High fuel consumption on my 260E - why?
 

Hello everybody

I have just bought a Mercedes-Benz 260E (W124 series), European Sportline model, in very good shape(the car is from 1990), with 97,000km(about 60,300 miles) on the odometer.

The sole problem is the fuel economy - MB gives 14-16 liters per 100km (16 to 14 mpg, roughly), yet in city traffic it took up to 23 liters / 100km (10 mpg, for our American friends).

What can cause the problem? Faulty oxygen sensor or clogged air mass sensor can give a false signal to the ECU and "ask" for more gas, but shouldn't the idle increase and the engine operation become erratic? The engine works just fine, at all rpm ranges, and no warning light comes on.

There is an economy indicator on board, yet I've not revved up the engine until it shows "high". How is the device wired, and which gas flow does it indicate - at the fuel pump or at the injectors?

~Thank you

Nautilus

PS I've changed the oil upon purchase, and put 10W40. IS it suitable?

would suggest usual ture up items, plugs, air filter, etc. and new
oxygen sensor.

Thank you

Air and oil filters are both new, I've changed them immediately after buying the car.

Not sure about the plugs yet...

How much would an oxygen sensor cost, and where is it exactly placed(I'm new to the W124 model)?

And a faulty sensor wouldn't trigger a warning light on board?

~Nautilus

PS how much fuel economy have you experienced with other W124s in city cycle(traffic jams, lights etc)

I have an 87 260E, there's another post going on with the same issue. try a search on it. but 260E seems to be giving me around 19 mpg and some have reported 25-30mpg.
I've replaced all that you mentioned plus the oxygen sensors, plugs, wires etc. nothing seemed to have changed the fuel consumption. w124 is a black hole for ur wallet when it comes to such things in terms of maintenance and repair. so be wise before u start shelling out cash on it.
good luck.

The economy gauge measure vacuume preesure in the intake manifold, if I recall correctly.


10w40 is fine.

Do not replace your O2 sensor until you check to see if its functioning. Search this site for info.

Is your car aligned correctly? Tire pressure? Decel shut-off working?

Check your duty cycel to make sure car is running around 45%-55% and this will also tell you if the 02 sensor is doing its job.

Thank you.

I've dealt with a M-B workshop around here... they will test it soon. (In the meantime, learnt how the O2 sensor looks like and works :D )

Two more questions: what spark plugs are recommended by the factory for a 260E model from 1990? (I do no longer have the owner's manual) On the other side, modern 3-electrode or 4-electrode spark plugs will do better or worse? (I don't want to unscrew one before I check)

~Nautilus

The preferred plug seems to be the Bosch HD9CO, copper with a single electrode. The debate seems to favor this plug over the platinum and multi-electrode models.

There are a lot of things that can cause poor fuel economy. You might as well take care of the simple stuff to get to a good baseline. I would do a basic tune-up (plugs, air filter, clean or replace the dist cap and rotor, and change the O2 sensor, and adjust the on/off ratio (plenty of posts on this). This stuff is cheap and simple and is more cost-effective to replace than diagnose.

If that did not improve the economy, then I would get a good shop to test the more expensive stuff like the EHA and injection system.

ratio?
 

Chuck,

What is the "on/off ratio"?


fahrgewehr2,

I keep reading about duty cycle but don't understand it. Is there only one place to check this?

thanks,

Duty cycle and on-off ratio are the same thing. They refer to the percentage of time that the frequency valve is closed or open. You measure it with a DMM with a "duty cycle" function at Pin 3 on the diagnostic socket.

The ideal is fluctuation around 50% as the computer reads the O2 sensor's voltage and "trims" the air/fuel mixture by diverting fuel from the flow that goes to the injectors.

Again, there are literally hundreds of good explanations by Steve B and others on how this works. Do a search.

thanks...
 

Thanks for the response Chuck. I'm beginning to understand the electronics of my car and where the parts are...

cheers,

plugs...
 

Nautilus,

DO NOT pull the ignition wires with the wire. You have to get hold of the jacket that's around the plug; otherwise, you'll have to shell more dough like I did... ouch...

Once removed, check the resistance of the wires... I think they're supposed to be 1 K-ohm... should be stamped on the jacket....

FYI: my '87 260E gets around 23 mpg highway and 'bout 20 combo.

good luck,

Re: plugs...
 

Thanks. I had the experience with silicone wires previously, from my VW Bug... pulled the wire apart from the jacket; luckily I've crimped it back on the wire with some pliers

~Nautilus

Have you checked whether there is a leak on the fuel circuitery?

Nope... but I wouldn't bet on it. It's obviously the engine is "overfed", even exhaust gases smell like unburnt gasoline. Something "gives" too much fuel, doesn't loose it. This week I plan to change the sparkplugs, and I'll bring it to a M-B workshop for diagnose.

~Nautilus

Do you smell fuel at intake in the flow mass sensor?
If yes the membran of the pressure regulator is leaking.