Recently replaced the plugs on my M104. Expected to see an improvement in gas mileage, since the old ones were 12kmi old, gapped a mile wide, and the wrong type of plug (resistors).
Just did my first fill up since the plug change:

15.4mpg average, about 50/50 highway city! (My first thought was that I should have sprung for an E420 since I wouldn't have lost anything in efficiency!)

OBSERVATIONS:
> She purrs like a kitten and pulls like a train with no stumbles or stalls.
> The idle is a liiittle rough, but mild enough that I can just barely feel it through the wheel. Coming from a diesel I have no meaningful baseline for expected idle vibration.
> Always catches on the second rotation, no surging at idle.
> RPMs start at 1k and ease down smoothly to around 500 within a second or two of startup in 60-70 degree ambients.
> There used to be a gasoline smell from the engine bay and tailpipe for a few seconds after startup, have not observed this since spark plug change.


This is my first gasser, so I don't really know where to start. I'm trying to ask as many questions as I can to get an idea of what to expect. Previously drove an E300D, and the world of spark plugs and throttles is entirely new to me.

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RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss)
1982 300D, Silberblau under a blue repaint 256k (sold for parts)
1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)

An in-line six is always smooth assuming it's in good health.

Do you know how many miles are on this car? What's the history??

Probably overlooked, it says 110K miles.

I am somewhat interested in this thread since mine does the same, but has high emissions.
Failed CA smog!
Mine runs between 13-14mpg (220K), no highway.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

my car has same engine,but i get 23 around town 28.5 highway and I use resistor plugs.
mines a 5 speed auto.I would check for a grabbing brake.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

anytime i had this issue it was a failing o2 sensor

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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


[/SIGPIC]

a grabbing brake caliper would most likely already be detected by a pull in the steering. Since it occurred immediately after the spark plug change that is the first thing to check. If the car is running smoothly, its unlikely to be down a cylinder but not impossible.

I love the o2 sensor, that means that the poor fuel mileage is from running too rich.

Reseating the plugs is the first thing I'd do, check them for fuel/Air mixture. the included picture is what I use for my motorcycle which is not fuel injected.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg

Oh, that is some good data right there. I kept two of the plugs that were installed when I got it, I remember them being brown and crummy looking. I'll take a look when I get home.

I may have a grabby caliper as well, but it's hard to tell because my front end is pretty loose. I've noticed cruise occasionally refusing to engage, which happened on my E300D when it had a caliper that would not retract all the way.

I have new (non-resistor!) plugs, an idler arm bushing kit, and new brake hoses in the mail, so we'll see what eliminating those variables does for the problem.

Personally, I too like the O2 sensor for this. She threw a code a while ago which I didn't bother to check as it cleared itself in a couple days, and I would expect a lot more difficulty starting, roughness at idle, or loss of power if it were unmetered air or fuel delivery problems. But as I said, I'm new to the stinky fuel world, so I appreciate all available advice!

__________________
RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss)
1982 300D, Silberblau under a blue repaint 256k (sold for parts)
1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)

you would need some sort of oscilloscope to measure the O2 output, if it gets weak you will lose serious gas mileage.

I have a chev impala 3.4 too in the family - it was drinking gas like its going out of style - no errors - replaced the o2 sensor and FPR - went right back to 20mpg.

a good starting point would be to check the exhaust for blockage (pre cat failure), o2 sensors gone blind or leaking FPR.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Check the fuel pressure regulator on fuel rail for fuel seeping in the vacuum hose, pull the vacuum hose off and see if any wet fuel comes out, a common missed item

okto, any update to your situation?

I just ran into this same scenario. Replaced my plugs, Bosch non-resistor, with OE Mercedes plugs and my mileage took a DIVE. I would consistently get around 310miles per a full tank. Now I can't even reach 285. Sure, there's a little variance there depending on how much the tank takes per fill but I could *always* rely on 310-320 prior to the change.

Edit: The plugs that were removed show no signs of any malfeasance.

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"Fubu" - 1993 500E 199/271 (103K) 124.036
"Patty" - 1994 E320 Touring "Sportline" 172/174 (239K) 124.092
"Punch" - 1997 Porsche 993TT (34K) Hang on!

The 95 E320 was once upon a time specified to do 18mpg City and 24mpg Highway.
If I take these two values and average them 50/50 we should get something like 21mpg, assuming the engine is in decent mechanical condition.
Therefore, 15mpg seem a bit too far. Best thing in my opinion is to pull the codes from 8 and 14.
Oldsinners better milage could be related to the more efficient 5 Speed tranny.
Kinda like the O2 sensor idea, trust me, I was measuring mine with all I had, including Oscope and did not see any abnormalies. My theory was that it was lazy and had no reference as to how fast the sensor should be.
I simply decided to replace the piece of art and viola, mine went to nice clean emmision and got about 16-17 mpg City only, damaging short trips. However, I feel guilty, did only about 2000 miles the past year. So, my mpg is absolutely no reference.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

I get 185 usually per full tank all city driving deliver food, just put new plugs as well

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1994 e320 (Charcoal and grey) 140, 000 miles

As far as I know the dealer will sometimes give you resistor plugs now when you ask for plugs for one of the older cars. Are you sure they are non-resistor? The counterman does not necessarily have any idea what that means.

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68 280SL - 70 280SL - 70 300SEL 3.5 - 72 350SL - 72 280SEL 4.5 - 72 220 - 72 220D - 73 450SL - 84 230GE - 87 200TD - 90 190E 2.0 - 03 G500

Nissan GTR - Nissan Skyline GTS25T - Toyota GTFour - Rover Mini - Toyota Land Cruiser HJ60 - Cadillac Eldorado - BMW E30 - BMW 135i