Bosch Mechanical Fuel Injection

A new Job into the shed is this lovely 220SE cabrio with 71,000 miles on the clock. As with a lot of these low mileage injected Mercedes , lack of use is a killer and this one is a prime example . The car has been hard to start with plug fouling and blowing smoke. The valve guide seals were replaced last but what is actually happening is that the injection pump oil seals have failed.

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Symkptoms were very typical, hard starting from cold, and sluggish running especially when cold. Sluggish aceleration was another red flag > Did the usal check by disconnecting the throttle and opened it up from idle, it stalled almost imeediately which,although the plugs indicated a rich mixture,the engine cut out. This meant it was running lean.

A quick easy check!.

Next,I unscrewed the cam box dipstick to reveal this  black substance oozing from the dip stick port….

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The oil seals in the pump have failed, sending oil into the cam box which slows down the governor leading to sluggish acceleration ,and this oil also gets into the fuel being injected leading to the plug fouling and intermittent exhaust smoke problems.

Next job is to examine the engines thermostat to make sure it is working correctly. A cold running engine with MFi will always run rich because the MFi’s own thermo control thinks it’s cold and keeps the pump delivering extra fuel. This rich running do0es and will cause bore wear,and very rapidly too.


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