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Old 03-25-2014, 03:26 PM
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Join Date: Apr 2006
Location: Mountain View CA.
Posts: 74
New Additional Symptom: Rough Idle When Hot

Hi All,

A few days ago I posted regarding an issue where my 1991 Mercedes 420SEL with 560 motor loses power when cold. I have not received any replies yet but since that post I have noticed some other odd behavior that may assist in a diagnosis. Yesterday, I drove the car on the highway for about 20 miles and then got caught in traffic. The A/C was on and the temperature crept up to almost 90C. The car started to occasionally misfire which felt like a hard kick here and there and was accompanied by a slight idle speed fluctuation. There was traffic all the way home so the temperature remained at 90C and the occasional missing was still happening. I shut it off for a few minutes and when I restarted it, the temperature immediately went to slightly over 90C and the idle was very rough. When I restarted it a few hours later, all of the symptoms were gone. Is there a link between the cold running behavior described below in my first post and the hot running behavior described above? I have driven the car recently on the highway without A/C and in milder weather and did not have any problems.

Any ideas?

Thanks,
David

560SEL Loses Power When Accelerating When Cold
Hi All,

I have recently noticed that my 1991 Mercedes 420SEL with 560 motor loses power when cold if I accelerate hard (not that hard). I am not sure how long this has been going on because I am very gentle with the car until it has reached operating temperature. Recently I had to accelerate harder than usual before the engine had reached operating temperature and the engine felt as if it had stalled but when I let off the gas and accelerated slowly it behaved normally. The engine idles slightly rough when cold but idles very smoothly at operating temperature and the problem completely disappears until the next time the car is started completely cold. For additional information: One of the fuel pumps was changed about a year ago though I have to check my records to see the last time that the fuel filter was changed. I think it was a few years ago.

Thanks in advance for your help and thanks for all your wisdom in the past.

David

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata
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