Question: My 1965 Mercedes Benz 230SL engine always starts during hot or cold weather, but stops idling when the water temperature reaches 180°F and the idle reaches 1000 rpm. I have replaced the automatic auxiliary start mechanism injectors and rebuilt the injection pump. I have not replaced the thermostat which is part of the auxiliary start mechanism, however I have bypassed it and left it out of the circuit by turning a screw located on the injection pump. I also adjusted the venturi control unit. Can you help?
Answer: I don’t think so. You have me stumped. Most early fuel injection units are mechanical, as opposed to the computer controlled units in use today. Although simple in theory, in reality they are a bit complicated and require the expertise of someone who has a good working knowledge of their operation. I called Dan at Aventura Motors in Southampton since I know is very familiar with these units.
His opinion is that even without any defects in the system, the adjustments on these units must be carried out accurately and methodically in order to achieve good results. Begin by checking the basics including ignition timing, mixture, and linkage adjustments. Other adjustments include the “venturi control unit” that you mention as well as an “air bypass” that you did not mention.
Because your unit starts malfunctioning when the engine reaches 180°F it is possible that the thermostat which you bypassed may be defective. This thermostat controls a plunger in the air intake which is supposed to open when the engine reaches operating temperature. A defective thermostat or plunger will result in the symptoms that you are experiencing. You rebuilt the injection pump, which when reinstalled must be done with the ignition timing retarded, likely to 20 degrees after top dead center, so make sure that you installed it properly.
Now that we’ve explored the logical causes for your problem, here’s one that comes out of the blue. A recent problem on a similar car was resolved when rust that originated in the fuel tank clogged the filters and restricted fuel flow to the engine. No one is sure why this would result in the symptoms you described, but it did. When the fuel tank was cleaned and the filters replaced, the problem went away.
Answer: I don’t think so. You have me stumped. Most early fuel injection units are mechanical, as opposed to the computer controlled units in use today. Although simple in theory, in reality they are a bit complicated and require the expertise of someone who has a good working knowledge of their operation. I called Dan at Aventura Motors in Southampton since I know is very familiar with these units.
His opinion is that even without any defects in the system, the adjustments on these units must be carried out accurately and methodically in order to achieve good results. Begin by checking the basics including ignition timing, mixture, and linkage adjustments. Other adjustments include the “venturi control unit” that you mention as well as an “air bypass” that you did not mention.
Because your unit starts malfunctioning when the engine reaches 180°F it is possible that the thermostat which you bypassed may be defective. This thermostat controls a plunger in the air intake which is supposed to open when the engine reaches operating temperature. A defective thermostat or plunger will result in the symptoms that you are experiencing. You rebuilt the injection pump, which when reinstalled must be done with the ignition timing retarded, likely to 20 degrees after top dead center, so make sure that you installed it properly.
Now that we’ve explored the logical causes for your problem, here’s one that comes out of the blue. A recent problem on a similar car was resolved when rust that originated in the fuel tank clogged the filters and restricted fuel flow to the engine. No one is sure why this would result in the symptoms you described, but it did. When the fuel tank was cleaned and the filters replaced, the problem went away.
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