1995 E320 terrible gas mileage
 

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.

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.

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.

anytime i had this issue it was a failing o2 sensor

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.

http://www.motocrossgear.com/wp/wp-c...2/plugdiag.jpg

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!

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.

gas mileage
 

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.

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.

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

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.