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1991 Mercedes 420 SEL
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6 Posts
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Hi, I am a newbie here. I have a 1991 420 SEL, I bought about a year ago. Has 200K miles on it, drives better than most brand new fords and chevrolets I've driven. Lately, it has been running rough. When I am in drive, foot on the brake not moving, the fuel usage gauge goes to about 1/4 and it stalled on me when I tried to turn the AC on. When I am driving, it seems to be using a lot more gas according to the fuel usage gauge and does not have all the power it used to. When I turn the AC on while I'm driving, it slows the car down a lot. I change my oil regularily and changed the plugs tonight. The plugs were really bad, but it does not seem to have fixed my problem. My air filter is clean and I had all the belts replaced last month. It had been getting great gas mileage about 18 miles to the gallon (of the most expensive gas of course)but now it is not doing so great. Any helpwould be great.
 

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1965 220S, 1999 Volvo V70 (wagon), 2006 Ford Crown Vic Police Interceptor, 72 350SL 4 Speed
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1,830 Posts
Your 'fuel usage gauge' is really a vacuum gauge. If it's not reading low enough (midway towards the red), then you have a vacuum leak somewhere and given the age and mileage, you need to replace all, and I mean ALL, the rubber bits in your intake manifold and surrounding systems. This job takes a lot of time and requires careful attention to detail, bolt length, size and position, for all the various systems. I've done this on my 420SEL.

You may also need to replace your fuel filter and make sure the fuel pressure regulator is working properly, ie not leaking fuel into the manifold.
 

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1987 420SEL
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48 Posts
If you haven't replaced your air distribution hoses on your 420 yet, I would do that first. They are cheap; usually the first rubber piece to go; and when they do, they will cause the symptoms you've got.

If you want to try to find the vacuum leak before you replace anything, get a 4-foot length of small-diameter hose, put one end in your ear, plug your other ear, and move the other end of the hose all around the intake manifold area as you listen for an air-sucking sound. Then try squirting your 420's rubber hoses with carb cleaner in the places where you hear the air leaking. If the engine starts to run better, you've found the air leak.
 
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