W124 '90 300E Misfire on Acceleration - PeachParts Mercedes-Benz Forum

AndreiFS · #1 03-13-2006, 12:23 PM

I have a '90 300E

My car misfires under sudden acceleration. Cold and hot start are fine.
Recently replaced ICV, OVP, air filter, spark plugs. I have a rotor, distributor cap, and coil on the way. I should mention that I think my car runs a little rich since I foolishly played with my CO mixture, I can smell unburned gas from the exhaust and there is grey smoke. Also, I think I can smell gas in the oil. Any ideas would be greatly appreciated. The car has no drivability problems. It just seems to misfire on acceleration. Sounds like it is missing but since I've never heard misfire I cannot tell if that's what it is. The noise comes from the engine bay, not the exhaust. The car gives off grey smoke at low RPM but none at all above 2000RPM. Any ideas yet?

Thanks, Andrei

G-Benz · #2 03-13-2006, 04:14 PM

Hard to determine without being there, but do you know if the engine wiring harness has ever been replaced? The factory wiring insulation (designed to deteriorate in order to be landfill-friendly) deteriorated far sooner than engineered, exposing bare wire to adjacent wires, creating short circuits that manifest in a variety of ways...including misfires.

AndreiFS · #3 03-13-2006, 05:16 PM

I doubt the harness has been changed. Anyway I thought that only affected '94-'95 models. I forgot to mention that this misfire only happens after the car is fully warmed up.

Mike Murrell · #4 03-13-2006, 05:23 PM

Quote:

Originally Posted by AndreiFS

I foolishly played with my CO mixture

Don't mean to sound like I'm flogging you, but it might be easier, quicker, etc. to pay a good MB independent tech to simply go over fuel/emission related settings.

Probably wouldn't put you out more than an hr. or two labor.

AndreiFS · #5 03-13-2006, 05:30 PM

Yeah, I know. I was getting desperate to solve another problem and I did that. Do you think reading the duty cycle and adjusting accordingly is good enough? Also, can an incorrect mixture cause misfire?

Mike Murrell · #6 03-13-2006, 05:36 PM

My experience with misfires has been limited to carburetted Asian muscle bikes.

If you believe the tech manuals, misfires are more the result of a lean condition.

I've never had to deal with this on my MB CIS/E system.

AndreiFS · #7 03-13-2006, 05:47 PM

Just to clarify, a misfire is when the engine is running and then there is a muffled thumping and vibration from the engine and there is no power, right?

A264172 · #8 03-13-2006, 06:12 PM

Quote:

Originally Posted by AndreiFS

Yeah, I know. I was getting desperate to solve another problem and I did that. Do you think reading the duty cycle and adjusting accordingly is good enough? Also, can an incorrect mixture cause misfire?

What problem were you trying to solve specifically?
You can only be effective adjusting the mix if the entire CIS-E system is working correctly.
Did you get a (Key On Engine Off) reading of 70 or 85% (aprox.)? at pins 3 and 4 with your test leads in their measurement positions?
Edit: Where the spark plugs R type? (Resistance type.)

AndreiFS · #9 03-13-2006, 06:28 PM

I was trying to solve a high idle condition. When I bought the car it had a low idle condition. I replaced the ICV and cleaned the buterfly. Afterwards I had a high idle problem. When I saw that the mixture adjustment had been tampered with (tower was broken), I assumed that the previous owner had raised the CO mix to compensate for low idle so I leaned it. Too much. Then the car barely ran so I enriched it again. I haven't measured duty cycle yet, the meter is on order. From what I understand I have to test at socket three and look for 50%. Is that with engine off and key on or engine on?

The previous spark plugs where the R type. I put old non-resistor type back in. I also have a new oxygen sensor which I haven't installed yet. Could that help?

A264172 · #10 03-13-2006, 07:00 PM

Quote:

Originally Posted by AndreiFS

I was trying to solve a high idle condition. When I bought the car it had a low idle condition. I replaced the ICV and cleaned the buterfly. Afterwards I had a high idle problem. When I saw that the mixture adjustment had been tampered with (tower was broken), I assumed that the previous owner had raised the CO mix to compensate for low idle so I leaned it. Too much. Then the car barely ran so I enriched it again. I haven't measured duty cycle yet, the meter is on order. From what I understand I have to test at socket three and look for 50%. Is that with engine off and key on or engine on?

The previous spark plugs where the R type. I put old non-resistor type back in. I also have a new oxygen sensor which I haven't installed yet. Could that help?

I am more familiar with the measurements on an 86'-89' M103.
You would turn the Key to the on position with the engine off. Then at the screw off cap (X11) you should connect your test leads to pins 3 and 2 and reverse them until you get a reading of roughly 70% (or 85% for a California car). This reading will not fluctuate.
Then You turn the engine on and let the coolant temp rise to 80°C.
Test in the pins that gave you 70% reading (with the engine running).
Your reading should fluctuate. If it does not fluctuate it is a fault code.
You want to achieve a fluctuateing reading around 50%
A reading of 50%/ non-fluctuateing, would mean a bad 0² sensor.
I'm not sure all this applies to 90' M103's completely but should be basicly similar.

A bad O2 sensor will not always show up in your duty cycle and can screw up your testing results and make your mixture rich as well. Try unpluging yours while you are waiting for your multi meter (it's in the passenger footwell under the carpet, under the padding, wire comming out of the transmission tunnel) to see if it has any benefit on preformance.
Also search this forum and read as much as you can on duty cycle and adjusting the mixture for your car. This link is a good start: http://www.landiss.com/mixture.htm

oliverb · #11 03-13-2006, 07:56 PM

Just to clarify, the wiring harness on your 1990 was not constructed with the biodegradable plastic. This is an issue from 1991-95.

AndreiFS · #12 03-13-2006, 09:16 PM

Alright, I will do this test. If the reading is off from the 50% should I adjust the mixture on the fly or adjust, then turn the engine off and start the entire process from the beginning? Is the X11 easy to find?

Mike Murrell · #13 03-13-2006, 11:59 PM

http://mercedes.braingears.com/

AndreiFS · #14 03-14-2006, 12:51 AM

Okay, I read the duty cycle. It read 70% at idle. Adjusted until it read approximately 50% with fluctuations between 46 and 53. The car no longer misfires at idle. On a test run it only misfired once. There is still light gray smoke from the back.

AndreiFS · #15 03-14-2006, 07:45 PM

The duty cycle at 2500RPM is 65%. Is that okay?

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