Bad Gas Mileage - PeachParts Mercedes-Benz Forum

dubadaddy · #1 06-20-2011, 10:31 AM

Hey guys,

Just bought a 1990 300CE with 100K and did a head gasket, new water pump etc...Car has been sitting in the garage from the previous owner.

Just made a trip from San Diego to Las Vegas, only got 20 mpg! What?

I did put in some injector cleaner hoping that it would improve the mileage but to no avail.

What now, any suggestions?

Thank you in advance.

tjts1 · #2 06-20-2011, 10:58 AM

Sounds about right.
http://www.fueleconomy.gov/feg/compx2008f.jsp?year=1990&make=Mercedes-Benz&model=300CE&hiddenField=Findacar

dubadaddy · #3 06-20-2011, 11:10 AM

I also have a E420 that gets 24 mpg at the same speed on the the same trip!

I would have thought that a 6 cylinder would get about 28mpg!!!

tjts1 · #4 06-20-2011, 11:33 AM

Quote:

Originally Posted by dubadaddy

I also have a E420 that gets 24 mpg at the same speed on the the same trip!

Yup, that also sounds about right.
http://www.fueleconomy.gov/feg/compx2008f.jsp?year=1997&make=Mercedes-Benz&model=E420&hiddenField=Findacar
There is a big technology gap between your 1990 and 1997 cars. EFI and the lockup torque converter make a huge difference. Whoever said 28mpg was either lying or using some very conservative driving techniques. Real world driving will never touch that number with Kjet.

mbdoc · #5 06-20-2011, 12:03 PM

The #1 reason for poor mileage on the 1990 300CE would be a non-functional O2 sensor.

Either the fuel mixture is out of adjustment OR the O2 sensor is NOT in control of the mixture.

dubadaddy · #6 06-20-2011, 12:15 PM

MB Doc,

Am I unreasonable to expect more than 20 mpg or?

Wouldn't the check engine light come on if there was a problem with the O2 sensor?

Not sure about the mixture. Is this something my indie can do or look at and repair??

oldsinner111 · #7 06-20-2011, 12:28 PM

gosh my big w140 gets 29 with a m104 engine.

mbdoc · #8 06-20-2011, 03:34 PM

I drove a 1991 300CE from Atlanta to Jax Florida & averaged about 26 MPG at 75+

So getting 20 is somewhat LOW!

dubadaddy · #9 06-20-2011, 03:48 PM

Quote:

Originally Posted by mbdoc

I drove a 1991 300CE from Atlanta to Jax Florida & averaged about 26 MPG at 75+

So getting 20 is somewhat LOW!

Again, if it was a O2 sensor problem would it not trigger the engine light?

Could my indie take care of a fuel/air mixture problem or is it a part I would need (such as throttle body or???)?

tbomachines · #10 06-20-2011, 04:05 PM

20 highway mpg is on the very low end, I would expect more and begin troubleshooting (beginning with examining your driving technique to see how much of a factor that plays into it before spending any money on it). See if you can find a mechanic who is experienced in Bosch CIS-E injection systems, MB guys and "foreign" auto repair specialists should be fairly well versed since Bosch stuck these on a lot of different cars over the years.

For the record, I regularly meet or exceed 28mpg in my 300E on the highway, its definitely not unrealistic. Mileage on my tank yesterday was 29.2, mostly highway driving. Flame suit on.

tbomachines · #11 06-20-2011, 04:08 PM

Quote:

Originally Posted by dubadaddy

Again, if it was a O2 sensor problem would it not trigger the engine light?

Could my indie take care of a fuel/air mixture problem or is it a part I would need (such as throttle body or???)?

If you have a DMM that can measure duty cycle you can do it yourself using the diagnostic port and adjust the lambda/AFR. You can also test the O2 sensor with the DMM, do a search for "lambda" and "duty cycle" within the tech section here, there are some great guides. The DIY links also has a couple crash-course type documents that are a good primer. Wish I could help more but I'm still learning this as well...

dubadaddy · #12 07-05-2011, 09:07 AM

Yup,

Air fuel mixture was the problem. Thanks for the tip MB Doc!

JohnM. · #13 07-05-2011, 07:41 PM

My 91 300E routinely gets 26.5mpg to 27mpg highway at 80mph. I'd imagine I could touch 28mpg at 65-70mph.

dubadaddy · #14 07-05-2011, 08:00 PM

Ok,

Thought all was good with the air/fuel mixture adjustment. I replaced the fuel filter at the same time I had G&G ******** in Las Vegas do the adjustment. They said everything else looked good (timing, cat converter, oxy sensor etc.) and I don't drive it hard.

Got 24 mpg running at 79 mph out from LV to San Diego. Not good enough in my book. With the next tank combo of highway and city is right at 20mpg.

Is there anything else that could give me better mileage? My opinion is, with a 6 cylinder I should be getting 27-29 highway and 23-24 city. Am I wrong? My E420 runs at 80 mph and I get 24mpg all day long!

Also, do they make an overdrive kit for the tranny on this model? Aftermarket?

tjts1 · #15 07-05-2011, 08:39 PM

Quote:

Originally Posted by dubadaddy

My opinion is, with a 6 cylinder I should be getting 27-29 highway and 23-24 city. Am I wrong?

Yes.

If you really care about MPGs, swap a 5 speed, swap EFI or both. Or just get another car. If its highway MPG that you're after, you can smooth out the airflow underneath the car for a couple more mpg for relatively little cost.

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