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Old 02-15-2003, 08:52 PM
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300E Gas Mileage -- 10 MPG City!


Before I spend more money on parts, I want to know more about what to look for in my hunt for much better mpg.

Car is a 1990 124. Fuel distribution systems was replaced about two years ago.

I have pulled the plugs (coppers) and replaced them about 1000 miles ago. Tan in color on the old plugs and they came off easy. So far, so good.

New air filter, oil (Castrol GTX 5w-30) changes with filter, new alternator, car starts first try on very cold winter days so I don't think the OVP is bad. No warning lights.

New vacuum hoses by idle control valve, air cleaner box to valve cover and from valve cover to manifold.

Car idles slightly rough at times and others it's smooth as silk. Car hesitates when starting off but only intermittently.

Gas mileage on highway about 20mpg in city about 10 to 12 mpg.

I've searched the archives but it seems that poor gas mileage is associated with hard starting. Not in my case as it starts usually on the first try.

I suspect a few items but I thought I'd ask first and see if I'm thinking in the right direction.

Any thoughts?

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Old 02-16-2003, 02:50 AM
haasman's Avatar
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Join Date: Jun 2001
Location: San Francisco, CA
Posts: 3,096

My first reaction is either a bad cap/rotor and/or oxygen sensor.

'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)
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Old 02-16-2003, 08:45 AM
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Thanks for the reply.

Would the fact that when i pull the spark plugs they look normal (no black deposits, just a good tan color) rule out the rotor? I have not pulled the rotor out.

As for the 02 sensor -- woldn't the check engine light come on if it was not working right? I know the light works during starting. Perhaps the 02 sensor can fail and not trigger an alert?

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Old 02-16-2003, 09:28 AM
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Join Date: Jul 2002
Posts: 44

You may want to replace your plug wires. My 300 began to run a little rough at idle and was surging a little. Replaced the plugs and wires and now runs, idles smooth as silk.
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Old 02-16-2003, 11:18 AM
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Replaced two years ago when fuel distibutor was replaced.

However, that's a thought. Thanks.

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Old 02-17-2003, 07:31 PM
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Further thoughts:

Perhaps the EGR or the EHA?

The confusing thing is the mixture. The plugs are perfect coming out of the engine. Gapped slightly wider than spec as per this forum.

The old ones were gapped to spec and were perfect. No corrosion or material on them at all.

Can an 02 sensor go bad without the light coming on?

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Old 02-18-2003, 01:55 AM
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Join Date: Sep 2001
Location: Austin, TX
Posts: 563
These are all things that can be tested. Look at the "Evaluating Electronic Engine Controls" article in the DIY section of this site. Steve explicitly went throught this for an M103 engine just within the last two or three months, so do a search.

My money is on the O2 sensor.
Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino
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