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Poor fuel mileage rough idle

8K views 14 replies 9 participants last post by  ahmedhossame 
#1 ·
For the past three or four months my 2001 E 320 (115,000) has had an occasional rough idle (not really bad) and the mileage is pretty mediocre, about 6-10 city and 18-20 highway. Power is fine with no hesitation at all and no smoke or smell from the exhaust. It was in for the B service about three weeks ago at which time I talked to my mechanic and he ran the diagnostics and said no codes showed up. I asked about the cats and he put the car in the lift and banged them around and said they were ok, also said if they were clogged the power would be down significantly.

The mileage never bothered me much since I came from a W140 S600 (damn that monster was thirsty!!!) but now that I bought a CLK 500 the mileage on the V8 CLK is better than the V6 on my E!

A friend suggested a bottle of sea foam but that sounds like too easy a fix.


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#2 ·
Just a couple of thoughts.

First, the service interval for spark plugs is 100K. Have those been changed?

Second, the service interval for O2 sensors is 100K. Have those been changed?

Third, banging on a cat will only tell you if it's breaking down inside, not if it's clogged.

A rough idle and poor fuel economy are potential symptoms of clogged cats. And they don't have to be badly clogged to have a negative effect on economy, but not a huge performance hit (particularly if you are a moderate driver).

Personally I'd take it to an indy or an exhaust shop to have them check the cats for clogging. Temperature differentials can speak to that, and there are inserts that can be threaded in where the O2 sensors go for a direct measurement.

I don't do a lot of city-type driving, but my combined mileage is around 26, and I usually see 30 or better on the highway running 60-65. A highway drive in rush-hour traffic usually nets me around 22-23, same vehicle as yours except without the elegance pack.

Good luck.
 
#3 ·
That's pretty bad mileage for a 320. My E430 is getting 20mpg mixed driving in Atlanta and I don't baby it. We had a 2000 E320 wagon but I can't remember the mileage...but I'm sure it was better than that. My experience has been that these cars are sensitive to issues and throw engine codes pretty easily. I would think that O2 sensors would cause a CEL.
 
#4 ·
"Bad Idle" - if coming to a braked stop, the RPM seems to dip below minimum, almost "choke", then recovers to run a bit "higher" than normal... then can also "hunt" mean drop a hair below then back up over normal idle... then you can have a MAF going out beyond spec .. if you consider replacement then use only genuine Bosch from a referred/credible internet source like oft mentioned autohausaz.com ... MAF going out beyond spec will not trigger MAF...

Often W210 air filters (use Mann only) are not replaced often enought... I replace every other oil change..

110K ... yes fuel filter replacement,,$35 Bosch or Mann.. and a few regular hose clamps to replace the MB trickster clamps..

100K, yes time for spark plug change... see threads about that so you don't waste money buying new-tech plugs especially multi-electrode that will not work well..

Fuel system cleaner... I recommend Stabil Marine Formula... $9 bottle has 2 doses 6 months apart

I have not had problems or inefficiencies running O2 sensors out to failure/CEL
 
#5 ·
I have 97 E320 114K runs rough on idle. I checked for vacuum leaks and couldn't find any. I replaced spark plugs and MAF (ebay one) 6 months ago and now replaced fuel filter. Still car idles rough. RPM will suddenly jump to 1500 then bavk to 650 and keep doing that until it shuts down. The car drives fine. Its just the idling. Any ideas? I have the M104 engine
 
#6 ·
I would start with a fresh set of spark plugs - only use OEM. NGK or Beru
if this does nothing have your fuel system properly cleaned along with the throttle body.
Sometimes simple carbon buildup will cause drivability issues. Please share what ends up working for you..
 
#7 ·
Redwood,

I used my ODBII scanner and was able to obtain live data as the car was running. I selected MAF (air flow in lbs/min) IAT (air temp), fuel amount, engine RPM, car speed to be live. The data was reading fine and suddenly as the idle RPM is low, the MAF will read a spike in amount of air entering thru (it goes from .8 to 1.5 lbs/min). Once this happens the car tries to compensate the virtual excess air by closing the throttle valve. Once I replace the MAF sensor I'll see if that was the problem or not.

I hope its not the computer sending wrong signals. I don't know if that can happen.
 
#12 ·
MAF reading on my 1999 E320 is 3.8 g/s = 0.5 lb/min @ idle (575 RPM). The MAF reading is fairly steady at idle (3.8 to 4 g/s) for my car.

Just tried it on a short city drive and it went up to 102 g/s (= 13.5 lb/min) on a quick acceleration (5 to 30 MPH) in one city block. From what I've gathered, I believe that a MAF's response to acceleration is a key measure of the MAF sensor's functionality.
 
#13 ·
New symptom

Yesterday while I was driving I had to floor the accelerator to get around a slow moving car and my car went into limp mode. No CEL or malfunction in the MFD or other symptoms. I stopped the car and restarted and it was back to normal. This morning I floored it after it was warmed up and the same thing happened with STILL no CEL or MFD malfunction. I stopped and restarted the car and everything was fine. I haven't had time to get it in to the workshop yet.....
 
#15 ·
Yesterday while I was driving I had to floor the accelerator to get around a slow moving car and my car went into limp mode. No CEL or malfunction in the MFD or other symptoms. I stopped the car and restarted and it was back to normal. This morning I floored it after it was warmed up and the same thing happened with STILL no CEL or MFD malfunction. I stopped and restarted the car and everything was fine. I haven't had time to get it in to the workshop yet.....
you probably have the 722.6 trans. Have you replaced the wire harness O-ring. Its a good practice to change trans oil, filter, and harness O-ring on those transmissions. Its a common Mercedes failure. Its an easy job unless you have any stripped head bolts which I had before. Be careful as all oil pan bolts are soft.
 
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