We have a great 1993 300E (3.2 liter) with 120K that uses almost no oil between oil changes at 3-4000 miles.
Gas mileage has dropped from just under 30mpg to just over 20mpg recently.
The other day the check engine light came on. About the same time the idle changed from a normally smooth 600-700rpm to a pulsing or surging 600rpm to 1100or 1200rpm (only after warm-up)
I checked the spark plugs, they were even but sooty black. I then retrieved the following trouble codes:
1. #2 oxygen sensor inoperative (replaced 7 months ago)
2. #4 air injection inoperative
3. #10 voltage at air mass sensor too high or too low
4. #26 up-shift delay switchover valve open or short (replaced 7 months ago)
5. checked for vacuum leaks, none found (yet)
6. read lots of archived threads on rough idle and other

I cleared codes but #10 is back.
Seems like the coolant temperature sensor might be the source of the problem. If it is sending the wrong signal saying the engine is still cold, thus the rich fuel mixture, bad gas mileage and rough idle.

Anyone have a value @ temperature chart for the coolant temperature sensor for this model?
Anyone have any other suggestions?
I really don’t want to start throwing parts at this problem. Thanks for your input!!!

Man, we have the same car, and I have never achieved fuel economy of just under 30 mpg. Even driving to Las Vegas with the cruise control pegged at 65 mph, I still got just 25-ish. I'm averaging about 21-22 mpg since I've owned the car, so I think your fuel economy is in the normal range.

A fluctuating idle is generally caused by either a vacum leak, bad mass air meter, or bad throttle actuator.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

Paul, could you please explain how I could better ascertain if the problem is the air mass meter or the throttle actuator. As I stated earlier no vacuum leaks found (yet).

BTW I have noticed that your vehicle seems similar. Ours really has gotten very good gas mileage (we keep a mileage log), we don’t push it to hard…

Anyone else like to comment?????

Would still like to know the test values @ temperatures for that coolant sensor....

Retreiving the codes in the DM could point you to the right direction.

The Mass Air Meter is so quick to replace that I think most shops would swap in a known good one to see if it solves the problem.

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Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".

I also have the same car. I also had the same problem you describe. I just replaced the MAF after much help from this site. I did a little testing on my own and found when I disconnect the MAF I would get the same symptoms. After I plugged it back in I tapped it a few times and the idle would surge. After a couple more taps it would return to normal. Mine also would get close to 30 mpg on the highway and around 21 everyday driving. When I had this problem the milage around town dropped to 18/19. Now after replacing the MAF it seems to run a little better and the idle is normal. Hope this helps.

I have the same engine in my '94 wagon with 233,miles.

1. My air flow meter caused the same symptoms last month. Was about $400 job at the dealer.

2. Even in the summer months I've never come close to 30 mpg. The best I get is 24 mpg. Share your secret (especially at $2.30 a gallon in CT!) !

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dtf
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (228,000)
1999 E300 Turbodiesel ( died @ 255,000)
2006 Toyota Tundra SR5 AC 4X4 (115,000 miles) rusted frame - sold to chop shop
2011 Audi A4 Avant (165,000 miles) Seized engine - donated to Salvation Army
BMW 330 xi 6 speed manual (175,034 miles)
2014 E350 4Matic Wagon 128,000 miles
2018 Dodge Ram 21,000 miles

Tell me please where is the Mass Air Flow located on a 93 2.8 300e