1992 300CE Fuel Issues

[thebighawk]

This is my last shot, kind readers. I bought, what I thought, was a nice example of a 1992 300CE. After a ton of work on this puppy, I still have bad fuel mileage.
Whats been changed trying to address this is, not in consecutive order: EHR valve, fuel distributor, air intake breather hose, to alleviate gas smell, A/C Acc temp sensor, water temp sensor, air temp sensor, dist cap, rotor, reference sensor, aux cooling fan relay, crank sensor, O2 sensor. Quite a list and still only getting combined mileage of 16/17. On a run on alligator alley at 80/85mph, I got 23mpg. Plug wires looked good, per my mechanic and the "specialty" benz guy I hadlook at car.
Is this the best I'm going to get, and quit griping, or is there something everyone has missed?
Also installed, ABS pump, timing chain, serpentine belt, alternator, compressor, dryer. More $$$$ than I paid for it into this so far.
Any one have a magic wand for this, or am I stuck. My 86 300e and 95 E320 both did much better.
Thanks in advance.
Embarrased

[Rick Hall]

This is the 104 motor, correct? I have the 103 motor and had the same issues but have been able to get the mileage up to about 20-22 in town and starting to see 25 or so on the road. Big 1400 mile road trip coming up in 3 weeks so I will know better after that what the highway mileage will be. These engines are susceptible to and are affected by vacuum leaks. Simple but hard to find leaks that are probably the biggest contributors to fuel mileage. This statement is based on both personal experience and in discussions with fellow 124 owners in MB Club tech sessions. If you have not done so already, start replacing your vacuum lines and connector fittings. Pay attention when your vacuum lines come to check valves and look closely at your check valves for cracks or heat stress. Replace the check valves. If they are not holding vacuum then they are also sucking air, not good. Also, these engines are sensitive to the type of spark plugs used. I think that engine still calls for Bosch COPPER plugs. If you have platinum plugs in there, idle quality and fuel mileage will be affected. Also, did you install a BOSCH Distributor cap and rotor? When I replaced the fuel distibutor on my 91 300E with a rebuilt from Performance Products ( a PYTHON brand if remember correctly) my fuel mileage did not get better and actually got worse before I finally replaced with a Bosch rebuild. Even though the first rebuild was cheaper than the Bosch rebuild, it was money wasted. I think it was close to $600 or so. Performance Products wouldn't do anything about that unit even though it was documented as a bad unit. One reason I dont do ANY business with Perf Prods now. There are several vacuum solenoids that are in your vacuum system that control ignition advance and the air pump that can affect mileage. What about you egr valve? You don't state how many miles are on this vehicle but if it is in the 100's then I would look at that also. The vacuum solenoids mad a difference to my milegae and driveability when I replaced them. Also, your wires may LOOK good but they could be bad internally. It might be time for a set of BERU wires. They aren't the cheapest but will last longer that the BOSCH or any other brands.
Hope I gave you some ideas. Good luck.

[dhjenkins]

http://www.fueleconomy.gov/feg/findacar.htm

EPA rating shows 15 city, 21 highway & 17 combined on premium gas.

Doesn't matter what improvements you make if you drive it like you stole it. Driving habits can make up to a 10% difference in mileage.

Low friction tires and the correct oil can make small differences as well.

[thebighawk]

Non resistive plugs per comments from this site. 104K is mileage now.
EGR valve was replaced. Bosch Dist cap and rotor used. Wires are only thing left, and the vacuum hoses. That will be next direction. Car is still very slow on acceleration. Doesn't buck or act up, just slow. On the road, acts like a Mercedes

[big dog 2]

My original Mercedes 1989 300E sticker states 18 city (15-21) and 22 highway (18-26). I get about those numbers and a bit better without running the A/C most time................

[lkchris]

25 mpg highway is normal for these cars.

[glenmore]

I have a 1991 300CE, so exact same engine. My city/short frwy commute mileage averages 18-21. Long trip highway mileage is marginally better at 25. My engine does have a small idle issue that I haven't tracked down yet but reading indicates a possible vacumn leak.

Your slow acceleration might provide a clue. I generally drive like an old lady but on the occassion I stomp on it, it really wails.

glenmore
1991 300CE
2000 C280

[bsmuwk]

Sounds like your fuel mixture is way rich. I have the same car and same engine. I just went through hell with the fuel system and after a few months I've nailed it down.

First thing I would do is get the car nice and warm. Drive it around until the exhaust gets hot so the oxygen sensor can start reading. Then adjust the fuel/air adjustment: http://www.landiss.com/mixture.htm

If you have a multimeter that has a duty cycle feature, adjust the system until it reaches about 47%-49%. That's the optimum percentage. Saying that you'll notice a slight change in the behavior of the car would be an understatement.

[ohiomike]

Daughter had a 92 300CE with the same problems. Replaced most all parts. In front of different techs. No one solved the running problems. When we put in a new oxygen sensor, it was like a new car. Did you try that even though the light is not lit. Seems the oxygen sensor goes out every 20,000 miles due to a rich mixture which is the prioblem.

[ps2cho]

Originally Posted by ohiomike

Daughter had a 92 300CE with the same problems. Replaced most all parts. In front of different techs. No one solved the running problems. When we put in a new oxygen sensor, it was like a new car. Did you try that even though the light is not lit. Seems the oxygen sensor goes out every 20,000 miles due to a rich mixture which is the prioblem.

And sometimes even faster due to the M103 tendency to leak oil through valve stems. If you have no oil usage, the O2 sensor should last much longer than 20k miles...

[bsmuwk]

Originally Posted by ps2cho

And sometimes even faster due to the M103 tendency to leak oil through valve stems. If you have no oil usage, the O2 sensor should last much longer than 20k miles...

92 300CE has early CIS-E M104. not a frequent oil burner like the m103.

Yes, these cars will go through oxygen sensors quickly when the car is not in proper tune. I check my fuel/air mixture frequently and keep my car in as much as a perfect state of tune as I can. Doing so keeps my car happy.

If the rest of the ignition system is in good functional condition, then the fuel/air mixture ratio is the only thing that's really preventing him from attaining the standard 25mpg hwy that the 92 300CE is capable of. I'm plenty positive that the air/fuel mixture is VERY rich and if not taken car of, he may begin the processes of destroying his cat converter. If he's replaced his O2 sensor recently, then the air/fuel mixture is a huge contributing factor to his fuel thirsty engine.

[thebighawk]

Thanks for all the replys and suggestions. O2 sensore recently replaced.
Was at a MBCA function Saturday in Pompano and got some good advice.
Try a resistive valve test for air flap potentiometer, and/or check fuek pressure readings on fuel distributor. OR....... check the 5mm vacuum line at base of throttle body.
These will be checked, and results posted.
thanks again forum folks!!!!!

[Rick Hall]

I think your big clue is "sluggish" on acceleration. Check the solenoid for the timing advance. Located on the pass side behind the windshield washer bottle. Solenoid out means no vacuum advance upon acceleration.