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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 54037
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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I have a 1983 mercedes 300sd. the fuel pump quit on me so i

Customer Question

i have a 1983 mercedes 300sd. the fuel pump quit on me so i replaced it with one from a spare motor.. in short when i try to start it. the motor goes full on run away.. i need to figure out why.
Submitted: 2 years ago.
Category: Mercedes
Expert:  Marty replied 2 years ago.

Hi, my name is***** the fuel shut off diaphragm is installed incorrectly it will cause the motor to run wide open. Remove the shut off valve and cover the opening and see if it still runs away. The hook arm on the shut off valve needs to engage into the linkage in the pump. If it is installed wrong it will push the linkage to the wide open throttle position.

Expert:  Marty replied 2 years ago.

Let me know if this is the problem.


Customer: replied 2 years ago.
ok i will check it out.
assuming the engine does not have excessive blow by and the source of the run away is the pump itself, is there anything else that can make it do this?the linkages move freely and stop at the right spots.
the pump i am working with is of unknown history. but it will run the motor, just at full throttletiming is set at 24 btdc. i may have forgotten to check the camshaft position when i put the pump on. but would it even run if the cam is 180 degrees off?
Expert:  Marty replied 2 years ago.

Correct, the engine will run away if there is excessive blow by or if the pump is defective but the usual problem is with the shut off diaphragm. It's also possible that the rack in the pump is seized. I think it will still run 180° out.