PeachParts Mercedes-Benz Forum

PeachParts Mercedes-Benz Forum (
-   Tech Help (
-   -   300E bad gas mileage (

John H 03-09-2000 02:34 PM


I noticed a rough idle on my '86 300E two weeks ago. It also has a noticable hesitation when taking off from a dead stop. Only happens when the motor is warm. Purr's like a kitten when cold. Now for the worse part, gas mileage has gone in the sewer just as prices hit $2+/gal. Was 20 MPG now its 12MPG.

I'm not a FI expert but my first thought is the temp sensor for the fuel enrichment. Anybody else have any thoughts on where to start? I do have the manuals and a good set of tools.


Benzmac 03-09-2000 09:07 PM

It could be that or an oxygen sensor...You need a lambda meter to see if it is online and in closed loop. This is where you get better fuel economy.

1981 280GE SWB
1987 16V

John H 03-10-2000 09:30 AM

Thanks Benzmac!

I checked the temp sensor and it works beautifully. I don't have a lamba meter. I think I'll just toss a new oxygen sensor in it since the shop charge to check it would be about half the cost of the part.

stevebfl 03-10-2000 11:58 PM

A good multimeter tells all on the 86 300e. The fault memory (only active no stored codes on 86-87) is read on pin #3 of the diagnostic socket. Set your meter on duty-cycle and read the value. If you have closed-loop oxygen sensor control you will have a roving duty-cycle; lean and rich about 50% if all is adjusted properly.

If you get a constant duty-cycle it will indicate a given problem 10%, 20%, 30%, etc.
Email if you need a code chart.

To guarantee proper meter set-up. Turn key on, engine off, and you should read 70% on a Federal version car and 85% on a CA version.

If you really want to watch the whole system use your amp-meter section and hook in series to read the EHA current. The EHA (electro-hydraulic actuator) is the mechanical result of all the electronic sensing. It is a controlled leak to the upper chamber pressure of the fuel distributtor.

The amp readings will be in the -60 to +100ma range. There will be a 40+ma current for starting enrichment, 20-30 for cold enrichment, maybe 10ma for acceleration enrichment AND about 10+ and 10- milliamps of O2 sensor correction.

Once warm the value of the current flow would be 0.0ma if the O2 sensor was disconnected. When hooked up the closed loop control should have the milliamp value hovering plus or minus one or two milliamps. If it ranges from -6 to -3 and back the system would be in closed loop but too rich so the mixture is being leaned out (-ma = too rich +ma = too lean.

The relative value of your oxygen sensor can be judged by the rate of the lean/rich correction during these cycles. The sensor should be hot and the engine speed at 2000rpm long enough to stabilize.


[This message has been edited by Webmaster (edited 03-14-2000).]

John H 03-14-2000 12:49 AM


Great response. Maybe MercedesShop needs another hired gun. It's evening now so I will hook up the meter tomorrow.

I can tell you that the problem did disapear today for a while. Is that possible for a failing O2 sensor? I wish I could say the problem vanished because I fiddled with something but all I did was wash it this weekend (LOL).

Lee, still waiting for the the O2 quote.

John H 03-16-2000 11:37 PM

Swaped out the O2 sensor today. The old unit would rattle when shook. The idle hesitation is gone. Looks like that was it. Many, many, thanks Benzmac and Stevebfl!!!!!!!

Can't wait to see if the milage is back up to twenty MPG (if not I'm going to start driving my 6 MPG 428CJ Fairlane convertible around because it's a lot more fun - at least in a straight line).

All times are GMT -4. The time now is 01:21 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website