W124 300E No good duty cycle reading - PeachParts Mercedes-Benz Forum

AndreiFS · #1 03-25-2006, 12:01 AM

'90 300E 110,000 miles

Hi guys, I just installed a new coil, rotor, distributor cap and wires. After some initial misfire which seems to have cleared up after some driving, the car seems fine. When I try to read duty cycle, however, I don't get a reading. With the engine off but key on I get a steady 51.1% which indicates oxygen sensor failure, correct?
When I switch the car on, the initial reading fluctuates about 50% but then after about 4 mins. jumps to about 85 then 95 then 103.0, where it remains steady. Unplugging the oxygen sensor doesn't seem to make a difference. Thick smoke also pours from the back of the car but it's hard to tell what colour it is under fluorescent lighting. It looks light gray and gets thicker when I accelerate.

Any ideas? Is the O2 sensor a likely culprit?

A264172 · #2 03-25-2006, 12:26 AM

with the o2 sensor unplugged... you should get a static (non-fluxuating) 50%

AndreiFS · #3 03-25-2006, 12:37 AM

You're right. I don't think I gave the engine enough time to adjust when I unplugged the O2 sensor.

AndreiFS · #4 03-25-2006, 09:53 PM

Okay, it misfires again. This happens when accelerating abruptly from idle. I unplugged the EHA and there was no noticable difference in performance. The duty cycle read 84.4 steady. After plugging the EHA back in the duty cycle steadily increased until topping at 103.0. With the car off but key "on" the duty cycle read 31.2 steady with the engine cold. What is going on?

PS. The car only misfires at full operating temperature.

A264172 · #5 03-26-2006, 10:59 AM

Quote:

Originally Posted by AndreiFS

... With the car off but key "on" the duty cycle read 31.2 steady with the engine cold. What is going on?

PS. The car only misfires at full operating temperature.

That means the leads are reversed (should read 70%) I had the same problem when I first tested my duty% the positive lead in the #2 pin and the negitive lead in the #3 pin will give the correct reading... (something about mercedes useing reverse duty cycle) So your real percetage is heading toward 0 (richening)
Does the car misfire with the o2 sensor unplugged? (fully warmed up) If not try unplugging the coolant temp sensor.

AndreiFS · #6 03-26-2006, 04:58 PM

I see. Thanks a lot.
The car misfires with the O2 sensor off as well as with the EHA off. The smoke also remains the same. There seems to be no difference. I will now try unplugging the coolant sensor. Is the coolant temp sensor also known as a coolant temperature switch? Since there is no difference with the EHA off, could this be pointing towards a mechanical fault such as the distrbutor or fuel pressure regulator?