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Old 05-05-2002, 03:46 AM
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Join Date: May 2002
Location: Connecticut
Posts: 1
Talking Fluctuation in Idle Speed

I have a idling problem where there is a significant change in idle speed Up & Down but after about 5 minutes it levels out. I have a 1973 280 SEL 4.5 Fuel Injected engine. This problem started out of the blue and I'm Freaked! At this point in time it has progressed to idling very rough & sputtering. To me it seems to be either a vaccum or some sort of switch malfunction, but I can't find any info on where to start. When I just had the idle fluctuation, I could not regulate the accelerator because it seemed to stick and the RPM's were very high and I had to actually pull back on the gas peddle to slow it down. It seemed like it wanted to take off on it's own. I haven't driven it in about a year because it was too dangerous. I replaced the Ignition Wires, Plugs, Points, Condenser & Checked the Timing and everything as far as I know was in spec. Like I said, this came out of nowhere. I used a dwell meter to set the point gap but I can't find out anywhere what the specification is in thousanths (Just to get an idea of what it should be manually). If you would please help me figure out where to start to trouble-shooting I woud appreciate it.
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Old 05-05-2002, 09:33 PM
Posts: n/a
I'm not sure if MB's are the same but my Bmw has a idle computer that can cause this type of thing or a Idle controll valve gone bad. Also I had this problem on my car because of the throttle postion sensor being off a little as well as a bad mass air flow unit.
I know it is not an MB but I hope this helps.
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Old 05-05-2002, 10:22 PM
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Join Date: Mar 2002
Location: Naperville, Illinois
Posts: 92
I'll admit that I am not familiar with your model MB, but the symptoms sound like a bad O2 sensor. The rough idle, surge problems during warm up are symtomatic of an O2 sensor that has gone south. Have it tested before replacing it, since they are not particularly cheap.
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Old 05-05-2002, 10:26 PM
Posts: n/a
The O2 sensor doesnt kick in untill 2 mintes into running on some cars once again not sure on MB's agian.
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Old 10-19-2002, 08:35 AM
Registered User
Join Date: Mar 2002
Posts: 33
I replaced both the idle control sesnor and the o2 sensor and the problem is gone for now.

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