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, 04:08 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 · #11 07-05-2011, 09:07 AM

Yup,

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

JohnM. · #12 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.

tbomachines · #13 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.

Hirnbeiss · #14 07-05-2011, 09:38 PM

tires and tire pressure

dubadaddy · #15 07-06-2011, 12:29 PM

Tires are new and tire pressure is correct.