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RMillerPhD, ASE Certified Technician
Category: Mercedes
Satisfied Customers: 374
Experience:  26 year of import auto repair, Owned and operated a Repair shop and Automotive Machine Shop
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1982 Mercedes Benz 380SEL: engage..rough idle..warms..2K rpm

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I have a 1982 Mercedes Benz 380SEL, 67K miles, well maintained. Recently, in cold starts the automatic choke does not engage and there is rough idle until the 84 -86 degree temp level is reached, normal operating temp for the car. When leaving it to warm, I keep the idle at about 12K but as it warms the idle will flucuate by 2K rpm rythmetically though I keep my foot very steady on the gas. After that the car runs very well but I do notice a bit more use of the power range on the economy guage and my mileage is down from 19 to 16 mpg average. At operating temp there is no hesitation or missing. The initial starting of the car is easy, no long cranking to get it started. Any suggestions?
This system is challenging to test. I have provided a chart with the preliminary test areas.

If you want me to download the potential tests for this chart let me know and I can accommodate you request.


It requires a lot of effort to edit and up load the instructions and I don't wish to waste yours or my time if you don't want to do the tests.

Let me know. The tests have a number 1-5 typically the MB manual see the 1 as the more likely and the higher the number the less likely. Let me know the number test you want to receive.

You have many options with today’s internet, so thank you for choosing



P.S. I take great pride in providing quality responses for the people I assist. If you believe that I have been professional and helpful in my assistance please don't forget to give a positive rating for this exchange and click the accept button. If your feel my answer does not address your issues, please just tell me and I will opt out, allowing another technician to address your situation. My Goal is to make this experience one you appreciate.
Customer: replied 9 years ago.
This chart is in the manual. What expierence have you had with this incident? Has this situation ever occured with one of your vehicles? If so, what was the most likely culprit. I have ordered a cold start relay switch as that seemed to be the most likely thing to have gone wrong, as at operating temp there is no problem.
The thermal time switch has been a common problem. It directly effects the pressure differential cold start/warm up valve. Which is also a common problem. You really need to test the pressures to determine the cause of this problem, but it requires a special tester. I have one and they are really helpful.

Did you want me to up load any of the test procedures from the chart above?

Customer: replied 9 years ago.
If you could download:
1. Cold Start System Test
2. On/Off Ratio Test
3. Ignition System Test

This would be most helpful.
Cold start Test

ON/Off Ratio Test

Ignition System Test

Let me know if the links are not correct and I will reload the files.
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