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Old 12-11-2010, 08:48 AM
1990 300se
Join Date: Oct 2005
Location: Honolulu, Hawaii
Posts: 111
Help, m103 with poor fuel economy!

Hi guys,

I have a '90 300se with the m103 engine with which I was getting around 18mpg for a while, but have found that it has recently dropped to around 10-11mpg. I've swapped the following items so far with the same results, and am hoping that someone sees something that I might have missed:

Ignition wires
Spark plugs (h9dc0)
Coolant temp sensor
Air intake sensor
O2 sensor

Duty cycle is set at 50%, fluctuating. I'm still reading up on testing and adjusting the EHA, so I haven't messed with that yet.

Recently, the engine has begun to "stumble" for a few seconds after starting, but starts fine otherwise. I'm guessing that some or all of the injectors may be leaking, though I don't know if they could cause such a huge drop in fuel economy.

The spark plugs looked okay but some were wet with "something," which I think is fuel. I'm going to pull the plugs again and try see if it is combustible. Also, the exhaust smells worse than before (cat is dead) but I don't feel that it smells like gas.

Anything else I should look into? Thanks for any input you may have!

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Old 12-11-2010, 10:44 AM
Registered User
Join Date: Dec 2010
Location: Austin, TX and LU, CH
Posts: 58
If you have the time You might want to read
CIS-E Tuning (W124 - 103.983)

While a member of another car club, one member bought a special ABS bleeding tool and then shared it with others in the community, they just shipped in on to the next person who needed it. It just kept moving around.

I'm very new here but I suspect a number of you know each other well enough to split the cost of an AFX wideband O2 sensor which you can then send around. It comes with a harness long enough to reach from the battery to the tailpipe, leaving the display in the passenger compartment. Simple to use, just put it in the tailpipe and connect to the battery.
With it hooked up you can unplug various sensors, switches, etc. to help you find your problem.

FYI, the mechanical injectors always fire/spray with the engine running. If they leak when the engine is turned off they will simply bleed down the residual pressure that the system holds, maybe an ounce, which is really no big deal. The next start might be longer to take since the system has to rebuild pressure, but that should be it.
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Old 12-11-2010, 10:46 AM
Registered User
Join Date: Dec 2010
Posts: 3
no idea
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Old 12-11-2010, 01:09 PM
Registered User
Join Date: Oct 2004
Location: Baton Rouge La
Posts: 2,631
i know you replaced it.... but this is a classic o2 sensor problem

1989 300ce 129k
( facelifted front,updated tail lights, lowered suspension,bilstein sports, lorinser front spoiler, MOMO steering wheel, remus exhaust,stainless steel brake lines). (Gone)

1997 s320 154k (what a ride). Sold with 179k miles. Replaced with Hyundai Equus

1994 e320 Cabriolet 108k

1972 280se 4.5 153k Owned for 12 yrs, sorry I sold it

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