EagleTribune.com, North Andover, MA


May 4, 2014

Right tool needed for fussy Benz

I have a 1985 Mercedes-Benz 380SL with 60,000 miles. It starts fine when cold or if after driving I stop and try to restart in 5 or 10 minutes. But if the car sits longer than about 10 minutes — this could be two hours — it will crank and crank and have a difficult time starting. When it does start, it stutters like it is not getting enough gas, or too much. I don’t notice any blue smoke from the exhaust, however. If it sits long enough to cool down, it starts fine.

I have been told it is vapor in the fuel line. I have not had a person tell me the problem and solution except to say it could be at least one of three parts and they would have to start replacing expensive parts with a process of elimination. Your help would be appreciated.

Whoa! Whatever happened to testing before replacing? Based on your symptoms, I’m leaning towards a fuel injection fault, as this system must properly adapt to engine temperature changes, whereas ignition and mechanical systems don’t.

Your Benz utilizes an old-school and almost bulletproof fuel injection system known as continuous injection system, or CIS. It’s a mechanical-hydraulic system, differing from more modern electronic fuel injection systems. The key to diagnosing a CIS vehicle is to have a special pressure gauge that allows a look at two different and really important values.

CIS utilizes an air-sensing device, sort of a flapper connected to a hydraulic piston. The more air that enters the engine, the more the piston moves, providing fuel flow to the fuel injectors via the attached fuel distributor — a mechanical-hydraulic marvel. The control pressure regulator, a sometimes buggy device, provides a counter force (a second and opposing pressure) to the top of the piston, altering movement, to allow for cold and warm engine requirements. The pressure gauge can check both overall system and control pressures.

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