89 300CE running rich/hesitation/Duty Cycle stuck at 82%
 

Hi guys,

This is my first post at PeachParts but some of you might know me from BW. Benzworld.org - Mercedes-Benz Discussion Forum - View Profile: visal911

Anyway as per title I'm having a few problems with my M103 powered 1989 300CE running rich and some lack of power.

This started being very noticeable after a new Spark plug, Distributor Cap and rotor change. I used Genuine Bosch parts and proper NGK non resistor plugs.

After Driving around for a bit like this I noticed that the car needed more push in the pedal to go and the economy gauge was in the middle at 60kmph when it usually was in the left hand corner. The new plugs were a bit dark after about 200kms. I will post pictures tomorrow when its day light.

So I tested the Duty cycle with a Multimeter and it was at 82% and 11.11V ~ according to what I was reading, higher V means That its running rich and the 82% is the proper OFF Mercedes Duty cycle.

I gave the screw a couple of tiny turns and a couple of big ones and waited for 10-20 seconds but nothing happened! The Reading did not go up OR down. It was like something else was enriching the mixture!!!

The car reads a 50% when started up and 50%~ at WOT.

Tested for Vacuum with a mittyVac - 15(hg?).

After reading lots of posts and other CIS material I have come to the conclusion that my O2 sensor has gone bad and is giving a wrong reading. Check engine light is not ON though. It flickered on and off a couple of months ago but haven't come on since. Car stinks of unburnt gas out of the exhaust.

What do you guys think? I don't want to throw parts at the car without knowing for sure and I cant really test the O2 or any other sensors because of the cold and I don't have a garage to work in.

Reasons for this happening after the tune up? City Mpg is pretty bad. I still get 10L/100kms on the highway.

What I have replaced

Rotor
Distributor Cap
Spark plugs
Fuel/Air filter within 30,000kms
EHA
Always running shell 91 without ethanol

Thanks a lot,
V

Hope to get to know people here and share my knowledge and experiences about these great cars.

At least on my 91 300SL, the screw needs to be pressed down to engage the actual adjuster. Before you go too far, press on the screw while running to lean the mixture then turn slightly to engage the adjuster and pull up slightly to richen. This will give you a idea how far you are off. Past that I have not done much with this system.

Also, on this car it had been sitting for 3 + years with old fuel. The fuel distributor plunger was sticking causing all sorts of running issues.

Hey, I can chime in here. I helped Visal adjust his duty cycle and he (we) were definitely getting the adjustment screw to turn. I've done this on my own car and I know how it feels when you get the screw, it is an easy thing to confuse when you've never done it before but I have and when you get the screw there is noticeable resistance. Anyway, we managed to change the nature of the idle (to a rougher idle) by messing with it so something is changing, but the meter read 82% no matter what. When you turn the car on it's 50% and slowly climbs up to 82 and stays there. WOT on the throttle it goes back to ~50% and then slowly climbs back up during idle. On my 190e 2.6 this worked text book, not so on his CE.

Sounds like similar problem I had on my 89 300E. Ran good when cold, but as it warmed up would run rough, the economy gauge would stay in the middle or peg to the right. Would only run good at wid open throttle. I cleanded the ICV and throttle body, changed a bunch of vacuum hoses. I tested the O2 sensor by dissconecting the wire under carpet in front of the front passenger seat with a jumper wire. While testing it it ran good when it got warmed up. The connection must have been bad. Connected it back up with a couple of twists and has been running good ever since. Got lucky on that one.

stuck at 82% is an open loop.
Check your altitude sensor for open circuit or faulty device. That's what the Mercedes repair manual says. I had the exact same situation and the manual was correct.
In your case, I would also check the microswitch just to make sure that your fault isn't on the other end of the CIS diagnosis spectrum.

Go to page 16 of document 073-121 code testing pdf file

Oh, make sure you adjust the screw back to where it should be by measuring the current through the EHA. You cannot just go crazy with that screw everytime you suspect a problem. Always make changes with the EHA current being measured to where you center the current to 0 mA and let the CIS handle the rich lean mixture from there. That current will then fluctuate up and down based on EHA compensation to regulate the fuel distributor.
Again, for the other posters, when it comes to CIS stop playing guessing games. Read up on how it works and what various adjustments you can do and when.

Professor is correct. It is in open loop.

It could be that prior adjustments with a now corrected ignition components have thrown the mixture off and it is outside the correction range of the EHA.

I have adjusted the mixture unit too far and it will max at 82% and 17% and only corrections the opposite way will allow it back into operating range.

Thank you everyone for all the replies

Where is the Altitude sensor located on this engine?

Last night I had access to my friends garage and I tested the two pronged coolant temp sensor. When one wire is removed the duty cycle goes down to 70-71%. What does that tell you?

I checked all the O2 sensor connections and they seem fine.

not sure about your CE, but the altitude sensor on my '89 SE is located @ passenger footwell, behind carpet and plastic bulkhead thing....just above 02 sensor

Duty cycle
 

I had an issue with my duty cycle running high but it did vary with throttle. It rose as revs increased. I did get cel code 17 o2sensor voltage short to ground or 12v. Look for vacuum leaks I found that the boot between the air plate and the intake manifold had gotten dry and hard and was leaking at the connection to the hoses that go to the cold start valve. Chech those two hoses also they dry out and leak vacuum

Tested the Throttle switch and air flow sensor potentiometer per Dan's page

Both tests were pretty much around ~.5%

So the Potentiometer is working fine. I tried to get it to go into closed loop by going CCW and CW till the car was barely running but i still couldn't find that sweet spot. Duty cycle stuck at 82% and fluctuating on the decimal figures.

I tested the O2 sensor per Oxygen Sensor / Lambda Sensor Voltage Testing - Benzworld.org - Mercedes-Benz Discussion Forum and all the values I got were around 4.5-7.5V. I had set the Multimeter to hundredths too.

Also there seems to be a short at my O2 sensor heater because the interior lights flicker when I move that connection around. Is that normal?

Anyways I'm buying a new O2 sensor today and see if it makes a difference.

Current at pin #5 was 12.7V and the voltage at battery was 13.6 - I'm guessing my OVP is in good shape.

Thanks everyone for all the help
Visal

UPDATE

Replaced the O2 sensor today and started the car.

Duty cycle at 66%.

Brought it down gradually and got it around 47-55% at Idle. Going to check it tomorrow at 2500rpm etc.

Car is kind of shaky at idle but runs good.

Thanks everyone for help....this was a very weird problem. I will keep updating the post as I go
:beerchug: