300E Fuel Woes... - PeachParts Mercedes-Benz Forum

Californian SE · #1 10-24-2011, 03:02 AM

Hey Everyone!

Recently I was driving a 200KM trip down to Geelong, and for some reason my fuel economy dropped when i stopped to fill up. One tank I'm on 450-500Ks (which i suspect was still too low anyway?) and now its down to 300-350 at best.

I got under her today, checked out the filter/pump and they're fine, cleaned the oxygen sensor while I was at it, and then had a quick look for leaks anywhere in the engine but nothing.

I'm fairly stumped as to where to look next, the obvious causes would not usually apply in such a quick manner? I always use premium unleaded fuel, (even at $1.60 a liter!) and run some injector cleaner through every few months (did run some through last fill up but still at the 300-350Ks per tank...)

Anyone improved economy with any different methods out there? Id be interested because I do usually long distance drives in the 300E, and I really like to keep her in tip top shape.

Thanks in advance folks!

benzme · #2 10-24-2011, 06:32 AM

I would test the O2 sensor before discounting it as being the culprit although I'm still not sure how much a dead sensor will affect fuel consumption. If it hasn't been replaced in 100k or so a fresh one couldn't hurt.

Oxygen Sensor / Lambda Sensor Voltage Testing - Benzworld.org - Mercedes-Benz Discussion Forum

My TE uses fuel like yours. Not happy. I have a new sensor and injectors waiting to go in so it will be interesting to see what the difference will be. Substantial I hope.

Keep us posted.

Californian SE · #3 10-25-2011, 01:13 AM

Thanks for that, I'll test it when I get a chance.

I think the issue is somewhere in fuel injection... Maybe blockages or leaks that evaporate to quickly to spot!

Also, there is a box attached to the injection unit "potentiometer"? or something like that?? Mine was falsely adjusted by a not so specialist mercedes mech. - perhaps this affects it also....

Thanks for the link, I was looking for that page a while ago

Cheers.

Californian SE · #4 10-26-2011, 12:51 AM

So I've been tinkering - tried to get a reading from the Oxygen Sensor - nothing.
Got a reading of between 6.0 and 10.5 in Volts for the X11 diagnosis pin between pins 2 and 3, (duty cycle) which I think comes out at anything from 50% to 25%?? with 50% being 6.0 - as max is about 14?

I then unplugged the O2 sensor, and it had NO effect whatsoever on the Duty Cycle reading?

On a side note, my distributor is adjusted well, the EHA may be a little high? And I lowered the idle speed right down as it was far to high (2000 in P sometimes). Trouble is now there is a rattle and the idle is slightly rough in Reverse and at 750RPM...

Is it possible that the O2 sensor is whats causing the mass fuel consumption? Seeing as the other things mainly focus on acceleration (EHA and Distributor settings) most of my traveling is highway KMS....

Hope this new info helps me somehow... Cheers folks.

benzme · #5 10-26-2011, 01:36 AM

Idle speed on a 300E is controlled by the ignition module and is not adjustable. If it was up around 2000rpm I can only think of a kinked or out of whack bowden cable causing it, or a massive vacuum leak.

How did you get it down? If you adjusted the duty cycle to do it then it must be sitting on a massively rich or lean stall speed. Either that or it was so far out in the first place. Not good

.

I would be looking for cracked or broken vacuum lines, especially the ones that run under the plenum chamber.

Californian SE · #6 10-26-2011, 04:47 AM

the cable for some reason was wound all the way out (thus the higher revs)

i cant seem to adjust the duty cycle too much anyway - too rich and it stalls, too lean and it just stalls again.

would the oxy sensor alter idle speeds? i am assuming that due to the fact it makes no difference to the car with or without my o2 - then it is faulty? ....

the problems just keep coming! thanks so much for your advice though i really appreciate the help!