure valve assemblies.
Details of fuel injection system of 230SL & 250SE engines:
A 6-element pump delivers gasoline to six injection nozzles located at the intake ports of the cylinder head, which in turn direct the fuel into the combustion chambers through the opened intake valves.
1 fuel tank
2 fuel feed pump
3 fuel line mesh filter
4 injection pump
5 fuel injection nozzles
6 coolant thermostat
7 auxiliary air line
8 accelerator pedal
9 control linkage
10 throttle duct or mixture controller
11 main starter switch
12 solenoid for rich fuel mixture on injection pump
13 electromagnetic starting valve
14 relay to solenoid for rich fuel mixture
15 thermal timing switch in coolant
16 timing switch
17 relay to electromagnetic starting valve
18 thermo-switch in coolant (from workshop manual)
Definition of Terms
Fuel feed pump: an electrical pump 2 located at the fuel tank 1 that sends fuel to the fuel injection pump 4.
Fuel injection pump: the mechanical pump 4 that pressurizes the fuel and then dispatches it as needed to the fuel injectors in the exact quantity and mixture required for the engine to idle smoothly, accelerate, or run at a fixed rate.
Pressure valves at the top of the fuel injection pump: the valves (not numbered) that maintain fuel in the fuel pipes under correct pressure so that the injectors can deliver the fuel to the engine during startup.
Injectors (Injection nozzles): parts 5 that create an atomized fuel spray which renders fuel readily combustible, thus allowing the spark to create ignition.
Important Notes
Warning: Pressure valves are not designed to be disassembled! Do not attempt to disassemble your pressure valve as it cannot be reassembled. The pictures of the disassembled valves shown here were made possible because Gus Pfister at PFI sacrificed two valve assemblies by taking them apart so I could document the components for this article. In general, the fuel injection system contains precision components that should not be worked on by the Do-It-Yourself (DIY) mechanic or any professional mechanic who thinks he or she knows how to adjust mechanical fuel injection, but really doesn’t have a clue. Adjusting the linkage is about as far as the DIY mechanic should go. It is very easy to get the system so far out of adjustment that the car will not run well – or not run at all. A word to the wise: Know your mechanic’s skills!
Think You Have a Fuel Injection Pump Problem? If your 230SL or early 250SL suffers from misfiring during startup and if you determine that the problem is in the pressure valve, PFI can convert the cone assembly to a ball assembly for the cost of the newer valves. As of 2013, each pressure valve costs $153. Multiply by six cylinders and the materials cost is $918. If this is done during the rebuild of your fuel injection pump, PFI adds no labor charge for machining the caps to fit into the earlier pump housing. If the pressure valves have to be retro-fitted after a fuel injection pump rebuild, then there will be a labor charge to machine and install the new pressure valves.
Pressure Valve Part Numbers
Bosch #D8C45CR1 / Daimler AG Part #000-078-0723:
For 230SLs, engine #127.981-10 or 12-00xxxx and 250SLs up to engine #129.982-10 or 12-001364 with cone-seals. This part is no longer available and not rebuildable.
Bosch #D8C45CR2 / Daimler AG Part #000-078-1123:
For 250SLs, engine #129.982-10 or 12-001365 onward) with ball-seals, available, but not rebuildable. All injection pumps with the ball seal pressure valves carry a letter “Z” at the end of the part number.