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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53940
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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I have a 1989 Mercedes 300E, hard start and low idle when cold

Resolved Question:

I have a 1989 Mercedes 300E, hard start and low idle when cold - once it gets to 80degrees, the idle is high 1200+. The mechanic adjusted the idle while it was cold, so I believe its hard to start from getting too much fuel.
Submitted: 7 years ago.
Category: Mercedes
Expert:  Marty replied 7 years ago.

HiCustomer the idle sped is not adjustable on your car so he has already made an error there. The adjustment now needs to be put back where it was before you go any further. I would test or replace the overvoltage protection relay located behind the battery. The relay provides power to your engine management system and your ABS brake system. The fuses in the relay should be checked first. When the relay fails your car becomes hard starting cold and runs poorly until it warms up. Let me know if you have any questions.






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Customer: replied 7 years ago.

Thank you for the quick response, very last question, bit of info that I missed earlier.


The overvoltage protection relay looks brand new and the fuses are good, is it possible to be the idle control?


Expert:  Marty replied 7 years ago.

I'd make sure you are getting voltage to the idle control valve with the key on. If you aren't then the relay is still in question. A bad idle valve is certainly a possibility if you are getting no fast idle when cold. I'd have him put the idle adjustments back to their factory settings and do a proper fuel mixture adjustment by monitoring the current to the EHA valve. Let me know if you need more help with it.